path: root/alphagen.c

/*
 * Copyright 2009 Michael Drake <tlsa@netsurf-browser.org>
 *
 * This file is part of AlphaGen.
 *
 * Licensed under the MIT License,
 * http://www.opensource.org/licenses/mit-license.php
 *
 * AlphaGen is a tool for obtaining a bitmap image with an alpha channel from
 * software which lacks this facility itself.
 *
 * Input:
 *     blackbg.png   - The image exported on a black background
 *     whitebg.png   - The image exported on a white background
 * Output
 *     alpha.png     - The resultant image with alpha channel
 *
 * Note that black.png and white.png must be the same size.
 */

#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

#include "png.h"


struct image {
	unsigned int width;
	unsigned int height;
	unsigned int channels;
	uint8_t **d;
};

/* image_* functions, so the libpng stuff is kept hidden */
bool image_init(struct image *img, int width, int height, int channels);
void image_free(struct image *img);
bool image_read_png(char *file_name, struct image *img);
bool image_write_png(char *file_name, struct image *img);

int main(int argc, char** argv)
{
	struct image b, w, a;
	unsigned int row_data_width;
	unsigned int z;
	uint8_t ar, ag, ab;

	/* Default background colour; 0 = black, 255 = white */
	const uint8_t bg = 255;

	/* Load input PNGs */
	if (!image_read_png("blackbg.png", &b)) {
		printf("Problem loading blackbg.\n");
		return EXIT_FAILURE;
	}
	if (!image_read_png("whitebg.png", &w)) {
		printf("Problem loading whitebg.\n");
		return EXIT_FAILURE;
	}
	/* Check both input images are the same size */
	if (b.width != w.width || b.height != w.height) {
		printf("Input images have different dimensions.\n");
		return EXIT_FAILURE;
	}

	/* Prepare output image */
	if (!image_init(&a, b.width, b.height, 4)) {
		printf("Couldn't create output image.\n");
		return EXIT_FAILURE;
	}

	/* Recover true image colours and alpha channel for the image */
	row_data_width = a.width * a.channels;
	for (unsigned int y = 0; y < a.height; y++) {
		z = 0; /* x-position in input images */
		for (unsigned int x = 0; x < row_data_width; x += 4) {
			/* Get alpha values first */
			ar = 255 - w.d[y][z]   + b.d[y][z];
			ag = 255 - w.d[y][z+1] + b.d[y][z+1];
			ab = 255 - w.d[y][z+2] + b.d[y][z+2];

			/* Set red, green and blue colour values
			 * use preset bg colour if fully transparent */
			a.d[y][x]   = (ar == 0) ? bg : (b.d[y][z]   * 255) / ar;
			a.d[y][x+1] = (ag == 0) ? bg : (b.d[y][z+1] * 255) / ag;
			a.d[y][x+2] = (ab == 0) ? bg : (b.d[y][z+2] * 255) / ab;

			/* Set alpha value */
			a.d[y][x+3] = (ar + ag + ab) / 3;

			z += 3;
		}
	}

	/* Save PNG with recovered colour and alpha data */
	if (!image_write_png("alpha.png", &a)) {
		printf("Couldn't write output image.\n");
		return EXIT_FAILURE;
	}

	/* Free image memory */
	image_free(&b);
	image_free(&w);
	image_free(&a);

	return EXIT_SUCCESS;
}

bool image_init(struct image *img, int width, int height, int channels)
{
	int size = sizeof(uint8_t**);
	int row_data_width;

	// Set frame dimensions
	img->width = width;
	img->height = height;
	img->channels = channels;

	// Allocate memory for row pointers
	img->d = (uint8_t**)malloc(height * size);
	if (img->d == NULL)
		return false;

	row_data_width = width * channels;
	for (int i = 0; i < height; i++) {
		// Allocate memory for row data
		img->d[i] = (uint8_t*)malloc(row_data_width);
		if (img->d[i] == NULL)
			return false;
	}
	return true;
}

void image_free(struct image *img)
{
	for (int i = 0; i < img->height; i++) {
		// Free row data
		free(img->d[i]);
	}
	// Free row pointers
	free(img->d);
}

/* Read RGB PNG with no alpha channel into img. */
bool image_read_png(char *file_name, struct image *img)
{
	png_structp png_ptr;
	png_infop info_ptr;
	unsigned int sig_read = 0;
	unsigned int row_data_width;
	png_uint_32 width, height;
	int bit_depth, color_type;
	FILE *fp;

	if ((fp = fopen(file_name, "rb")) == NULL)
		return false;

	/* Create and initialize the png_struct. */
	png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
			NULL, NULL, NULL);

	if (png_ptr == NULL) {
		fclose(fp);
		return false;
	}

	/* Allocate/initialize the memory for image information. */
	info_ptr = png_create_info_struct(png_ptr);
	if (info_ptr == NULL) {
		fclose(fp);
		png_destroy_read_struct(&png_ptr, png_infopp_NULL,
				png_infopp_NULL);
		return false;
	}

	/* Set error handling if you are using the setjmp/longjmp method
	 * because I left stuff as NULL in png_create_read_struct(). */
	if (setjmp(png_jmpbuf(png_ptr))) {
		/* Free all of the memory associated with the png_ptr and
		 * info_ptr */
		png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
		fclose(fp);
		/* If we get here, we had a problem reading the file */
		return false;
	}

	/* Set up the input control */
	png_init_io(png_ptr, fp);

	/* If we have already read some of the signature */
	png_set_sig_bytes(png_ptr, sig_read);

	/* Read in the entire image at once */
	png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_PACKING, png_voidp_NULL);

	width = png_get_image_width(png_ptr, info_ptr);
	height = png_get_image_height(png_ptr, info_ptr);
	bit_depth = png_get_bit_depth(png_ptr, info_ptr);
	color_type = png_get_color_type(png_ptr, info_ptr);

	if (color_type != PNG_COLOR_TYPE_RGB && bit_depth != 8)
		return false;

	if (!image_init(img, width, height, 3))
		return false;

	png_bytepp row_pointers = png_get_rows(png_ptr, info_ptr);

	row_data_width = img->width * img->channels;
	for (int i = 0; i < img->height; i++) {
		memcpy(img->d[i], row_pointers[i], row_data_width);
	}

	/* clean up after the read, and free any memory allocated */
	png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);

	/* close the file */
	fclose(fp);

	return true;
}



/* Save output PNG file with alpha channel with img data */
bool image_write_png(char *file_name, struct image *img)
{
	FILE *fp;
	png_structp png_ptr;
	png_infop info_ptr;

	if (img->channels != 4)
		/* Only rgba can be saved, as that is the point of this */
		return false;

	/* open the file */
	fp = fopen(file_name, "wb");
	if (fp == NULL)
		return false;

	/* Create and initialize the png_struct */
	png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
			NULL, NULL, NULL);

	if (png_ptr == NULL) {
		fclose(fp);
		return false;
	}

	/* Allocate/initialize the image information data. */
	info_ptr = png_create_info_struct(png_ptr);
	if (info_ptr == NULL) {
		fclose(fp);
		png_destroy_write_struct(&png_ptr, png_infopp_NULL);
		return false;
	}
	/* Set error handling, needed because I gave NULLs to
	 * png_create_write_struct. */
	if (setjmp(png_jmpbuf(png_ptr))) {
		/* If we get here, we had a problem reading the file */
		fclose(fp);
		png_destroy_write_struct(&png_ptr, &info_ptr);
		return false;
	}

	/* set up the output control */
	png_init_io(png_ptr, fp);

	/* Set the image information. */
	png_set_IHDR(png_ptr, info_ptr, img->width, img->height, 8,
			PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_ADAM7,
			PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);

	/* TODO: ??? Maybe I should set gamma stuff, but I dunno what */
	/* png_set_gAMA(png_ptr, info_ptr, gamma); */

	/* Write the file header information. */
	png_write_info(png_ptr, info_ptr);

	/* pack pixels into bytes */
	png_set_packing(png_ptr);

	if (img->height > PNG_UINT_32_MAX/png_sizeof(png_bytep)) {
		png_error (png_ptr, "Image is too tall to process in memory");
		return false;
	}

	png_write_image(png_ptr, (png_byte **)img->d);

	/* finish writing the rest of the file */
	png_write_end(png_ptr, info_ptr);

	/* clean up after the write, and free any memory allocated */
	png_destroy_write_struct(&png_ptr, &info_ptr);

	/* close the file */
	fclose(fp);

	return true;
}