3 * COPYRIGHT: Written by John Cunningham Bowler, 2015.
4 * To the extent possible under law, the author has waived all copyright and
5 * related or neighboring rights to this work. This work is published from:
8 * Generate a PNG with an alpha channel, correctly.
10 * This is a test case generator; the resultant PNG files are only of interest
11 * to those of us who care about whether the edges of circles are green, red,
14 * The program generates an RGB+Alpha PNG of a given size containing the given
15 * shapes on a transparent background:
17 * genpng width height { shape }
18 * shape ::= color width shape x1 y1 x2 y2
22 * black white red green yellow blue brown purple pink orange gray cyan
24 * The point is to have colors that are linguistically meaningful plus that old
25 * bugbear of the department store dress murders, Cyan, the only color we argue
32 * line: a straight line
34 * Each shape is followed by four numbers, these are two points in the output
35 * coordinate space (as real numbers) which describe the circle, square, or
36 * line. The shape is filled if it is preceded by 'filled' (not valid for
37 * 'line') or is drawn with a line, in which case the width of the line must
40 * The whole set of information can be repeated as many times as desired:
42 * shape ::= color width shape x1 y1 x2 y2
44 * color ::= black|white|red|green|yellow|blue
45 * color ::= brown|purple|pink|orange|gray|cyan
48 * shape ::= circle|square|line
54 * The output PNG is generated by down-sampling a 4x supersampled image using
55 * a bi-cubic filter. The bi-cubic has a 2 (output) pixel width, so an 8x8
56 * array of super-sampled points contribute to each output pixel. The value of
57 * a super-sampled point is found using an unfiltered, aliased, infinite
58 * precision image: Each shape from the last to the first is checked to see if
59 * the point is in the drawn area and, if it is, the color of the point is the
60 * color of the shape and the alpha is 1, if not the previous shape is checked.
62 * This is an aliased algorithm because no filtering is done; a point is either
63 * inside or outside each shape and 'close' points do not contribute to the
64 * sample. The down-sampling is relied on to correct the error of not using
67 * The line end-caps are 'flat'; they go through the points. The square line
68 * joins are mitres; the outside of the lines are continued to the point of
77 /* Normally use <png.h> here to get the installed libpng, but this is done to
78 * ensure the code picks up the local libpng implementation:
80 #include "../../png.h"
82 #if defined(PNG_SIMPLIFIED_WRITE_SUPPORTED) && defined(PNG_STDIO_SUPPORTED)
84 static const struct color
91 /* color ::= black|white|red|green|yellow|blue
92 * color ::= brown|purple|pink|orange|gray|cyan
99 { "yellow", 1, 1, 0 },
101 { "brown", .5, .125, 0 },
102 { "purple", 1, 0, 1 },
103 { "pink", 1, .5, .5 },
104 { "orange", 1, .5, 0 },
105 { "gray", 0, .5, .5 },
108 #define color_count ((sizeof colors)/(sizeof colors[0]))
110 static const struct color *
111 color_of(const char *arg)
113 int icolor = color_count;
115 while (--icolor >= 0)
117 if (strcmp(colors[icolor].name, arg) == 0)
118 return colors+icolor;
121 fprintf(stderr, "genpng: invalid color %s\n", arg);
126 width_of(const char *arg)
128 if (strcmp(arg, "filled") == 0)
134 double w = strtod(arg, &ep);
136 if (ep != NULL && *ep == 0 && w > 0)
140 fprintf(stderr, "genpng: invalid line width %s\n", arg);
145 coordinate_of(const char *arg)
148 double w = strtod(arg, &ep);
150 if (ep != NULL && *ep == 0)
153 fprintf(stderr, "genpng: invalid coordinate value %s\n", arg);
157 struct arg; /* forward declaration */
159 typedef int (*shape_fn_ptr)(const struct arg *arg, double x, double y);
160 /* A function to determine if (x,y) is inside the shape.
162 * There are two implementations:
164 * inside_fn: returns true if the point is inside
166 * -1: the point is outside the shape by more than the filter width (2)
167 * 0: the point may be inside the shape
168 * +1: the point is inside the shape by more than the filter width
175 const struct color *color;
176 shape_fn_ptr inside_fn;
177 shape_fn_ptr check_fn;
178 double width; /* line width, 0 for 'filled' */
179 double x1, y1, x2, y2;
182 /* IMPLEMENTATION NOTE:
184 * We want the contribution of each shape to the sample corresponding to each
185 * pixel. This could be obtained by super sampling the image to infinite
186 * dimensions, finding each point within the shape and assigning that a value
187 * '1' while leaving every point outside the shape with value '0' then
188 * downsampling to the image size with sinc; computationally very expensive.
190 * Approximations are as follows:
192 * 1) If the pixel coordinate is within the shape assume the sample has the
193 * shape color and is opaque, else assume there is no contribution from
196 * This is the equivalent of aliased rendering or resampling an image with
197 * a block filter. The maximum error in the calculated alpha (which will
198 * always be 0 or 1) is 0.5.
200 * 2) If the shape is within a square of size 1x1 centered on the pixel assume
201 * that the shape obscures an amount of the pixel equal to its area within
204 * This is the equivalent of 'pixel coverage' alpha calculation or resampling
205 * an image with a bi-linear filter. The maximum error is over 0.2, but the
206 * results are often acceptable.
208 * This can be approximated by applying (1) to a super-sampled image then
209 * downsampling with a bi-linear filter. The error in the super-sampled
210 * image is 0.5 per sample, but the resampling reduces this.
212 * 3) Use a better filter with a super-sampled image; in the limit this is the
215 * 4) Do the geometric calculation; a bivariate definite integral across the
216 * shape, unfortunately this means evaluating Si(x), the integral of sinc(x),
217 * which is still a lot of math.
219 * This code uses approach (3) with a bi-cubic filter and 8x super-sampling
220 * and method (1) for the super-samples. This means that the sample is either
221 * 0 or 1, depending on whether the sub-pixel is within or outside the shape.
222 * The bi-cubic weights are also fixed and the 16 required weights are
223 * pre-computed here (note that the 'scale' setting will need to be changed if
224 * 'super' is increased).
226 * The code also calculates a sum to the edge of the filter. This is not
227 * currently used by could be used to optimize the calculation.
233 if (x <= 1) return (1.5*x - 2.5)*x*x + 1;
234 if (x < 2) return (((2.5 - 0.5*x)*x - 4)*x + 2);
240 while (x < 2*super) {
241 s = s + bicubic(x/super);
248 b = bicubic(x/super);
251 print " /*", x, "*/ { ", b, ", ", s, " }";
257 if (x < 2*super) print ","
263 #define BICUBIC1(x) /* |x| <= 1 */ ((1.5*(x)* - 2.5)*(x)*(x) + 1)
264 #define BICUBIC2(x) /* 1 < |x| < 2 */ (((2.5 - 0.5*(x))*(x) - 4)*(x) + 2)
265 #define FILTER_WEIGHT 9 /* Twice the first sum below */
266 #define FILTER_WIDTH 2 /* Actually half the width; -2..+2 */
267 #define FILTER_STEPS 8 /* steps per filter unit */
271 /* These numbers are exact; the weight for the filter is 1/9, but this
272 * would make the numbers inexact, so it is not included here.
275 /* 0*/ { 1.0000000000, 4.5000000000 },
276 /* 1*/ { .9638671875, 3.5000000000 },
277 /* 2*/ { .8671875000, 2.5361328125 },
278 /* 3*/ { .7275390625, 1.6689453125 },
279 /* 4*/ { .5625000000, .9414062500 },
280 /* 5*/ { .3896484375, .3789062500 },
281 /* 6*/ { .2265625000, -.0107421875 },
282 /* 7*/ { .0908203125, -.2373046875 },
283 /* 8*/ { 0, -.3281250000 },
284 /* 9*/ { -.0478515625, -.3281250000 },
285 /*10*/ { -.0703125000, -.2802734375 },
286 /*11*/ { -.0732421875, -.2099609375 },
287 /*12*/ { -.0625000000, -.1367187500 },
288 /*13*/ { -.0439453125, -.0742187500 },
289 /*14*/ { -.0234375000, -.0302734375 },
290 /*15*/ { -.0068359375, -.0068359375 }
294 alpha_calc(const struct arg *arg, double x, double y)
296 /* For [x-2..x+2],[y-2,y+2] calculate the weighted bicubic given a function
297 * which tells us whether a point is inside or outside the shape. First
298 * check if we need to do this at all:
300 switch (arg->check_fn(arg, x, y))
303 return 0; /* all samples outside the shape */
306 return 1; /* all samples inside the shape */
313 # define FILTER_D (FILTER_WIDTH*FILTER_STEPS-1)
314 for (dy=-FILTER_D; dy<=FILTER_D; ++dy)
316 double wy = bicubic[abs(dy)][0];
323 for (dx=-FILTER_D; dx<=FILTER_D; ++dx)
325 double wx = bicubic[abs(dx)][0];
327 if (wx != 0 && arg->inside_fn(arg, x+dx/16, y+dy/16))
331 alpha += wy * alphay;
335 /* This needs to be weighted for each dimension: */
336 return alpha / (FILTER_WEIGHT*FILTER_WEIGHT);
341 /* These are the shape functions. */
343 * { inside_square_filled, check_square_filled },
344 * { inside_square, check_square }
347 square_check(double x, double y, double x1, double y1, double x2, double y2)
348 /* Is x,y inside the square (x1,y1)..(x2,y2)? */
350 /* Do a modified Cohen-Sutherland on one point, bit patterns that indicate
353 * x<x1 | x<y1 | x<x2 | x<y2
354 * 0 x 0 x To the right
355 * 1 x 1 x To the left
359 * So 'inside' is (x<x1) != (x<x2) && (y<y1) != (y<y2);
361 return ((x<x1) ^ (x<x2)) & ((y<y1) ^ (y<y2));
365 inside_square_filled(const struct arg *arg, double x, double y)
367 return square_check(x, y, arg->x1, arg->y1, arg->x2, arg->y2);
371 square_check_line(const struct arg *arg, double x, double y, double w)
372 /* Check for a point being inside the boundaries implied by the given arg
373 * and assuming a width 2*w each side of the boundaries. This returns the
374 * 'check' INSIDE/OUTSIDE/0 result but note the semantics:
382 * And '0' means within the line boundaries.
385 double cx = (arg->x1+arg->x2)/2;
386 double wx = fabs(arg->x1-arg->x2)/2;
387 double cy = (arg->y1+arg->y2)/2;
388 double wy = fabs(arg->y1-arg->y2)/2;
390 if (square_check(x, y, cx-wx-w, cy-wy-w, cx+wx+w, cy+wy+w))
392 /* Inside, but maybe too far; check for the redundant case where
397 if (wx > 0 && wy > 0 && square_check(x, y, cx-wx, cy-wy, cx+wx, cy+wy))
398 return INSIDE; /* between (inside) the boundary lines. */
400 return 0; /* inside the lines themselves. */
403 return OUTSIDE; /* outside the boundary lines. */
407 check_square_filled(const struct arg *arg, double x, double y)
409 /* The filter extends +/-FILTER_WIDTH each side of each output point, so
410 * the check has to expand and contract the square by that amount; '0'
411 * means close enough to the edge of the square that the bicubic filter has
412 * to be run, OUTSIDE means alpha==0, INSIDE means alpha==1.
414 return square_check_line(arg, x, y, FILTER_WIDTH);
418 inside_square(const struct arg *arg, double x, double y)
420 /* Return true if within the drawn lines, else false, no need to distinguish
421 * INSIDE vs OUTSIDE here:
423 return square_check_line(arg, x, y, arg->width/2) == 0;
427 check_square(const struct arg *arg, double x, double y)
429 /* So for this function a result of 'INSIDE' means inside the actual lines.
431 double w = arg->width/2;
433 if (square_check_line(arg, x, y, w+FILTER_WIDTH) == 0)
435 /* Somewhere close to the boundary lines. If far enough inside one of
436 * them then we can return INSIDE:
440 if (w > 0 && square_check_line(arg, x, y, w) == 0)
443 /* Point is somewhere in the filter region: */
447 else /* Inside or outside the square by more than w+FILTER_WIDTH. */
452 * { inside_circle_filled, check_circle_filled },
453 * { inside_circle, check_circle }
455 * The functions here are analoguous to the square ones; however, they check
456 * the corresponding ellipse as opposed to the rectangle.
459 circle_check(double x, double y, double x1, double y1, double x2, double y2)
461 if (square_check(x, y, x1, y1, x2, y2))
463 /* Inside the square, so maybe inside the circle too: */
464 const double cx = (x1 + x2)/2;
465 const double cy = (y1 + y2)/2;
466 const double dx = x1 - x2;
467 const double dy = y1 - y2;
472 /* It is outside if the distance from the center is more than half the
475 return x*x+y*y < .25;
478 return 0; /* outside */
482 inside_circle_filled(const struct arg *arg, double x, double y)
484 return circle_check(x, y, arg->x1, arg->y1, arg->x2, arg->y2);
488 circle_check_line(const struct arg *arg, double x, double y, double w)
489 /* Check for a point being inside the boundaries implied by the given arg
490 * and assuming a width 2*w each side of the boundaries. This function has
491 * the same semantic as square_check_line but tests the circle.
494 double cx = (arg->x1+arg->x2)/2;
495 double wx = fabs(arg->x1-arg->x2)/2;
496 double cy = (arg->y1+arg->y2)/2;
497 double wy = fabs(arg->y1-arg->y2)/2;
499 if (circle_check(x, y, cx-wx-w, cy-wy-w, cx+wx+w, cy+wy+w))
501 /* Inside, but maybe too far; check for the redundant case where
506 if (wx > 0 && wy > 0 && circle_check(x, y, cx-wx, cy-wy, cx+wx, cy+wy))
507 return INSIDE; /* between (inside) the boundary lines. */
509 return 0; /* inside the lines themselves. */
512 return OUTSIDE; /* outside the boundary lines. */
516 check_circle_filled(const struct arg *arg, double x, double y)
518 return circle_check_line(arg, x, y, FILTER_WIDTH);
522 inside_circle(const struct arg *arg, double x, double y)
524 return circle_check_line(arg, x, y, arg->width/2) == 0;
528 check_circle(const struct arg *arg, double x, double y)
530 /* Exactly as the 'square' code. */
531 double w = arg->width/2;
533 if (circle_check_line(arg, x, y, w+FILTER_WIDTH) == 0)
537 if (w > 0 && circle_check_line(arg, x, y, w) == 0)
540 /* Point is somewhere in the filter region: */
544 else /* Inside or outside the square by more than w+FILTER_WIDTH. */
549 * { NULL, NULL }, There is no 'filled' line.
550 * { inside_line, check_line }
553 line_check(double x, double y, double x1, double y1, double x2, double y2,
554 double w, double expand)
556 /* Shift all the points to (arg->x1, arg->y1) */
559 double len2 = lx*lx + ly*ly;
565 /* The dot product is the distance down the line, the cross product is
566 * the distance away from the line:
568 * distance = |cross| / sqrt(len2)
570 cross = x * ly - y * lx;
572 /* If 'distance' is more than w the point is definitely outside the line:
575 * |cross| >= w * sqrt(len2)
576 * cross^2 >= w^2 * len2:
578 if (cross*cross >= (w+expand)*(w+expand)*len2)
579 return 0; /* outside */
581 /* Now find the distance *along* the line; this comes from the dot product
582 * lx.x+ly.y. The actual distance (in pixels) is:
584 * distance = dot / sqrt(len2)
586 dot = lx * x + ly * y;
588 /* The test for 'outside' is:
590 * distance < 0 || distance > sqrt(len2)
591 * -> dot / sqrt(len2) > sqrt(len2)
594 * But 'expand' is used for the filter width and needs to be handled too:
596 return dot > -expand && dot < len2+expand;
600 inside_line(const struct arg *arg, double x, double y)
602 return line_check(x, y, arg->x1, arg->y1, arg->x2, arg->y2, arg->width/2, 0);
606 check_line(const struct arg *arg, double x, double y)
608 /* The end caps of the line must be checked too; it's not enough just to
609 * widen the line by FILTER_WIDTH; 'expand' exists for this purpose:
611 if (line_check(x, y, arg->x1, arg->y1, arg->x2, arg->y2, arg->width/2,
614 /* Inside the line+filter; far enough inside that the filter isn't
617 if (arg->width > 2*FILTER_WIDTH &&
618 line_check(x, y, arg->x1, arg->y1, arg->x2, arg->y2, arg->width/2,
631 shape_fn_ptr function[2/*fill,line*/][2];
637 { { inside_square_filled, check_square_filled },
638 { inside_square, check_square } }
641 { { inside_circle_filled, check_circle_filled },
642 { inside_circle, check_circle } }
646 { inside_line, check_line } }
650 #define shape_count ((sizeof shape_defs)/(sizeof shape_defs[0]))
653 shape_of(const char *arg, double width, int f)
657 for (i=0; i<shape_count; ++i) if (strcmp(shape_defs[i].name, arg) == 0)
659 shape_fn_ptr fn = shape_defs[i].function[width != 0][f];
664 fprintf(stderr, "genpng: %s %s not supported\n",
665 width == 0 ? "filled" : "unfilled", arg);
669 fprintf(stderr, "genpng: %s: not a valid shape name\n", arg);
674 parse_arg(struct arg *arg, const char **argv/*7 arguments*/)
676 /* shape ::= color width shape x1 y1 x2 y2 */
677 arg->color = color_of(argv[0]);
678 arg->width = width_of(argv[1]);
679 arg->inside_fn = shape_of(argv[2], arg->width, FN_INSIDE);
680 arg->check_fn = shape_of(argv[2], arg->width, FN_CHECK);
681 arg->x1 = coordinate_of(argv[3]);
682 arg->y1 = coordinate_of(argv[4]);
683 arg->x2 = coordinate_of(argv[5]);
684 arg->y2 = coordinate_of(argv[6]);
688 read_wh(const char *name, const char *str)
689 /* read a PNG width or height */
692 unsigned long ul = strtoul(str, &ep, 10);
694 if (ep != NULL && *ep == 0 && ul > 0 && ul <= 0x7fffffff)
695 return (png_uint_32)/*SAFE*/ul;
697 fprintf(stderr, "genpng: %s: invalid number %s\n", name, str);
702 pixel(png_uint_16p p, struct arg *args, int nargs, double x, double y)
704 /* Fill in the pixel by checking each shape (args[nargs]) for effects on
705 * the corresponding sample:
707 double r=0, g=0, b=0, a=0;
709 while (--nargs >= 0 && a != 1)
711 /* NOTE: alpha_calc can return a value outside the range 0..1 with the
714 const double alpha = alpha_calc(args+nargs, x, y) * (1-a);
716 r += alpha * args[nargs].color->red;
717 g += alpha * args[nargs].color->green;
718 b += alpha * args[nargs].color->blue;
722 /* 'a' may be negative or greater than 1; if it is, negative clamp the
723 * pixel to 0 if >1 clamp r/g/b:
735 /* And fill in the pixel: */
736 p[0] = (png_uint_16)/*SAFE*/round(r * 65535);
737 p[1] = (png_uint_16)/*SAFE*/round(g * 65535);
738 p[2] = (png_uint_16)/*SAFE*/round(b * 65535);
739 p[3] = (png_uint_16)/*SAFE*/round(a * 65535);
743 p[3] = p[2] = p[1] = p[0] = 0;
747 main(int argc, const char **argv)
749 int convert_to_8bit = 0;
751 /* There is one option: --8bit: */
752 if (argc > 1 && strcmp(argv[1], "--8bit") == 0)
753 --argc, ++argv, convert_to_8bit = 1;
760 # define max_shapes 256
761 struct arg arg_list[max_shapes];
763 /* The libpng Simplified API write code requires a fully initialized
766 memset(&image, 0, sizeof image);
767 image.version = PNG_IMAGE_VERSION;
769 image.width = read_wh("width", argv[1]);
770 image.height = read_wh("height", argv[2]);
771 image.format = PNG_FORMAT_LINEAR_RGB_ALPHA;
773 image.colormap_entries = 0;
775 /* Check the remainder of the arguments */
776 for (nshapes=0; 3+7*(nshapes+1) <= argc && nshapes < max_shapes;
778 parse_arg(arg_list+nshapes, argv+3+7*nshapes);
780 if (3+7*nshapes != argc)
782 fprintf(stderr, "genpng: %s: too many arguments\n", argv[3+7*nshapes]);
786 /* Create the buffer: */
787 buffer = malloc(PNG_IMAGE_SIZE(image));
793 /* Write each row... */
794 for (y=0; y<image.height; ++y)
798 /* Each pixel in each row: */
799 for (x=0; x<image.width; ++x)
800 pixel(buffer + 4*(x + y*image.width), arg_list, nshapes, x, y);
803 /* Write the result (to stdout) */
804 if (png_image_write_to_stdio(&image, stdout, convert_to_8bit,
805 buffer, 0/*row_stride*/, NULL/*colormap*/))
808 return 0; /* success */
812 fprintf(stderr, "genpng: write stdout: %s\n", image.message);
818 fprintf(stderr, "genpng: out of memory: %lu bytes\n",
819 (unsigned long)PNG_IMAGE_SIZE(image));
824 /* Wrong number of arguments */
825 fprintf(stderr, "genpng: usage: genpng [--8bit] width height {shape}\n"
826 " Generate a transparent PNG in RGBA (truecolor+alpha) format\n"
827 " containing the given shape or shapes. Shapes are defined:\n"
829 " shape ::= color width shape x1 y1 x2 y2\n"
830 " color ::= black|white|red|green|yellow|blue\n"
831 " color ::= brown|purple|pink|orange|gray|cyan\n"
832 " width ::= filled|<number>\n"
833 " shape ::= circle|square|line\n"
834 " x1,x2 ::= <number>\n"
835 " y1,y2 ::= <number>\n"
837 " Numbers are floating point numbers describing points relative to\n"
838 " the top left of the output PNG as pixel coordinates. The 'width'\n"
839 " parameter is either the width of the line (in output pixels) used\n"
840 " to draw the shape or 'filled' to indicate that the shape should\n"
841 " be filled with the color.\n"
843 " Colors are interpreted loosely to give access to the eight full\n"
844 " intensity RGB values:\n"
846 " black, red, green, blue, yellow, cyan, purple, white,\n"
848 " Cyan is full intensity blue+green; RGB(0,1,1), plus the following\n"
849 " lower intensity values:\n"
851 " brown: red+orange: RGB(0.5, 0.125, 0) (dark red+orange)\n"
852 " pink: red+white: RGB(1.0, 0.5, 0.5)\n"
853 " orange: red+yellow: RGB(1.0, 0.5, 0)\n"
854 " gray: black+white: RGB(0.5, 0.5, 0.5)\n"
856 " The RGB values are selected to make detection of aliasing errors\n"
857 " easy. The names are selected to make the description of errors\n"
860 " The PNG is written to stdout, if --8bit is given a 32bpp RGBA sRGB\n"
861 " file is produced, otherwise a 64bpp RGBA linear encoded file is\n"
867 #endif /* SIMPLIFIED_WRITE && STDIO */