CN109949384B - Method for instantly generating color stippling based on increment Voronoi sequence - Google Patents
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Abstract
The method for instantly generating the color stippling based on the incremental Voronoi sequence comprises the following steps: randomly generating seed points in a sampling area, dividing the sampling area into Voronoi diagrams according to the generated seed points, obtaining sampling points by using the furthest point strategy according to the current Voronoi diagrams, adding the sampling points into the Voronoi division, and continuously iterating an algorithm to obtain a required sampling sequence; initializing parameters of an input image I and an output image O, determining the number n of sampling points, selecting n sampling points from the generated IVS to form a sequence S, and calculating the size A of a sampling area of a picture; determining the radius of a pure color point corresponding to the sampling point; dividing the sampling sequence into a plurality of subsets; calculating the position of the sampling point on the image I in the subset for each sampling point; each channel adopts a threshold strategy to determine the color selected by each sampling point in the set; and outputting the finally generated image O. The color stippling generation method has high efficiency and can generate color stippling in near real time.
Description
Technical Field
The invention belongs to the technical field of computer graphics, and particularly relates to a method for instantly generating a color stippling based on an incremental Voronoi sequence.
Background
Color stippling is a pictorial representation of a uniform and unordered placement of pure color dots on a canvas that produces a pictorial representation of a large number of primary color dots directly on a picture that is consistent with an intended impression. The color stippling was first proposed by the French impression Pai painter George, repair and draw, which is a painting technique that uses the principle of color decomposition and combination in physics, using only solid color dots, without the need to blend multiple complex colors.
The basis of color stippling is color separation, and in computer graphics, there are RGB color space, CMY color space, and RYB color space, which are common color spaces related to this. The RGB color space uses additive color models with red, green, and blue colors as three primary colors, and is commonly applied in display devices; the CMY color space uses subtractive color models using cyan, magenta and yellow as three primary colors, and is generally used in printing apparatuses; the RYB color space uses subtractive color models with red, yellow and blue colors as the three primary colors, which are commonly used in painting. Computer graphics has been studied for stippling synthesis for more than two decades, with most of the work focused on black and white stippling simulation. In color stippling, since the points of different primary colors belong to different sets, how to balance the point sets of different primary colors is a problem of high calculation overhead, so the color stippling generating method in the prior art has the problems of low generating speed or low quality of the generated stippling.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a method for instantly generating a color stippling based on an incremental Voronoi sequence.
The invention discloses a method for instantly generating a color stippling based on an increment Voronoi sequence, which comprises the following steps:
s1, generating IVS: randomly generating seed points in a sampling area, dividing the sampling area into Voronoi diagrams according to the generated seed points, obtaining sampling points by using the furthest point strategy according to the current Voronoi diagrams, adding the sampling points into the Voronoi division, and continuously iterating an algorithm to obtain a required sampling sequence; in the IVS generation process, the sequence number of a sampling point in the IVS is inversely proportional to the area of the corresponding Voronoi region;
s2, initializing parameters of an input image I and an output image O, determining the number n of sampling points, selecting n sampling points from the IVS generated in the step S1 to form a sequence S, and calculating the size A of a sampling area of a picture;
s3, determining the radius r of the pure color point corresponding to the sampling point by the step S1; dividing the sequence S into m subsets S j Where j=1, 2, m; for each ofSampling point S j，i At subset S j In calculating the position p of the sampling point on the image I i The method comprises the steps of carrying out a first treatment on the surface of the At position p of image I i Three tone values are taken, the three tone values are respectively projected into the space corresponding to the stippling, and the values becomeWherein k=1, 2,3; each channel adopts a threshold strategy to determine the color selected by each sampling point in the set; the color selected on the corresponding position point of the output image O is b;
s4, outputting the finally generated image O.
In the method as described above, in step S1, the serial number of the sampling point in the IVS is inversely proportional to the area of the Voronoi region corresponding to the sampling point, specifically:
a i ≈A 0 /i
wherein i is the serial number of the sampling point, a i For the Voronoi area corresponding to the ith sampling point, A 0 Is the total area of the sampling area.
The method as described above, wherein the relation between the size and the number of sampling points in the image local area and the image tone of the area is:
gA＝nδ
where g is the image tone, a is the image local area, n is the number of sampling points, the area of delta pure color points, delta = pi r 2 R is the radius of the pure color point.
The method as described above, wherein each channel in step 3 determines the color selected by each sampling point in the set by using a threshold strategy, specifically:
when (when)When select=1; when->When select=0; the sample point is selected when the threshold select=1, otherwise the sample point is discarded.
The method for instantly generating the color stippling based on the incremental Voronoi sequence has the following advantages:
1. generation of IVS: the number of the generated sampling points can be changed at any time; the distribution of sampling points has good blue noise characteristics; the sampling algorithm does not depend on the resolution of the sampling area; the sampling points may be generated off-line. Therefore, by using the IVS as a sampling method, stippling can be efficiently generated.
2. Color space conversion: the present invention is expressed using three color spaces of RGB, CMY, and RYB, and since black is hardly visually mixed by its three primary colors in the three color spaces, adding black (K) expands the three spaces to RGBK, CMYK, and RYBK. The color stippling is to place pure color points on a white canvas, and for CMY and RYB, the color space meeting the subtractive color model is directly placed on the canvas, and for RGB additive color model, the operation is performed after the numerical values of all channels are reversed.
3. Threshold strategy: the threshold strategy is critical to the final generated color stippling. After the sampling points are generated, how to select the stay-away problem of each sampling point and the color corresponding to the sampling point in the sampling area according to the tone value of the picture, which determines the quality of the finally generated color stippling.
4. The technical scheme provided by the invention is a method for instantly generating the color stippling by using a threshold strategy based on IVS, and avoids using a too complex color stippling generation algorithm. The color stippling technology supporting different primary color spaces is provided, and the problem that the model is too single and cannot be effectively popularized is solved. The technical scheme can generate high-quality colored stippling at a speed close to real time.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention and do not constitute an undue limitation. In the drawings:
FIG. 1 is a flow chart of a method of instant generation of color stippling based on incremental Voronoi sequences of the present invention;
FIG. 2 is a color stippling graph generated by selecting different numbers of points in an IVS according to an embodiment of the present invention;
FIG. 3 is a color point drawing generated under RGBK, CMYK, RYBK and RGBCMYK corresponding to different pictures according to an embodiment of the present invention;
FIG. 4 is a color stippling diagram generated by selecting different numbers of dots in an IVS using seven primary colors of R, G, B, C, M, Y, K according to an embodiment of the present invention;
FIG. 5 is a graph of performance analysis using different numbers of samples according to an embodiment of the present invention;
FIG. 6 is a graph of PSNR versus the number of points selected in an IVS for a stippled point generated in an embodiment of the present invention;
FIG. 7 is a PSNR diagram comparing different pictures in different spaces according to an embodiment of the present invention;
FIG. 8 is a graph comparing stippling with seven-color stippling in an IVS of an embodiment of the present invention;
fig. 9 is a color stippling diagram of an embodiment of the present invention using rgbcyk on a white background.
Detailed Description
The following will describe embodiments of the present invention in detail by referring to examples and drawings, so that the implementation process of how to apply the technical means to solve the technical problems and achieve the technical effects of the present invention can be fully understood and implemented accordingly.
Halftoning refers to the technique of quantizing a continuous tone image (such as a gray-scale image and a color image) into a binary image or a color image of only a few colors with a small number of colors, and the quantized image has a visual effect similar to that of the original image over a certain distance. The present invention uses a color image as an input and utilizes a halftone technique to quantize the color image into a stippled image of finite primary color points.
Fig. 1 is a flowchart of a method for instantly generating a color stippling based on an incremental Voronoi sequence provided by the present invention. As shown in fig. 1, the method for instantly generating a color stippling based on an incremental Voronoi sequence of the present invention may include the following steps (S1 to S4):
s1, generating IVS: randomly generating seed points in a sampling area, dividing the sampling area into Voronoi diagrams according to the generated seed points, obtaining sampling points by using the furthest point strategy according to the current Voronoi diagrams, adding the sampling points into the Voronoi division, and continuously iterating an algorithm to obtain a required sampling sequence; in the IVS generation process, the sequence number of the sampling points in the IVS is inversely proportional to the area of the corresponding Voronoi region.
Specifically, in step S1, the serial number of the sampling point in the IVS is inversely proportional to the area of the Voronoi region corresponding to the sampling point, specifically:
a i ≈A 0 /i
wherein i is the serial number of the sampling point, a i For the Voronoi area corresponding to the ith sampling point, A 0 Is the total area of the sampling area.
The relation between the size and the number of sampling points in the local area of the image and the hue of the image in the area is as follows:
gA＝nδ
where g is the image tone, a is the image local area, n is the number of sampling points, the area of delta pure color points, delta = pi r 2 R is the radius of the pure color point.
S2, initializing parameters of an input image I and an output image O, determining the number n of sampling points, selecting n sampling points from the IVS generated in the step S1 to form a sequence S, and calculating the size A of a sampling area of a picture.
S3, determining the radius of the pure color point corresponding to the sampling point by the step S1; dividing the sequence S into a plurality of subsets; calculating the position of the sampling point on the image I in the subset for each sampling point; three tone values are taken at the position of the image I, and the three tone values are respectively projected into a space corresponding to the stippling; each channel adopts a threshold strategy to determine the color selected by each sampling point in the set; the color selected at the corresponding position point of the output image O is b.
Specifically, the radius r of the pure color point corresponding to the sampling point is determined by the step S1; dividing the sequence S into m subsets S j Where j=1, 2, m; for each sampling point S j，i At subset S j In calculating the position p of the sampling point on the image I i The method comprises the steps of carrying out a first treatment on the surface of the In image IPosition p i Three tone values are taken, the three tone values are respectively projected into the space corresponding to the stippling, and the values becomeWherein k=1, 2,3; each channel adopts a threshold strategy to determine the color selected by each sampling point in the collection, whenWhen select=1; when->When select=0; selecting the sample point when the threshold select=1, otherwise discarding the sample point; the color selected at the corresponding position point of the output image O is b.
S4, outputting the finally generated image O.
The following is an example of a method for instantly generating a color stippling based on an incremental Voronoi sequence provided by the present invention.
A first part:
the complete steps of this embodiment are as follows:
step 1: generating IVS: randomly generating seed points in a sampling area, dividing the sampling area into Voronoi diagrams according to the generated seed points, obtaining sampling points by using the furthest point strategy according to the current Voronoi diagrams, adding the sampling points into the Voronoi division, and continuously iterating an algorithm to obtain a required sampling sequence; in the process of generating the IVS, the following relation exists that the serial numbers of sampling points in the IVS are inversely proportional to the areas of the Voronoi areas corresponding to the IVS, namely the areas a of the Voronoi areas corresponding to the ith sampling point i Total area A of the sampling area 0 The relation of (2) is:
a i ≈A 0 /i (1)
step 2: the size and number of points in a local area of the image should be related to the image tone of that area. An image area of tone g with area a can be represented by area δ=pi r 2 Is represented by n solid-colored dots:
gA＝nδ
(2)
step 3: initializing parameters of an input image I and an output image O, determining the number n of sampling points, selecting n sampling points from the IVS generated in the step 1 to form a sequence S, and calculating the sampling area size A of a picture.
Step 4: the radius size of the stippling can be determined to be r by the step (1).
Step 5: dividing the sequence S into m subsets S j Where j=1, 2,..m.
Step 6: for each subset S j do
Step 7: for each sampling point S j，i At subset S j In do
Step 8: calculating the position p of the sampling point on the image I i 。
Step 9: at position p of image I i Three tone values are taken.
Step 10: projecting the three tone values into the space corresponding to the stippling, respectively, the values becomeWhere k=1, 2,3.
Step 11: and each channel adopts a threshold strategy to determine the color selected by each sampling point in the set.
The sample point is selected when select=1, otherwise the sample point is discarded.
Step 12: and the color b selected at the corresponding position point of the output image O.
Step 13: end for
Step 14: end for
Step 15: and outputting the finally generated image O.
A second part:
first, for a sample-based approach, an IVS is generated offline. Thus, more time is saved when the color stippling is generated, and the number of generated IVS can be changed at any time; the distribution of sampling points has good blue noise characteristics; the sampling algorithm does not depend on the characteristics of the resolution of the sampling area, etc. The expanded threshold strategy can be used later to effectively classify sampling points in a sampling area in the picture. Finally, the embodiment of the invention generates high-quality color stippling at a speed close to real time in different spaces. For different color spaces, the quality of the color stippling technique was tested and the stippling effects with different primary colors were compared in depth according to PNSR. The data result shows that RGBCMYK multi-tone color stippling generates high-quality stippling results without increasing the computational complexity, and can be compared with the color stippling computational results of the prior art for up to several hours.
Third section:
in the performance test of all experimental data, the data of each result is tested for multiple times and the result is averaged, the relation between the points with different numbers selected in the IVS and the time is shown in figure 5, gray represents a single CPU thread, black represents 8 CPU threads, and the time scale is in a logarithmic form. It is observed that a single CPU thread and 8 CPU threads have the same trend, and the computation time increases linearly with the use of more IVS sampling points. It is also noted that the single thread run time is approximately 4 times the 8 thread run time.
The number of different points is selected in the IVS to generate a color stippling result as shown in figure 3. Fig. 3 shows color stippling generated in different spaces (RGBK, CMYK, and RYBK). In the algorithm for generating the colored stippling by using RGBK, CMYK and RYBK, some pictures have good quality of the stippling generated in a certain space or two spaces, like colored stippling generated by Apple in RGBK and RYBK in FIG. 3, but the stippling generated in CMYK has larger chromatic aberration. To solve this problem and improve the quality of the colored stippling, the number of available colors may be further increased. Adding additional colors does not increase the complexity of the algorithm, i.e., the number of points selected in the IVS and the time it takes for the algorithm to take are exactly the same as the color space used above. The final result was synthesized using CMY pairwise blending to produce R, G, B three colors. Thereby extending the four colors of CMYK to seven colors of rgbrcmyk. Fig. 2 and 4 show two sets of results of this method, with the radius of the points gradually decreasing from left to right.
In black and white stippling, the method of measuring the similarity of two pictures is to compare their peak signal to noise ratios. The invention extends the way of calculating the PSNR of the gray picture to the color picture. The specific practice is to convert the color space of the generated stippling into YCbCr space and calculate its PSNR on the Y channel separately. Tables 1,2 and 3 apply this calculation to compare the difference in number of points, time, PSNR and different color spaces for the threshold tradeoff strategy of the present invention and other IVS based random throwing strategies. In this experiment, the number of points selected in the IVS for the input pictures in the table are 1048k,696k and 696k, each run with 8 threads. The result shows that the threshold selection strategy used by the invention has obvious advantages in both operation efficiency and result quality.
Table 1 comparison table of number of stippled points, time and PSNR generated in random throwing and thresholding strategy algorithm for different pictures in RGBK color
Table 2 comparison table of number of stippled dots, time and PSNR generated by different pictures in CMYK color in random throw and threshold strategy algorithm
TABLE 3 comparison of the number of stippled points, time and PSNR for different pictures in random tossing and thresholding strategy algorithm under RYBK space
Fig. 6 analyzes the relationship between the number of selected points in the IVS and the number of points in the generated stippling and PSNR using the same inputs as in fig. 2. It can be seen that for the same plot, the number of stippling points and PSNR increase simultaneously as the number of points selected in the IVS increases, while the quality of the colored stippling can also be improved using more colors. The results show that the PSNR of the results using seven colors of rgcmyk remains around the maximum of the other colors used only.
Table 4 comparison of the maximum PSNR in rgb, cmy and RYB spaces with the PSNR of rgbrcmyk.
The two groups of comparison results can clearly find that when the brightness of the pictures is close but the colors are different, the distinction degree of the black and white stippling is obviously insufficient. The colored stippling of the present invention, however, has a significant distinction in color, such as lotus and lotus leaves in the first set of drawings, and flowers and branches in the second set of drawings. Further, as can be seen from fig. 8, the black and white stippling using the IVS is significantly inferior in visual effect to the algorithm of the present embodiment.
The scheme of the embodiment can generate high-quality color stippling results under the efficiency close to real time. The result in fig. 9 is a color stippling on a white background using rgbcyk with a drawing named "sunday afternoon on the dawanda" as input. The number of color point drawing points in the figure is approximately 1181k, and the 8-thread run time is approximately 724ms. In the case of up to 2801k points in the IVS, the 8-thread run time is only 724ms. Meanwhile, the color stippling generated by the algorithm of the embodiment has very high quality no matter the color stippling is a global visual effect or a local representation detail.
While the foregoing description illustrates and describes the preferred embodiments of the present invention, it is to be understood that the invention is not limited to the forms disclosed herein, but is not to be construed as limited to other embodiments, and is capable of numerous other combinations, modifications, and variations within the spirit and scope of the present invention, as defined by the above teachings or by the appended claims. And that modifications and variations such as would be obvious to one skilled in the art are intended to be included within the spirit and scope of the present invention, without departing from the spirit and scope of the following claims.
Claims (1)
1. A method for instantly generating a color stippling based on an incremental Voronoi sequence, comprising:
s1, generating IVS;
randomly generating seed points in a sampling area, dividing the sampling area into Voronoi diagrams according to the generated seed points, obtaining sampling points by using the furthest point strategy according to the current Voronoi diagrams, adding the sampling points into the Voronoi division, and continuously iterating an algorithm to obtain a required sampling sequence; in the process of generating the IVS, the serial number of the sampling point in the IVS is inversely proportional to the area of the Voronoi region corresponding to the serial number of the sampling point in the IVS to be a i ≈A 0 I, wherein i is the serial number of the sampling point, a i For the Voronoi area corresponding to the i th sampling point, A 0 Is the total area of the sampling area;
the relation between the size and the number of sampling points in the image local area and the image tone of the area is gA=ndelta, wherein g is the image tone, A is the area of the image local area, n is the number of sampling points, delta is the area of a pure color point, and delta=pi r 2 R is the radius of the pure color point;
s2, initializing parameters of the input image I and the output image O,
determining the number n of sampling points, selecting n sampling points from the generated IVS to form a sequence S, and calculating the size A of an image sampling area;
s3, determining the radius r of the pure color point corresponding to the sampling point,
dividing the sequence S into m subsets S j Where j=1, 2, … m;
subset S j Each sampling point S of (a) j，i Calculating the position p of the sampling point on the image I;
three hue values are taken at the position p of the image I, which are projected into the color space corresponding to the stippling,
the color space is specifically RGB, CMY and RYB;
determining a set by each channel using a threshold strategyColor selected by each sampling point; outputting the selected color at the corresponding position point of the output image O; each channel adopts a threshold strategy to determine the color selected by each sampling point in the collection, and the rule is thatWhen select=1;
when (when)When select=0; selecting the sampling point when the threshold value select=1, and outputting the three selected color channel values +.>Wherein k=1, 2,3; otherwise the sample point is discarded and,
the color stippling is to place pure color points on a white canvas, and to place points on the canvas directly for CMY and RYB color spaces meeting the color reduction model, and to operate after reversing the numerical values of all channels for RGB additive model;
s4, outputting the finally generated image O.
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