Seeing beautiful marbled papers in a documentary about Venice triggered an immediate desire to try to write some code to procedurally recreate them. In this article I'll share how I did a small CLI app using LibCapy and examples of the result it produces.

First, the usual boilerplate for the app. Including LibCapy and defining the command line arguments. I've choosen to allow selection of the image dimensions (in cm with dpi selection), number of colors, number of pass (cf later), output path of course, RNG seed for reproducibility, and a pastel mode for the random colors (just cause I like it).

#include "LibCapy/capy.h" ... // App structure typedef struct App { // Return code at the end of execution int ret; CapyPad(int, ret); // Arguments parser CapyArgParser* parser; ... } App; ... int main(int argc, char** argv) { ... // Create the instance of the application struct App app = { .ret = EXIT_SUCCESS, ... }; try { // Definition of the arguments from the command line CapyArg args[] = { { .lbl = argv[0], .nbVal=capyArg_variableNbVal, .help = "Create random venetian marbled paper.\n" }, { .lbl = "--dims", .shortLbl="-d", .nbVal=2, .help = "Dimensions (width, height in cm) of the paper (default: 21x29.7).", }, { .lbl = "--dpi", .shortLbl="-dpi", .nbVal=1, .help = "Dpi (default: 300).", }, { .lbl = "--output", .shortLbl="-o", .nbVal=1, .help = "Path to the output image (PNG format, default: ./output.png).", }, { .lbl = "--size-palette", .shortLbl="-p", .nbVal=1, .help = "Number of colors in the palette (default: 10)", }, { .lbl = "--seed", .shortLbl="-s", .nbVal=1, .help = "Seed for the RNG (default: current time)", }, { .lbl = "--nb-pass", .shortLbl="-n", .nbVal=1, .help = "Number of stretching passes (default: 2)", }, { .lbl = "--pastel", .shortLbl="-pa", .nbVal=0, .help = "Use a pastel color palette", }, }; // Create the argument parser and parse the arguments app.parser = CapyArgParserAlloc(argc, argv, args); ... } endCatch; return app.ret; }

The app structure's properties are just what it takes to memorize the command line arguments, and are initialised as follow

... // App structure typedef struct App { ... // Dimensions in cm double dims[2]; // Dpi double dpi; // Path to the output image char const* pathImg; // Size of the color palette size_t sizePalette; // RNG seed CapyRandomSeed_t seed; // Number of pass for color stretching size_t nbPass; // Flag for pastel color palette bool flagPastel; CapyPad(bool, flagPastel); } App; ... int main(int argc, char** argv) { // Create the instance of the application struct App app = { .ret = EXIT_SUCCESS, .dims = {21.0, 29.7}, .dpi = 300.0, .pathImg = "./output.png", .sizePalette = 10, .seed = (CapyRandomSeed_t)time(NULL), .nbPass = 2, .flagPastel = false, }; try { ... // Process the command line arguments if($(app.parser, isSet)("--dims")) loop(i, (size_t)2) app.dims[i] = $(app.parser, getAsDouble)("--dims", i); if($(app.parser, isSet)("--dpi")) app.dpi = $(app.parser, getAsDouble)("--dims", 0); if($(app.parser, isSet)("--output")) app.pathImg = $(app.parser, getAsStr)("--output", 0); if($(app.parser, isSet)("--size-palette")) app.sizePalette = (size_t)$(app.parser, getAsInt)("--size-palette", 0); if($(app.parser, isSet)("--seed")) app.seed = (CapyRandomSeed_t)$(app.parser, getAsInt)("--seed", 0); if($(app.parser, isSet)("--nb-pass")) app.nbPass = (size_t)$(app.parser, getAsInt)("--nb-pass", 0); app.flagPastel = $(app.parser, isSet)("--pastel"); // Create the paper Render(&app); ... } endCatch; return app.ret; }

A few user-defined exceptions to manage invalid arguments:

... // App exceptions enum AppExceptions { excInvalidDims = CapyExc_LastID, excInvalidDpi, lastAppException }; // Label for the AppExceptions char const* exceptionStr[] = { "The dimensions in argument are invalid", "The dpi in argument is invalid", }; char const* ExcToStr(CapyException_t exc) { // If the exception ID is one of AppExceptions, // return the exception's label if (exc >= CapyExc_LastID && exc < lastAppException) { return exceptionStr[exc - CapyExc_LastID]; } // Else, the exception ID is not one of AppExceptions, return NULL else return NULL; } ... int main(int argc, char** argv) { // Set the conversion function for raised exceptions CapyAddExcToStrFun(ExcToStr); ... try { ... // Check the parameters if(app.dims[0] <= 0.0 || app.dims[1] <= 0.0) raiseExc(excInvalidDims); if(app.dpi <= 0.0) raiseExc(excInvalidDpi); ... } catchDefault { // Inform the user about the raised exception $(&capyDecoratorRed, fprintf)( stdout, "Sorry, an exception occured. Exiting.\n"); $(&capyDecoratorRed, fprintf)( stdout, "%s\n", CapyExcToStr(CapyGetLastExcId())); app.ret = EXIT_FAILURE; } endCatch; return app.ret; }

Then we're ready for the interesting stuff. I need a random number generator, a Gaussian blur kernel (to smooth the image and somehow replicate the mixing of pigments in the real process), a palette of random colors, and an image to draw on. The image size is initially a bit larger than the result one to accomodate the Gaussian blur kernel. The image background is arbitrarily assigned the first color in the palette.

// Render the paper void Render(App* const app) { // Create the kernel for blurring size_t blurKernelSize = 5; CapyImgKernel kernel = CapyImgKernelCreate(blurKernelSize); // Create the RNG CapyRandom rng = CapyRandomCreate(app->seed); // Create the color palette CapyColorPalette palette = CapyColorPaletteCreate(app->sizePalette); if(app->flagPastel) $(&palette, setRandomPastelRGB)(&rng); else $(&palette, setRandomRGB)(&rng); // Calculate the image dimensions CapyImgDims dims = {0}; loop(i, 2) dims.vals[i] = (CapyImgDims_t)CapyCm2Pixel((double)(app->dims[i]), app->dpi) + 2 * blurKernelSize; printf("Image dimensions: %ux%u\n", dims.width, dims.height); // Allocate memory for the image CapyImg* img = CapyImgAlloc(capyImgMode_rgb, dims); // Set the background color of the image printf("Set background to color #0\n"); CapyColorData bgColor = $(&palette, get)(0); $(img, fillWithColor)(&bgColor); ... }

As for the real marble paper, I start with some random drops on a surface. I want them to be placed randomly but nicely spread on the whole image. That's a perfect use case for Poisson disk sampling. For the sampling parameter I choose 2cm as the minimum distance between disk. It gives results I like for A4 size paper, but not so much for other sizes. So I tried to parameterized the distance with the paper size. I'm not convinced with the result but I kept it like this anyway. Getting satisfying results for A4 is sufficient for now as I'm just having fun here. The blurring strength is then set equal to the sampling distance.

For drawing, I use a circle (that doesn't matter much if the real drop are not perfect circles), and a 9B CapyPen (almost no bleeding of colors between drop but a little bit). The radius of each drop is randomly set somewhere between 0.75 and 1.0 of the sampling distance, and that range decreases proportional to the golden ratio (because why not) for each color in the palette. Reducing the range avoids the last drawn color to overwhelm the first ones. I also skip the first color in the palette, which has been used for the background.

... // Create the Poisson sampler over the image CapyPoissonSampling* sampling = CapyPoissonSamplingAlloc(app->seed, 2); sampling->domains[0].max = dims.width - 1; sampling->domains[1].max = dims.height - 1; double dropSpace = 0.0; if(app->dims[0] < app->dims[1]) dropSpace = app->dims[0] * 0.1; else dropSpace = app->dims[1] * 0.1; sampling->r = CapyCm2Pixel(dropSpace, app->dpi); // Set the blur strength double blurStrength = dropSpace; $(&kernel, setToGaussianBlur)(blurStrength); // Create the pen and circle shape for drawing CapyPen pen = CapyPenCreate(); pen.hardness = capyPenHardness_9B; CapyCircle circle = CapyCircleCreate(); // Initial range of size for the drops, relative to the sampling radius CapyRangeDouble rangeRadius = {.min = 0.75, .max = 1.0}; // For each color in the palette loop(iColor, palette.size - 1) { // Set the color of the pen pen.color = $(&palette, get)(iColor + 1); // Sample the position of drops on the paper CapySamples* samples = $(sampling, getSamples)(); printf("Drop %lu samples of color #%lu\n", samples->size, iColor + 1); // For each drop loop(iSample, samples->size) { CapySample const sample = $(samples, get)(iSample); // Draw the drop with a random size loop(i, 2) circle.center.coords[i] = sample.vals[i]; circle.radius = $(&rng, getDoubleRange)(&rangeRadius) * sampling->r; $(&pen, drawFilledCircle)(&circle, img); } // Reduce the range of size for the drops to avoid predominance of // foreground color loop(i, 2) rangeRadius.vals[i] /= 1.618; // Free memory CapySamplesFree(&samples); } ...

This gives something like this:

Then I reproduce the stretching of colors with a comb like for the real paper. The number of times ('pass') the drops are stretched is set by the user, and all other parameters are randomised: the number of teeth, their distribution, the direction of stretching, and the stretching's strength and shape.

The stretching axis (x or y) is randomly choosen at each pass.

... // Loop on the stretching passes loop(iStretching, app->nbPass) { printf("Stretching pass #%lu\n", iStretching); // Get the stretching axis uint8_t const stretchingAxis = ($(&rng, getUInt8)()) & 1; printf("Stretching axis: %u\n", stretchingAxis); ... } ...

While we are at playing with Poisson disk sampling, lets use it for the teeth distribution too. The distance in this case is randomly choosen in a range proportional to the distance of the drops sampling, and increasing with each pass. I also ensure that the first and last teeth are at the borders of the paper for convenience.

// Comparator for stretching samples int CmpSample(void const* const a, void const* const b) { CapySample const* const sampleA = (CapySample const*)a; CapySample const* const sampleB = (CapySample const*)b; if(sampleA->vals[0] < sampleB->vals[0]) return -1; else if(sampleA->vals[0] > sampleB->vals[0]) return 1; else return 0; } ... void Render(App* const app) { ... loop(iStretching, app->nbPass) { ... // Get the average space between stretching axis CapyRangeDouble rangeStretchingSpace = { .min = dropSpace * 0.5 * (double)(1 + iStretching), .max = dropSpace * 1.5 * (double)(1 + iStretching) }; double stretchingSpaceCm = $(&rng, getDoubleRange)(&rangeStretchingSpace); printf("Stretching space %lf\n", stretchingSpaceCm); // Sample stretching positions CapyPoissonSampling* stretchSampling = CapyPoissonSamplingAlloc(app->seed, 1); stretchSampling->domains[0].max = dims.vals[1 - stretchingAxis] - 1; stretchSampling->r = CapyCm2Pixel(stretchingSpaceCm * 0.75, app->dpi); CapySamples* stretchSamples = $(stretchSampling, getSamples)(); CapyComparator cmpSample = CapyComparatorCreate(); cmpSample.eval = CmpSample; $(stretchSamples, qsort)(&cmpSample); $(&cmpSample, destruct)(); printf("%lu stretching positions\n", stretchSamples->size); // Force the first and last sample to be at the borders of the paper CapySample firstStretchSample = $(stretchSamples, get)(0); firstStretchSample.vals[0] = 0.0; CapySample lastStretchSample = $(stretchSamples, get)(stretchSamples->size - 1); lastStretchSample.vals[0] = stretchSampling->domains[0].max; ... } ... }

The stretching itself consists of shifting pixels along the stretching axis, warping around the edges of the image if needed. The shifting amount follows a \(|sin|\) pattern controlled by the stretching direction \(d\) (randomly set to -1 or 1), strength \(s\) (starting within [2, 7] times the distance in the drop sampling and decreased accordingly to the golden ratio at each pass) and power \(p\) (randomly set within [0.25, 0.75] at each pass). That pattern applies between each teeth: I prepare an array of shifting value for each pixel, with the value equal to \(d.s.sin(t\pi)^p\) where \(t\) (in [0, 1]) is the relative position of the pixel between the two nearest teeth of the comb.

... loop(iStretching, app->nbPass) { ... // Stretching power double const stretchingPower = 0.25 + 0.75 * $(&rng, getDouble)(); printf("Stretching power %lf\n", stretchingPower); // Create the stretching value array CapyImgPos_t* stretchingVals = NULL; safeMalloc(stretchingVals, dims.vals[1 - stretchingAxis]); loop(iVal, dims.vals[1 - stretchingAxis]) { size_t iStretchPos = 0; CapySample stretchSample = $(stretchSamples, get)(iStretchPos + 1); while(stretchSample.vals[0] < iVal) { ++iStretchPos; stretchSample = $(stretchSamples, get)(iStretchPos + 1); } double c[2] = {0.0, stretchSample.vals[0]}; stretchSample = $(stretchSamples, get)(iStretchPos); c[0] = stretchSample.vals[0]; double x = ((double)iVal - c[0]) / (c[1] - c[0]); double y = pow(sin(x * M_PI), stretchingPower) * stretchingStrength * coeffDir; stretchingVals[iVal] = (CapyImgPos_t)y; } // Create a clone of the image CapyImg* cloneImg = CapyImgClone(img); // Loop on the pixels of the image forEach(pixel, img->iter) { // Calculate the position of the stretched pixel CapyImgPos srcPos = pixel.pos; srcPos.coords[stretchingAxis] += stretchingVals[srcPos.coords[1 - stretchingAxis]]; if(srcPos.coords[stretchingAxis] < 0) srcPos.coords[stretchingAxis] += (CapyImgPos_t)(dims.vals[stretchingAxis]); else if( srcPos.coords[stretchingAxis] >= (CapyImgPos_t)(dims.vals[stretchingAxis]) ) { srcPos.coords[stretchingAxis] -= (CapyImgPos_t)(dims.vals[stretchingAxis]); } // Set the color of the pixel in the image with the color of the // stretched pixel in the original image CapyColorData const* color = $(cloneImg, getColor)(&srcPos); *(pixelPtr->color) = *color; } ... // Reduce the stretching strength stretchingStrength /= 1.618; printf("Stretching strength %lf\n", stretchingStrength); } ...

Finally I apply blurring between each pass, crop the provisional border at the end, and save the result.

void Render(App* const app) { ... loop(iStretching, app->nbPass) { ... // Apply blurring printf("Blurring\n"); CapyImg* imgBlur = $(&kernel, apply)(img); CapyImgFree(&img); img = imgBlur; ... } // Crop the unblurred border loop(i, 2) dims.vals[i] -= 2 * blurKernelSize; CapyImg* cropImg = CapyImgAlloc(capyImgMode_rgb, dims); CapyImgPos posTopLeft = {.x = -(CapyImgPos_t)blurKernelSize, .y = -(CapyImgPos_t)blurKernelSize}; $(img, pasteInto)(cropImg, &posTopLeft); CapyImgFree(&img); img = cropImg; // Save the image printf("Save to %s\n", app->pathImg); $(img, saveToPath)(app->pathImg); ... }

The effect of 3 successive passes on the example given above are shown below:

Some more examples:

And the complete code source:

#include "LibCapy/capy.h" // App exceptions enum AppExceptions { excInvalidDims = CapyExc_LastID, excInvalidDpi, lastAppException }; // Label for the AppExceptions char const* exceptionStr[] = { "The dimensions in argument are invalid", "The dpi in argument is invalid", }; char const* ExcToStr(CapyException_t exc) { // If the exception ID is one of AppExceptions, // return the exception's label if (exc >= CapyExc_LastID && exc < lastAppException) { return exceptionStr[exc - CapyExc_LastID]; } // Else, the exception ID is not one of AppExceptions, return NULL else return NULL; } // App structure typedef struct App { // Return code at the end of execution int ret; CapyPad(int, ret); // Arguments parser CapyArgParser* parser; // Dimensions in cm double dims[2]; // Dpi double dpi; // Path to the output image char const* pathImg; // Size of the color palette size_t sizePalette; // RNG seed CapyRandomSeed_t seed; // Number of pass for color stretching size_t nbPass; // Flag for pastel color palette bool flagPastel; CapyPad(bool, flagPastel); } App; // Comparator for stretching samples int CmpSample(void const* const a, void const* const b) { CapySample const* const sampleA = (CapySample const*)a; CapySample const* const sampleB = (CapySample const*)b; if(sampleA->vals[0] < sampleB->vals[0]) return -1; else if(sampleA->vals[0] > sampleB->vals[0]) return 1; else return 0; } // Render the paper void Render(App* const app) { // Create the kernel for blurring size_t blurKernelSize = 5; CapyImgKernel kernel = CapyImgKernelCreate(blurKernelSize); // Create the RNG CapyRandom rng = CapyRandomCreate(app->seed); // Create the color palette CapyColorPalette palette = CapyColorPaletteCreate(app->sizePalette); if(app->flagPastel) $(&palette, setRandomPastelRGB)(&rng); else $(&palette, setRandomRGB)(&rng); // Calculate the image dimensions CapyImgDims dims = {0}; loop(i, 2) dims.vals[i] = (CapyImgDims_t)CapyCm2Pixel((double)(app->dims[i]), app->dpi) + 2 * blurKernelSize; printf("Image dimensions: %ux%u\n", dims.width, dims.height); // Allocate memory for the image CapyImg* img = CapyImgAlloc(capyImgMode_rgb, dims); // Set the background color of the image printf("Set background to color #0\n"); CapyColorData bgColor = $(&palette, get)(0); $(img, fillWithColor)(&bgColor); // Create the Poisson sampler over the image CapyPoissonSampling* sampling = CapyPoissonSamplingAlloc(app->seed, 2); sampling->domains[0].max = dims.width - 1; sampling->domains[1].max = dims.height - 1; double dropSpace = 0.0; if(app->dims[0] < app->dims[1]) dropSpace = app->dims[0] * 0.1; else dropSpace = app->dims[1] * 0.1; sampling->r = CapyCm2Pixel(dropSpace, app->dpi); // Set the blur strength double blurStrength = dropSpace; $(&kernel, setToGaussianBlur)(blurStrength); // Create the pen and circle shape for drawing CapyPen pen = CapyPenCreate(); pen.hardness = capyPenHardness_9B; CapyCircle circle = CapyCircleCreate(); // Initial range of size for the drops, relative to the sampling radius CapyRangeDouble rangeRadius = {.min = 0.75, .max = 1.0}; // For each color in the palette loop(iColor, palette.size - 1) { // Set the color of the pen pen.color = $(&palette, get)(iColor + 1); // Sample the position of drops on the paper CapySamples* samples = $(sampling, getSamples)(); printf("Drop %lu samples of color #%lu\n", samples->size, iColor + 1); // For each drop loop(iSample, samples->size) { CapySample const sample = $(samples, get)(iSample); // Draw the drop with a random size loop(i, 2) circle.center.coords[i] = sample.vals[i]; circle.radius = $(&rng, getDoubleRange)(&rangeRadius) * sampling->r; $(&pen, drawFilledCircle)(&circle, img); } // Reduce the range of size for the drops to avoid predominance of // foreground color loop(i, 2) rangeRadius.vals[i] /= 1.618; // Free memory CapySamplesFree(&samples); } // Get the stretching strength double stretchingStrength = CapyCm2Pixel((2.0 + 5.0 * $(&rng, getDouble)()) * dropSpace, app->dpi); printf("Stretching strength %lf\n", stretchingStrength); // Loop on the stretching passes loop(iStretching, app->nbPass) { printf("Stretching pass #%lu\n", iStretching); // Get the stretching axis uint8_t const stretchingAxis = ($(&rng, getUInt8)()) & 1; printf("Stretching axis: %u\n", stretchingAxis); // Get the average space between stretching axis CapyRangeDouble rangeStretchingSpace = { .min = dropSpace * 0.5 * (double)(1 + iStretching), .max = dropSpace * 1.5 * (double)(1 + iStretching) }; double stretchingSpaceCm = $(&rng, getDoubleRange)(&rangeStretchingSpace); printf("Stretching space %lf\n", stretchingSpaceCm); // Sample stretching positions CapyPoissonSampling* stretchSampling = CapyPoissonSamplingAlloc(app->seed, 1); stretchSampling->domains[0].max = dims.vals[1 - stretchingAxis] - 1; stretchSampling->r = CapyCm2Pixel(stretchingSpaceCm * 0.75, app->dpi); CapySamples* stretchSamples = $(stretchSampling, getSamples)(); CapyComparator cmpSample = CapyComparatorCreate(); cmpSample.eval = CmpSample; $(stretchSamples, qsort)(&cmpSample); $(&cmpSample, destruct)(); printf("%lu stretching positions\n", stretchSamples->size); // Force the first and last sample to be at the borders of the paper CapySample firstStretchSample = $(stretchSamples, get)(0); firstStretchSample.vals[0] = 0.0; CapySample lastStretchSample = $(stretchSamples, get)(stretchSamples->size - 1); lastStretchSample.vals[0] = stretchSampling->domains[0].max; // Get the stretching direction uint8_t const stretchingDirection = ($(&rng, getUInt8)()) & 1; double const coeffDir = (stretchingDirection == 0 ? 1.0 : -1.0); printf("Stretching direction: %u\n", stretchingDirection); // Stretching power double const stretchingPower = 0.25 + 0.75 * $(&rng, getDouble)(); printf("Stretching power %lf\n", stretchingPower); // Create the stretching value array CapyImgPos_t* stretchingVals = NULL; safeMalloc(stretchingVals, dims.vals[1 - stretchingAxis]); loop(iVal, dims.vals[1 - stretchingAxis]) { size_t iStretchPos = 0; CapySample stretchSample = $(stretchSamples, get)(iStretchPos + 1); while(stretchSample.vals[0] < iVal) { ++iStretchPos; stretchSample = $(stretchSamples, get)(iStretchPos + 1); } double c[2] = {0.0, stretchSample.vals[0]}; stretchSample = $(stretchSamples, get)(iStretchPos); c[0] = stretchSample.vals[0]; double x = ((double)iVal - c[0]) / (c[1] - c[0]); double y = pow(sin(x * M_PI), stretchingPower) * stretchingStrength * coeffDir; stretchingVals[iVal] = (CapyImgPos_t)y; } // Create a clone of the image CapyImg* cloneImg = CapyImgClone(img); // Loop on the pixels of the image forEach(pixel, img->iter) { // Calculate the position of the stretched pixel CapyImgPos srcPos = pixel.pos; srcPos.coords[stretchingAxis] += stretchingVals[srcPos.coords[1 - stretchingAxis]]; if(srcPos.coords[stretchingAxis] < 0) srcPos.coords[stretchingAxis] += (CapyImgPos_t)(dims.vals[stretchingAxis]); else if( srcPos.coords[stretchingAxis] >= (CapyImgPos_t)(dims.vals[stretchingAxis]) ) { srcPos.coords[stretchingAxis] -= (CapyImgPos_t)(dims.vals[stretchingAxis]); } // Set the color of the pixel in the image with the color of the // stretched pixel in the original image CapyColorData const* color = $(cloneImg, getColor)(&srcPos); *(pixelPtr->color) = *color; } // Free memory CapyPoissonSamplingFree(&stretchSampling); CapyImgFree(&cloneImg); free(stretchingVals); CapySamplesFree(&stretchSamples); CapyPoissonSamplingFree(&stretchSampling); // Apply blurring printf("Blurring\n"); CapyImg* imgBlur = $(&kernel, apply)(img); CapyImgFree(&img); img = imgBlur; // Reduce the stretching strength stretchingStrength /= 1.618; printf("Stretching strength %lf\n", stretchingStrength); } // Crop the unblurred border loop(i, 2) dims.vals[i] -= 2 * blurKernelSize; CapyImg* cropImg = CapyImgAlloc(capyImgMode_rgb, dims); CapyImgPos posTopLeft = {.x = -(CapyImgPos_t)blurKernelSize, .y = -(CapyImgPos_t)blurKernelSize}; $(img, pasteInto)(cropImg, &posTopLeft); CapyImgFree(&img); img = cropImg; // Save the image printf("Save to %s\n", app->pathImg); $(img, saveToPath)(app->pathImg); // Free memory CapyImgFree(&img); $(&circle, destruct)(); $(&pen, destruct)(); CapyPoissonSamplingFree(&sampling); $(&palette, destruct)(); $(&rng, destruct)(); $(&kernel, destruct)(); } int main(int argc, char** argv) { // Set the conversion function for raised exceptions CapyAddExcToStrFun(ExcToStr); // Create the instance of the application struct App app = { .ret = EXIT_SUCCESS, .dims = {21.0, 29.7}, .dpi = 300.0, .pathImg = "./output.png", .sizePalette = 10, .seed = (CapyRandomSeed_t)time(NULL), .nbPass = 2, .flagPastel = false, }; try { // Definition of the arguments from the command line CapyArg args[] = { { .lbl = argv[0], .nbVal=capyArg_variableNbVal, .help = "Create random venetian marbled paper.\n" }, { .lbl = "--dims", .shortLbl="-d", .nbVal=2, .help = "Dimensions (width, height in cm) of the paper (default: 21x29.7).", }, { .lbl = "--dpi", .shortLbl="-dpi", .nbVal=1, .help = "Dpi (default: 300).", }, { .lbl = "--output", .shortLbl="-o", .nbVal=1, .help = "Path to the output image (PNG format, default: ./output.png).", }, { .lbl = "--size-palette", .shortLbl="-p", .nbVal=1, .help = "Number of colors in the palette (default: 10)", }, { .lbl = "--seed", .shortLbl="-s", .nbVal=1, .help = "Seed for the RNG (default: current time)", }, { .lbl = "--nb-pass", .shortLbl="-n", .nbVal=1, .help = "Number of stretching passes (default: 2)", }, { .lbl = "--pastel", .shortLbl="-pa", .nbVal=0, .help = "Use a pastel color palette", }, }; // Create the argument parser and parse the arguments app.parser = CapyArgParserAlloc(argc, argv, args); // Process the command line arguments if($(app.parser, isSet)("--dims")) loop(i, (size_t)2) app.dims[i] = $(app.parser, getAsDouble)("--dims", i); if($(app.parser, isSet)("--dpi")) app.dpi = $(app.parser, getAsDouble)("--dims", 0); if($(app.parser, isSet)("--output")) app.pathImg = $(app.parser, getAsStr)("--output", 0); if($(app.parser, isSet)("--size-palette")) app.sizePalette = (size_t)$(app.parser, getAsInt)("--size-palette", 0); if($(app.parser, isSet)("--seed")) app.seed = (CapyRandomSeed_t)$(app.parser, getAsInt)("--seed", 0); if($(app.parser, isSet)("--nb-pass")) app.nbPass = (size_t)$(app.parser, getAsInt)("--nb-pass", 0); app.flagPastel = $(app.parser, isSet)("--pastel"); // Check the parameters if(app.dims[0] <= 0.0 || app.dims[1] <= 0.0) raiseExc(excInvalidDims); if(app.dpi <= 0.0) raiseExc(excInvalidDpi); // Create the paper Render(&app); } catchDefault { // Inform the user about the raised exception $(&capyDecoratorRed, fprintf)( stdout, "Sorry, an exception occured. Exiting.\n"); $(&capyDecoratorRed, fprintf)( stdout, "%s\n", CapyExcToStr(CapyGetLastExcId())); app.ret = EXIT_FAILURE; } endCatch; return app.ret; }

Compile with gcc -o main main.c `pkg-config --cflags gtk+-3.0` -lcapy -lm -lpng (need LibCapy version v0.9.0 or higher). Comments or questions are always welcome by email.