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;
}