CN111156928B - Grating three-dimensional scanner moire fringe eliminating method based on DLP projection - Google Patents
Grating three-dimensional scanner moire fringe eliminating method based on DLP projection Download PDFInfo
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Abstract
The invention provides a moire fringe elimination method of a grating three-dimensional scanner based on DLP projection, which comprises the steps of acquiring a standard plane multi-frequency multi-step phase shift image to obtain a standard plane absolute phase diagram; removing the plane effect according to the absolute phase diagram to obtain an absolute phase diagram of the plane effect; removing the distortion effect according to the absolute phase diagram of the deplanation effect to obtain an absolute phase diagram of the depolarisation effect; carrying out parameter space transformation on the absolute phase diagram and the absolute phase diagram with the distortion removal effect to obtain a phase value-phase error diagram; obtaining a phase value-phase error mapping table according to the phase value-phase error map; collecting multi-frequency multi-step phase shift images of the measured object, correcting an absolute phase diagram according to a phase value-phase error mapping table, and then performing three-dimensional reconstruction on the measured object. The invention can eliminate moire fringes existing in the grating three-dimensional scanning of DLP projection under the condition of ensuring the reconstruction precision and the reconstruction details, and improves the measurement effect of the grating three-dimensional scanner based on the DLP projection.
Description
Technical Field
The invention relates to the field of three-dimensional measurement, in particular to a moire fringe eliminating method of a grating three-dimensional scanner based on DLP projection.
Background
Digital Light Processing (DLP) processes image signals digitally and projects the Light. DLP employs Digital MicroMirror Device (DMD) developed by TI (texas instruments, usa) as a main key processing element to implement Digital optical processing. DMD is a digital optical switch consisting of a plurality of small square mirrors with a side length of 16 μm, each of which represents one pixel of a projected image, as shown in fig. 1, and DLP is mainly characterized by the capability of producing high-brightness, high-resolution and high-definition digital video images. With the programmability of the DMD, the image projected by the DLP can be generated by computer software as needed, which is extremely useful in grating three-dimensional measurement: the phase shift grating with a certain phase difference in required frames is generated in advance by computer software, and then DLP is projected to an object in sequence according to the phase shift sequence to realize phase shift.
However, in practical applications, since the DLP projects an image in a pixel manner, the grating projected when projecting the grating image has a certain gray scale level in one period, each gray scale level corresponds to one pixel in the DLP, and each pixel has a certain width due to the size of the small mirror in the DMD. When a Charge-Coupled Device (CCD) collects a deformed grating image, if there is a small frequency difference between spatial sampling frequencies of a DLP and a CCD image sensor, a modulation fringe will appear in an image obtained by the CCD, and the modulation fringes are moire fringes, as shown in fig. 2, a schematic diagram of moire fringes existing in an image collected by a grating three-dimensional scanner based on DLP projection is shown. Phase errors generated by the moire fringes have a relatively large influence on the three-dimensional measurement precision of the grating.
In order to reduce the influence of moire effect on the three-dimensional grating measurement precision, one method is to ensure that the spatial sampling frequency of the DLP and the CCD image sensor has larger frequency difference, however, the DLP and the CCD image sensor are modulated by an object and an attitude in the actual operation, and the larger frequency difference means larger resolution difference, which is not beneficial to high-precision measurement, so that the method has no actual operability; the other method is a defocusing projection method, which suppresses high-frequency components generated by DLP sampling in a projection light field and reduces Moire interference fringes.
Disclosure of Invention
The invention provides a moire fringe eliminating method without loss of precision and detail aiming at moire fringe phenomenon existing in grating three-dimensional scanning of DLP projection, comprising the following steps:
step 1, collecting a standard plane multi-frequency multi-step phase shift image, obtaining a main phase diagram of each frequency by using the multi-step phase shift image of each frequency, obtaining a whole-period phase diagram of each frequency by combining the main phase diagrams of all frequencies, and overlapping the whole-period phase diagram of each frequency with the main phase diagram to obtain an absolute phase diagram of each frequency;
step 2, performing plane fitting on the absolute phase diagram of each frequency to obtain a plane fitting phase diagram, and subtracting the plane fitting phase diagram from the absolute phase diagram of each frequency to obtain an absolute phase diagram of each frequency with the plane effect removed;
step 3, carrying out surface fitting for three times on each frequency plane effect-removed absolute phase diagram to obtain a surface fitting phase diagram, and subtracting the surface fitting phase diagram from each frequency plane effect-removed absolute phase diagram to obtain each frequency distortion effect-removed absolute phase diagram;
step 4, respectively taking the absolute phase value of each frequency as an X axis, taking the absolute phase value without distortion effect as a Y axis to carry out parameter space transformation, and projecting each point on a phase diagram pixel by pixel into a defined parameter space to obtain a phase value-phase error diagram;
step 5, segmenting the phase value according to the phase value-phase error graph of each frequency to obtain a segmented phase value-phase error set, eliminating the rough phase error of the segmented phase value-phase error set, then calculating the average value, and constructing a phase value-phase error mapping table by taking the segmented serial number as a key and the average value of the phase error as values;
and 6, acquiring multi-frequency multi-step phase shift images of the measured object to calculate an absolute phase image of each frequency, correcting the absolute phase value of each frequency pixel by pixel according to a phase value-phase error mapping table, superposing and averaging the absolute phase images after correction of a plurality of frequencies to obtain an average corrected absolute phase image, and performing three-dimensional reconstruction on the measured object by using the average corrected absolute phase image.
On the basis of the above technical solution, the preferable step 1 specifically includes the following steps:
step 1.1, DLP projects multi-frequency multi-step grating images onto a standard plane, and a CCD image sensor shoots multi-frequency multi-step grating images modulated by the standard plane from a position different from the DLP; the width of the CCD image sensor is w, and the height of the CCD image sensor is h; the number of the multi-frequency phase shift frequencies is n, and the ith frequency is represented as lambda i Maximum decoding space xi max ＝LCM(λ 1 ，…，λ n ) Where LCM (. Cndot.) represents the least common multiple; the number of phase shift steps is m; the gray-scale matrix sequence of the collected phase-shift image is G 1,1 ，G 1,2 ，…,G n,m-1 ,G n,m (ii) a The invention adopts n =3,m =4;
in the step 1.2, the method comprises the following steps of,
for the main phase value of pixel coordinate (u, v) on the phase-shifted image of the ith frequency, u ∈ [0,w ], v ∈ [0,h), the main phase map for each frequency is calculated by the formula:
wherein:
step 1.3, the integral period number of the ith frequency pixel coordinate (u, v) is p i (u,v)，-λ i ≤p i (u,v)≤λ 1 The number of the whole cycles satisfies the following formula:
wherein Round (·) denotes a rounding operation; simultaneously solving all frequencies to obtain the periodicity of each frequency in pixel coordinates (u, v), and obtaining a whole-period phase diagram I of each frequency according to the periodicity i (u,v)：
I i (u,v)＝p i (u,v)λ i ,i∈[1,n],u∈[0,w),v∈[0,h)
Step 1.4, calculating the absolute phase diagram of the ith frequency as A i (u,v)：
The unit of the absolute phase value is a pixel, and represents a pixel coordinate value of a projector column number corresponding to the current image.
On the basis of the above technical solution, a preferred embodiment of step 2 is: and (3) respectively carrying out plane fitting on the absolute phase diagram of each frequency:
a i u+b i v+c i ＝A i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
wherein (a) i ,b i ,c i ) Substituting the absolute phase value of the ith frequency into the formula pixel by pixel for the plane fitting coefficient to be solved of the ith frequency, and calculating the plane fitting coefficient by adjustment; obtaining a plane fitting phase diagram Y of the ith frequency according to the plane fitting coefficient i (u, v) are:
Y i (u,v)＝a i u+b i v+c i ,i∈[1,n],u∈[0,w),v∈[0,h)
subtracting the plane fitting phase diagram from the absolute phase diagram of each frequency to obtain an absolute phase diagram P of each frequency with the plane effect removed i (u, v) are as follows:
P i (u,v)＝A i (u,v)-Y i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
on the basis of the above technical solution, a preferred embodiment of step 3 is: and (3) carrying out cubic surface fitting on the absolute phase diagram of each frequency removing plane effect respectively:
d i u 3 +e i u 2 v+f i uv 2 +g i v 3 +h i u 2 +j i uv+k i v 2 +l i u+m i v+n i ＝P i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
wherein (d) i ,e i ,f i ,g i ,h i ,j i ,k i ,l i ,m i ,n i ) Substituting the absolute phase value of the pixel-by-pixel deplanation effect of the ith frequency into the formula for the curved surface fitting coefficient to be solved of the ith frequency, and calculating the curved surface fitting coefficient by using the adjustment; obtaining a surface fitting phase diagram Z of the ith frequency according to the surface fitting coefficient i (u, v) are:
Z i (u,v)＝d i u 3 +e i u 2 v+f i uv 2 +g i v 3 +h i u 2 +j i uv+k i v 2 +l i u+m i v+n i ,i∈[1,n],u∈[0,w),v∈[0,h)
subtracting the surface fitting phase diagram from the absolute phase diagram of each frequency removing plane effect to obtain an absolute phase diagram D of each frequency removing distortion effect i (u, v) are:
D i (u,v)＝P i (u,v)-Z i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
on the basis of the above technical solution, a preferred embodiment of step 4 is: each point (x, y) on the phase error map for the ith frequency is taken from the absolute phase value of each pixel (u, v) on the phase map and the absolute phase value to remove the distortion effect, where the value of x is A i (u, v) y is D i (u, v), phase value-phase error map M i (x, y) is collectively expressed as:
M i (x,y)＝{(x,y)|x＝A i (u,v)∩y＝D i (u,v),u∈[0,w),v∈[0,h)},i∈[1,n]
on the basis of the above technical solution, the preferable step 5 specifically includes the following steps:
step 5.1, the segmentation step length of the ith frequency is s i Phase value range x of jth segment ij Comprises the following steps:
counting the phase value-phase error set M in the j segment ij (x,y)：
M ij (x,y)＝{(x,y)|M i (x,y)∩x∈x ij }
Step 5.2, take out M ij The phase errors in (x, y) are sorted according to the magnitude of the phase error value to obtain a sorted phase error sequence
Wherein k is the number of phase error values; the phase error values of the front and the rear quarter are removed, the phase error value of the middle half is kept for averaging, and the average value y of the phase errors is obtained ij ：
Step 5.3, taking the serial number j of the jth segment as a key of the mapping table, and calculating the phase error average value y in the segment ij As a value, a pair of key value pairs is obtained, all segments are processed to obtain a phase value-phase error mapping table T i ：
On the basis of the above technical solution, the preferable step 6 specifically includes the following steps:
step 6.1, collecting multi-frequency multi-step phase shift images of the object to be measured to calculate an absolute phase diagram of each frequency, wherein the calculation step is the same as the step 1;
step 6.2, calculating the key k of the phase value-phase error mapping table pixel by pixel according to the absolute phase value of the ith frequency i (u,v)：
Wherein Floor (·) represents a rounding operation; according to key k i (u, V) to find the corresponding value V in the phase-to-phase error map i (u,v)：
V i (u,v)＝T i [k i (u,v)],i∈[1,n],u∈[0,w),v∈[0,h)
Subtracting the corresponding correction value from the absolute phase of each frequency to obtain a corrected absolute phase value C i (u,v)：
C i (u,v)＝A i (u,v)-V i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
Step 6.3, the multiple frequency corrected absolute phase diagrams are superposed and averaged to obtain an average corrected absolute phase diagram C (u, v), which is shown as the following formula:
and 6.4, performing three-dimensional reconstruction on the measured object by using the average corrected absolute phase diagram.
Compared with the prior art, the method for eliminating the moire fringes of the grating three-dimensional scanner based on DLP projection has the following beneficial effects:
firstly, the pose and the spatial sampling frequency of a DLP and CCD image sensor are not required, and the method has practical operability;
secondly, the details and the precision of phase shift measurement are not reduced while the moire fringe effect is suppressed;
thirdly, the operation is convenient, the calculated amount is small, and the influence on the traditional phase shift measurement process is small. The phase value-phase error mapping table is obtained by off-line calculation and stored, and does not occupy the time of real-time measurement; in real-time measurement, only an absolute phase correction flow needs to be added after an absolute phase calculation flow and before a three-dimensional reconstruction flow, the correction flow only comprises table look-up operation and subtraction operation, the calculation amount is small, the flow change amount is small, and any change on a hardware structure is not needed.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the prior art descriptions will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.
FIG. 1 is a schematic diagram of a DMD and one of its mirrors;
FIG. 2 is a schematic representation of moire fringes;
FIG. 3 is a flow chart of a moire fringe elimination method of a grating three-dimensional scanner based on DLP projection according to the present invention;
FIG. 4 is a schematic diagram of absolute phase;
FIG. 5 is a schematic diagram of absolute phase with planar effect removed;
FIG. 6 is a diagram of absolute phase to remove distortion effects;
FIG. 7 is a partial schematic diagram of phase values versus phase errors before moire fringe cancellation;
FIG. 8 is a schematic diagram of the local network formation effect of the reconstructed point cloud before moire fringe elimination;
FIG. 9 is a partial diagram of phase values versus phase errors after moire cancellation;
fig. 10 is a schematic diagram of a local network formation effect of reconstructed point cloud after moire fringe elimination.
Detailed Description
The technical solutions of the present invention are described in detail below with reference to the drawings and examples, and the technical solutions in the embodiments of the present invention are clearly and completely described. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
The invention provides a moire fringe eliminating method without loss of precision and detail aiming at a moire fringe phenomenon existing in grating three-dimensional scanning of DLP projection, as shown in figure 3, comprising the following steps:
step 1, collecting a standard plane multi-frequency multi-step phase shift image, obtaining a main phase diagram of each frequency by using the multi-step phase shift image of each frequency, obtaining a whole-period phase diagram of each frequency by combining the main phase diagrams of all frequencies, and overlapping the whole-period phase diagram of each frequency with the main phase diagram to obtain an absolute phase diagram of each frequency.
Step 1.1, DLP projects multi-frequency multi-step grating images onto a standard plane, and a CCD image sensor shoots the multi-frequency multi-step grating images modulated by the standard plane from a position different from the DLP. The width of the CCD image sensor is w, and the height of the CCD image sensor is h; the number of the multi-frequency phase shift frequencies is n, and the ith frequency is represented as lambda i Maximum decoding space xi max ＝LCM(λ 1 ，…，λ n ) Where LCM (. Cndot.) represents the least common multiple; number of phase shift stepsThe number is m; the gray-scale matrix sequence of the collected phase-shift image is G 1,1 ，G 1,2 ，…,G n,m-1 ,G n,m . The invention adopts n =3,m =4.
And 1.2, obtaining a main phase diagram of each frequency by using the multi-step phase-shift image of each frequency. Suppose that
For the main phase value of pixel coordinate (u, v) on the phase-shifted image of the ith frequency, u ∈ [0,w ], v ∈ [0,h), the main phase map for each frequency is calculated by the formula:
wherein:
and step 1.3, combining the main phase values of all frequencies to obtain a whole-cycle phase diagram of each frequency. The number of whole periods of the ith frequency pixel coordinate (u, v) is p i (u,v)，-λ i ≤p i (u,v)≤λ 1 The whole period number satisfies the following formula:
where Round (·) denotes a rounding operation. According to the formula (3), all frequencies are solved simultaneously to obtain the period number of each frequency in the pixel coordinate (u, v), and the whole period phase diagram I of each frequency is obtained according to the period number i (u, v) are as follows:
I i (u,v)＝p i (u,v)λ i ,i∈[1,n],u∈[0,w),v∈[0,h) (4)
and step 1.4, superposing the whole-period phase diagram of each frequency with the main phase diagram to obtain an absolute phase diagram of each frequency. The absolute phase diagram of the ith frequency is A i (u, v), the calculation formula is as follows:
the unit of the absolute phase value is a pixel, and represents a pixel coordinate value of a projector column number corresponding to the current image. Fig. 4 shows an absolute phase diagram.
And 2, performing plane fitting on the absolute phase diagram of each frequency to obtain a plane fitting phase diagram, and subtracting the plane fitting phase diagram from the absolute phase diagram of each frequency to obtain the absolute phase diagram of each frequency without the plane effect.
Since the phase shift encoding and the standard plane spatially modulate the projected grating phase, the absolute phase diagram has a large trend that increases from left to right, which annihilates the moire effect and is not beneficial to analysis. Therefore, the absolute phase map of each frequency is subjected to plane fitting, and the formula is as follows:
a i u+b i v+c i ＝A i (u,v),i∈[1,n],u∈[0,w),v∈[0,h) (6)
wherein (a) i ,b i ,c i ) And substituting the absolute phase value of the ith frequency into the formula pixel by pixel for the plane fitting coefficient to be solved of the ith frequency, and calculating the plane fitting coefficient by adjustment. Obtaining a plane fitting phase diagram Y of the ith frequency according to the plane fitting coefficient i (u, v) are:
Y i (u,v)＝a i u+b i v+c i ,i∈[1,n],u∈[0,w),v∈[0,h) (7)
subtracting the plane fitting phase diagram from the absolute phase diagram of each frequency to obtain an absolute phase diagram P of each frequency with the plane effect removed i (u, v) are as follows:
P i (u,v)＝A i (u,v)-Y i (u,v),i∈[1,n],u∈[0,w),v∈[0,h) (8)
fig. 5 is a diagram showing an absolute phase diagram of removing the planar effect. It can be seen that after the planar effect is removed, the absolute phase diagram has an obvious curved surface deformation trend, and the moire fringe effect cannot be directly seen.
And 3, carrying out surface fitting for three times on each frequency plane effect removed absolute phase diagram to obtain a surface fitting phase diagram, and subtracting the surface fitting phase diagram from each frequency plane effect removed absolute phase diagram to obtain each frequency distortion effect removed absolute phase diagram.
Due to distortion of a projector and a camera and modulation of phases by geometrical relations, an absolute phase diagram without a plane effect still has a trend of curved surface deformation, and the distortion trend annihilates a moire fringe effect and is not beneficial to analysis. Therefore, cubic surface fitting is performed on the absolute phase diagram of each frequency removing the plane effect, and the formula is as follows:
d i u 3 +e i u 2 v+f i uv 2 +g i v 3 +h i u 2 +j i uv+k i v 2 +l i u+m i v+n i ＝P i (u,v),i∈[1,n],u∈[0,w),v∈[0,h) (9)
wherein (d) i ,e i ,f i ,g i ,h i ,j i ,k i ,l i ,m i ,n i ) Substituting the absolute phase value of the pixel-by-pixel deplanation effect of the ith frequency into the formula, and calculating the surface fitting coefficient by the adjustment. Obtaining a surface fitting phase diagram Z of the ith frequency according to the surface fitting coefficient i (u, v) are:
Z i (u,v)＝d i u 3 +e i u 2 v+f i uv 2 +g i v 3 +h i u 2 +j i uv+k i v 2 +l i u+m i v+n i ,i∈[1,n],u∈[0,w),v∈[0,h) (10)
subtracting the surface fitting phase diagram from the absolute phase diagram of each frequency removing plane effect to obtain an absolute phase diagram D of each frequency removing distortion effect i (u, v) are as follows:
D i (u,v)＝P i (u,v)-Z i (u,v),i∈[1,n],u∈[0,w),v∈[0,h) (11)
fig. 6 is a diagram showing the absolute phase of the distortion effect. It is observed that after the distortion effect is removed, the absolute phase diagram is not obviously systematically regular, but a local oblique fringe phenomenon exists, which is a reflection of the moire effect on the absolute phase diagram.
Step 4, respectively using the absolute phase value of each frequency as an X axis and the absolute phase value without distortion effect as a Y axis to carry out parameter space transformation, projecting each point on a phase diagram pixel by pixel into a defined parameter space, and obtaining a phase value-phase error diagram M i (x,y)。
Each point (x, y) on the phase error map of the ith frequency is taken from the absolute phase value of each pixel (u, v) on the phase map and the absolute phase value to remove distortion effects, where x is a i (u, v) y is D i (u, v), the phase value-phase error map M i (x, y) is collectively expressed as:
M i (x,y)＝{(x,y)|x＝A i (u,v)∩y＝D i (u,v),u∈[0,w),v∈[0,h)},i∈[1,n] (12)
FIG. 7 is a partial diagram (x e [600,610 ]) showing the relationship between phase value and phase error of the standard plane before moire fringe elimination, in which the jagged pattern of 1 pixel period can be seen, which is a visual reflection of moire effect on the relationship between phase value and phase error. Theoretically, the phase value-phase error relation graph obtained by the standard plane should be randomly distributed around 0; however, due to the influence of moire effect, a saw-tooth distribution exceeding ± 0.04 pixel appears, which not only affects the accuracy of phase measurement, but also causes the final reconstructed point cloud to have a fringe-like phenomenon, affecting data details.
Fig. 8 is a schematic diagram showing a local network formation effect of a standard plane reconstructed point cloud before moire fringe elimination, which is used for performing plane fitting on the reconstructed standard plane point cloud and displaying the fitting plane and the network formation result in an overlapping manner. It can be seen from the figure that some cavities appear in the net-forming result without moire fringe elimination operation, and obvious fringe-like phenomenon can be seen after the holes are superposed with the fitting plane.
And step 5, segmenting the phase value according to the phase value-phase error graph of each frequency to obtain a segmented phase value-phase error set, eliminating the coarse phase error of the segmented phase value-phase error set, then calculating the average value, and constructing a phase value-phase error mapping table by taking the segment serial number as a key and the phase error average value as a value.
And 5.1, respectively segmenting the phase value of each frequency phase value-phase error graph to obtain a segmented phase value-phase error set. The step size of the segment of the ith frequency is s i Phase value range x of jth segment ij Comprises the following steps:
counting the phase value-phase error set M in the j segment ij (x,y)：
M ij (x,y)＝{(x,y)|M i (x,y)∩x∈x ij } (14)
And 5.2, eliminating the rough difference of the phase error from the segmented phase value-phase error set, and then calculating the average value.
Taking out M ij The phase errors in (x, y) are sorted according to the magnitude of the phase error value to obtain a sorted phase error sequence
Where k is the number of phase error values. Removing the phase error value of each quarter of the front and the back, keeping the phase error value of the middle half for averaging, and obtaining the average value y of the phase errors ij The expression is:
and 5.3, constructing a phase value-phase error mapping table by taking the segment serial numbers as keys and the average value of the phase errors as values. Taking the serial number j of the jth segment as a key of a mapping table, and calculating the phase error average value y in the segment ij As values, a pair of key-value pairs is obtained, proceeding for all segmentsProcessing the obtained phase value to phase error mapping table T i Comprises the following steps:
and 6, acquiring a multi-frequency multi-step phase shift image of the measured object to calculate an absolute phase image of each frequency, correcting the absolute phase value of each frequency pixel by pixel according to a phase value-phase error mapping table, superposing and averaging the corrected absolute phase images of a plurality of frequencies to obtain an average corrected absolute phase image, and performing three-dimensional reconstruction on the measured object by using the average corrected absolute phase image.
And 6.1, acquiring multi-frequency multi-step phase shift images of the measured object to calculate an absolute phase diagram of each frequency. The calculation procedure is identical to step 1.
And 6.2, correcting the absolute phase value of each frequency according to the phase value-phase error mapping table. Calculating a key k of a phase value-phase error map pixel by pixel based on an absolute phase value of an ith frequency i (u, v), the calculation method is as follows:
where Floor (·) represents a round-off operation. According to key k i (u, V) to find the corresponding value V in the phase-to-phase error map i (u, v) as shown in the following formula:
V i (u,v)＝T i [k i (u,v)],i∈[1,n],u∈[0,w),v∈[0,h) (18)
subtracting the corresponding correction value from the absolute phase of each frequency to obtain a corrected absolute phase value C i (u, v) as shown in the following formula:
C i (u,v)＝A i (u,v)-V i (u,v),i∈[1,n],u∈[0,w),v∈[0,h) (19)
and 6.3, superposing and averaging the plurality of frequency-corrected absolute phase maps to obtain an average corrected absolute phase map C (u, v). As shown in the following formula:
and 6.4, performing three-dimensional reconstruction on the measured object by using the average corrected absolute phase diagram. As shown in fig. 9, which is a local schematic diagram (x ∈ [600,610 ]) of a phase value-phase error relationship of a standard plane after moire fringe elimination, it can be seen that, after being corrected by the method provided by the present invention, the phase value-phase error relationship diagram obtained by the standard plane is randomly distributed around 0 (the phase error distribution is relatively thick around an integer coordinate because of the dispersion of the phase error distribution), which effectively improves the accuracy of phase measurement and the details of point cloud reconstruction.
Fig. 10 is a schematic diagram of local network formation effect of standard plane reconstructed point cloud after moire fringe elimination, which is used for performing plane fitting on reconstructed standard plane point cloud and displaying the fitting plane and network formation result in an overlapping manner. As can be seen from the figure, after the moire fringe elimination operation, the network construction result is relatively complete, and meanwhile, the fringe phenomenon is not very obvious after the network construction result is superposed with the fitting plane, and the network construction result basically presents random distribution.
From the implementation steps, compared with the traditional method, the method has the following remarkable advantages:
firstly, the pose and the spatial sampling frequency of a DLP and CCD image sensor are not required, and the method has practical operability;
secondly, the details and the precision of phase shift measurement are not reduced while the moire fringe effect is suppressed;
thirdly, the operation is convenient, the calculated amount is small, and the influence on the traditional phase shift measurement process is small. The phase value-phase error mapping table is obtained by off-line calculation and stored, and does not occupy the time of real-time measurement; in real-time measurement, only an absolute phase correction flow needs to be added after an absolute phase calculation flow and before a three-dimensional reconstruction flow, the correction flow only comprises table look-up operation and subtraction operation, the calculation amount is small, the flow change amount is small, and any change on a hardware structure is not needed.
In addition, the invention is suitable for a single-camera grating structure light three-dimensional measurement system and is also suitable for a multi-camera grating structure light three-dimensional measurement system.
In specific implementation, the above processes can be automatically operated by adopting a computer software mode, and a system device for operating the method also needs to be in a protection range.
The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.
Claims (6)
1. A moire fringe eliminating method of a grating three-dimensional scanner based on DLP projection is characterized in that: the moire fringe eliminating method of the grating three-dimensional scanner based on DLP projection comprises the following steps:
step 1, collecting a standard plane multi-frequency multi-step phase shift image, obtaining a main phase diagram of each frequency by using the multi-step phase shift image of each frequency, obtaining a whole-period phase diagram of each frequency by combining the main phase diagrams of all frequencies, and overlapping the whole-period phase diagram of each frequency with the main phase diagram to obtain an absolute phase diagram of each frequency;
step 2, performing plane fitting on the absolute phase diagram of each frequency to obtain a plane fitting phase diagram, and subtracting the plane fitting phase diagram from the absolute phase diagram of each frequency to obtain an absolute phase diagram of each frequency with the plane effect removed;
step 3, carrying out surface fitting for three times on each frequency plane effect-removed absolute phase diagram to obtain a surface fitting phase diagram, and subtracting the surface fitting phase diagram from each frequency plane effect-removed absolute phase diagram to obtain each frequency distortion effect-removed absolute phase diagram;
step 4, respectively taking the absolute phase value of each frequency as an X axis, taking the absolute phase value without distortion effect as a Y axis to carry out parameter space transformation, and projecting each point on a phase diagram pixel by pixel into a defined parameter space to obtain a phase value-phase error diagram;
step 5, segmenting the phase value according to the phase value-phase error graph of each frequency to obtain a segmented phase value-phase error set, eliminating the rough phase error of the segmented phase value-phase error set, then calculating the average value, and constructing a phase value-phase error mapping table by taking the segmented serial number as a key and the average value of the phase error as values;
step 6, collecting multi-frequency multi-step phase shift images of the measured object to calculate an absolute phase diagram of each frequency, calculating keys of a phase value-phase error mapping table pixel by pixel according to the absolute phase value of each frequency, finding corresponding values according to the keys to the phase-phase error mapping table, subtracting corresponding correction values from the absolute phase of each frequency to obtain corrected absolute phase values, superposing and averaging the absolute phase diagrams after correction of a plurality of frequencies to obtain an average corrected absolute phase diagram, and performing three-dimensional reconstruction of the measured object by using the average corrected absolute phase diagram;
the step 6 specifically comprises the following steps:
step 6.1, collecting multi-frequency multi-step phase shift images of the measured object to calculate an absolute phase diagram of each frequency, wherein the calculation step is the same as the step 1;
step 6.2, calculating the key k of the phase value-phase error mapping table pixel by pixel according to the absolute phase value of the ith frequency i (u,v)：
Wherein Floor (·) represents a rounding operation; according to key k i (u, V) to find the corresponding value V in the phase-to-phase error map i (u,v)：
V i (u,v)＝T i [k i (u,v)],i∈[1,n],u∈[0,w),v∈[0,h)
Subtracting the corresponding correction value from the absolute phase of each frequency to obtain a corrected absolute phase value C i (u,v)：
C i (u,v)＝A i (u,v)-V i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
Step 6.3, the multiple frequency corrected absolute phase diagrams are superposed and averaged to obtain an average corrected absolute phase diagram C (u, v), which is shown as the following formula:
and 6.4, performing three-dimensional reconstruction on the measured object by using the average corrected absolute phase diagram.
2. The method for eliminating moire fringes of a grating three-dimensional scanner based on DLP projection as claimed in claim 1, wherein: the step 1 specifically comprises the following steps:
step 1.1, DLP projects multi-frequency multi-step grating images onto a standard plane, and a CCD image sensor shoots multi-frequency multi-step grating images modulated by the standard plane from a position different from the DLP; the width of the CCD image sensor is w, and the height of the CCD image sensor is h; the number of the multi-frequency phase shift frequencies is n, and the ith frequency is represented as lambda i Maximum decoding space xi max ＝LCM(λ 1 ，L，λ n ) Where LCM (. Cndot.) represents the least common multiple; the number of phase shift steps is m; the gray-scale matrix sequence of the collected phase-shift image is G 1，1 ，G 1，2 ，…，G n，m-1 ，G n，m (ii) a The invention adopts n =3,m =4;
in the step 1.2, the method comprises the following steps of,
for the main phase value of pixel coordinate (u, v) on the phase-shifted image of the ith frequency, u ∈ [0,w ], v ∈ [0,h), the main phase map for each frequency is calculated by the formula:
wherein:
step 1.3, the integral period number of the ith frequency pixel coordinate (u, v) is p i (u,v)，-λ i ≤p i (u,v)≤λ 1 The number of the whole cycles satisfies the following formula:
wherein Round (·) denotes a rounding operation; simultaneously solving all frequencies to obtain the period number of each frequency in pixel coordinates (u, v), and obtaining a whole period phase diagram I of each frequency according to the period number i (u,v)：
I i (u,v)＝p i (u,v)λ i ,i∈[1,n],u∈[0,w),v∈[0,h)
Step 1.4, calculating the absolute phase diagram of the ith frequency as A i (u,v)：
The unit of the absolute phase value is a pixel, and represents a pixel coordinate value of a projector column number corresponding to the current image.
3. The method for eliminating moire fringes of a grating three-dimensional scanner based on DLP projection as claimed in claim 1, wherein: the implementation of the step 2 is as follows:
and (3) respectively carrying out plane fitting on the absolute phase diagram of each frequency:
a i u+b i v+c i ＝A i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
wherein (a) i ,b i ,c i ) Substituting the absolute phase value of the ith frequency into the formula pixel by pixel for the plane fitting coefficient to be solved of the ith frequency, and calculating the plane fitting coefficient by adjustment; obtaining a plane fitting phase diagram Y of the ith frequency according to the plane fitting coefficient i (u, v) are:
Y i (u,v)＝a i u+b i v+c i ,i∈[1,n],u∈[0,w),v∈[0,h)
subtracting the plane fitting phase map from the absolute phase map of each frequency to obtain an absolute phase map of each frequency with the plane effect removed:
P i (u,v)＝A i (u,v)-Y i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
wherein P is i (u, v) represents an absolute phase diagram of the deplanation effect.
4. The method for eliminating moire fringes of a grating three-dimensional scanner based on DLP projection as claimed in claim 1, wherein: the implementation of the step 3 is as follows:
and (3) carrying out cubic surface fitting on the absolute phase diagram of each frequency removing plane effect respectively:
d i u 3 +e i u 2 v+f i uv 2 +g i v 3 +h i u 2 +j i uv+k i v 2 +l i u+m i v+n i ＝P i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
wherein (d) i ,e i ,f i ,g i ,h i ,j i ,k i ,l i ,m i ,n i ) Substituting the absolute phase value of the pixel-by-pixel deplanation effect of the ith frequency into the formula for the curved surface fitting coefficient to be solved of the ith frequency, and calculating the curved surface fitting coefficient by using the adjustment; obtaining a surface fitting phase diagram Z of the ith frequency according to the surface fitting coefficient i (u, v) are:
Z i (u,v)＝d i u 3 +e i u 2 v+f i uv 2 +g i v 3 +h i u 2 +j i uv+k i v 2 +l i u+m i v+n i ,i∈[1,n],u∈[0,w),v∈[0,h)
subtracting the surface fitting phase diagram from the absolute phase diagram of each frequency removing plane effect to obtain the absolute phase diagram of each frequency removing distortion effect:
D i (u,v)＝P i (u,v)-Z i (u,v),i∈[1,n],u∈[0,w),v∈[0,h)
wherein D is i (u, v) shows the absolute phase diagram of the undistorted effect.
5. The method for eliminating moire fringes of a grating three-dimensional scanner based on DLP projection as claimed in claim 1, wherein: the implementation of the step 4 is as follows:
each point (x, y) on the phase error map of the ith frequency is taken from the absolute phase value of each pixel (u, v) on the phase map and the absolute phase value to remove distortion effects, where x is a i (u, v) y is D i (u, v), the phase value-phase error map is collectively represented as:
M i (x,y)＝{(x,y)|x＝A i (u,v)∩y＝D i (u,v),u∈[0,w),v∈[0,h)},i∈[1,n]
wherein M is i (x, y) represents a phase value-phase error map.
6. The method for eliminating moire fringes of a grating three-dimensional scanner based on DLP projection as claimed in claim 1, wherein: the step 5 specifically comprises the following steps:
step 5.1, the segmentation step length of the ith frequency is s i Phase value range x of jth segment ij Comprises the following steps:
counting the phase value-phase error set M in the j segment ij (x,y)：
M ij (x,y)＝{(x,y)|M i (x,y)∩x∈x ij }
Step 5.2, take out M ij The phase errors in (x, y) are sorted according to the magnitude of the phase error value to obtain a sorted phase error sequence
L,/>
Wherein k is the number of phase error values; the phase error values of the front and the rear quarter are removed, the phase error value of the middle half is kept for averaging, and the average value y of the phase errors is obtained ij ：
Step 5.3, taking the serial number j of the jth segment as a key of the mapping table, and calculating the phase error average value y in the segment ij As values, a pair of key value pairs is obtained, all segments are processed to obtain a phase value-phase error mapping table:
wherein, T i A phase value-phase error map is represented.
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