A simple libpng example program

PNG格式读写离不开PNG库libpng 是一套免费的、公开源代码的程序库，支持对 PNG 图形文件的创建、读写等操作。可以到www.libpng.org 上下载源代码。libpng 使用 zlib 程序库作为压缩引擎，zlib 也是著名的 gzip (GNU zip) 所采用的压缩引擎。zlib 是通用的压缩库，提供了一套 in-memory 压缩和解压函数，并能检测解压出来的数据的完整性(integrity)。zlib 也支持读写 gzip (.gz) 格式的文件这些都是免费和公开的。在下载了pbglib，并且要下载zlib，如何才能加到代码中呢？以VC6.为例来说明。其他CBuild等编译器相信也一样的。

       比如我们从网上下载了一个lpng1210.zip和zlib-.2.3.tar.tar。首先把他们解压。在lpng的目录中寻找projects\visualc6\libpng.dsw。并在zlib中寻找zlib-1.2.3\projects\visualc6\zlib.dsw。用vc打开libpng工程文件，把zlib的工程文件也添加进去。设置好在setting中设置好的zlib.lib和pbg.lib的输出路径，以及include路径（这个简单方法相信大家都明白吧）。运行后，就生成了两个静态库了。

      lib做好以后，就可以将lib加到我们要做的工程中去。在link中设置好，并将png.h和zlib.h都加进去。这样就可以正式的使用libpng了！如果还有不明白的地方，在这里留言或者在百度上搜一下。

下面将将到重点内容了。如何用pnglib实现对文件的读写？A simple libpng example program（http://zarb.org/~gc/html/libpng.html）

 

/* * Copyright 2002-2010 Guillaume Cottenceau. * * This software may be freely redistributed under the terms * of the X11 license. * */#include <unistd.h>#include <stdlib.h>#include <stdio.h>#include <string.h>#include <stdarg.h>#define PNG_DEBUG 3#include <png.h>void abort_(const char * s, ...){        va_list args;        va_start(args, s);        vfprintf(stderr, s, args);        fprintf(stderr, "\n");        va_end(args);        abort();}int x, y;int width, height;png_byte color_type;png_byte bit_depth;png_structp png_ptr;png_infop info_ptr;int number_of_passes;png_bytep * row_pointers;void read_png_file(char* file_name){        char header[8];    // 8 is the maximum size that can be checked        /* open file and test for it being a png */        FILE *fp = fopen(file_name, "rb");        if (!fp)                abort_("[read_png_file] File %s could not be opened for reading", file_name);        fread(header, 1, 8, fp);        if (png_sig_cmp(header, 0, 8))                abort_("[read_png_file] File %s is not recognized as a PNG file", file_name);        /* initialize stuff */        png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);        if (!png_ptr)                abort_("[read_png_file] png_create_read_struct failed");        info_ptr = png_create_info_struct(png_ptr);        if (!info_ptr)                abort_("[read_png_file] png_create_info_struct failed");        if (setjmp(png_jmpbuf(png_ptr)))                abort_("[read_png_file] Error during init_io");        png_init_io(png_ptr, fp);        png_set_sig_bytes(png_ptr, 8);        png_read_info(png_ptr, info_ptr);        width = png_get_image_width(png_ptr, info_ptr);        height = png_get_image_height(png_ptr, info_ptr);        color_type = png_get_color_type(png_ptr, info_ptr);        bit_depth = png_get_bit_depth(png_ptr, info_ptr);        number_of_passes = png_set_interlace_handling(png_ptr);        png_read_update_info(png_ptr, info_ptr);        /* read file */        if (setjmp(png_jmpbuf(png_ptr)))                abort_("[read_png_file] Error during read_image");        row_pointers = (png_bytep*) malloc(sizeof(png_bytep) * height);        for (y=0; y<height; y++)                row_pointers[y] = (png_byte*) malloc(png_get_rowbytes(png_ptr,info_ptr));        png_read_image(png_ptr, row_pointers);        fclose(fp);}void write_png_file(char* file_name){        /* create file */        FILE *fp = fopen(file_name, "wb");        if (!fp)                abort_("[write_png_file] File %s could not be opened for writing", file_name);        /* initialize stuff */        png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);        if (!png_ptr)                abort_("[write_png_file] png_create_write_struct failed");        info_ptr = png_create_info_struct(png_ptr);        if (!info_ptr)                abort_("[write_png_file] png_create_info_struct failed");        if (setjmp(png_jmpbuf(png_ptr)))                abort_("[write_png_file] Error during init_io");        png_init_io(png_ptr, fp);        /* write header */        if (setjmp(png_jmpbuf(png_ptr)))                abort_("[write_png_file] Error during writing header");        png_set_IHDR(png_ptr, info_ptr, width, height,                     bit_depth, color_type, PNG_INTERLACE_NONE,                     PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);        png_write_info(png_ptr, info_ptr);        /* write bytes */        if (setjmp(png_jmpbuf(png_ptr)))                abort_("[write_png_file] Error during writing bytes");        png_write_image(png_ptr, row_pointers);        /* end write */        if (setjmp(png_jmpbuf(png_ptr)))                abort_("[write_png_file] Error during end of write");        png_write_end(png_ptr, NULL);        /* cleanup heap allocation */        for (y=0; y<height; y++)                free(row_pointers[y]);        free(row_pointers);        fclose(fp);}void process_file(void){        if (png_get_color_type(png_ptr, info_ptr) == PNG_COLOR_TYPE_RGB)                abort_("[process_file] input file is PNG_COLOR_TYPE_RGB but must be PNG_COLOR_TYPE_RGBA "                       "(lacks the alpha channel)");        if (png_get_color_type(png_ptr, info_ptr) != PNG_COLOR_TYPE_RGBA)                abort_("[process_file] color_type of input file must be PNG_COLOR_TYPE_RGBA (%d) (is %d)",                       PNG_COLOR_TYPE_RGBA, png_get_color_type(png_ptr, info_ptr));        for (y=0; y<height; y++) {                png_byte* row = row_pointers[y];                for (x=0; x<width; x++) {                        png_byte* ptr = &(row[x*4]);                        printf("Pixel at position [ %d - %d ] has RGBA values: %d - %d - %d - %d\n",                               x, y, ptr[0], ptr[1], ptr[2], ptr[3]);                        /* set red value to 0 and green value to the blue one */                        ptr[0] = 0;                        ptr[1] = ptr[2];                }        }}int main(int argc, char **argv){        if (argc != 3)                abort_("Usage: program_name <file_in> <file_out>");        read_png_file(argv[1]);        process_file();        write_png_file(argv[2]);        return 0;}