如何解码一个PNG图像为原始字节？

Question

我正在寻找一种方法来解码一些PNG文件，我听说过libpng，但我不明白这是如何工作的。它是否将png文件转换为ARGB8888格式的字节数组或其他内容？

Answer 1

Runnable的例子

这种读取和现有PNG，修改它，并写入修改后的版本到磁盘。

修改部分在原始字节上完成。

用法：

./a.out [<old-png> [<new-png>]] 

• old-png默认为a.png
• new-png默认为b.png

测试在Ubuntu 15.10，51年2月1日了libpng，编译：

gcc main.c -lpng 

改编自：https://gist.github.com/niw/5963798

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

#include <png.h> 

unsigned int width; 
unsigned int height; 
png_bytep *row_pointers; 

static void read_png_file(char *filename) { 
    FILE *fp = fopen(filename, "rb"); 
    png_byte bit_depth; 
    png_byte color_type; 
    unsigned int y; 

    png_structp png = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL); 
    if (!png) abort(); 
    png_infop info = png_create_info_struct(png); 
    if (!info) abort(); 
    if (setjmp(png_jmpbuf(png))) abort(); 
    png_init_io(png, fp); 
    png_read_info(png, info); 
    width = png_get_image_width(png, info); 
    height = png_get_image_height(png, info); 
    color_type = png_get_color_type(png, info); 
    bit_depth = png_get_bit_depth(png, info); 
    /* Read any color_type into 8bit depth, RGBA format. */ 
    /* See http://www.libpng.org/pub/png/libpng-manual.txt */ 
    if (bit_depth == 16) 
     png_set_strip_16(png); 
    if (color_type == PNG_COLOR_TYPE_PALETTE) 
     png_set_palette_to_rgb(png); 
    /* PNG_COLOR_TYPE_GRAY_ALPHA is always 8 or 16bit depth. */ 
    if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8) 
     png_set_expand_gray_1_2_4_to_8(png); 
    if (png_get_valid(png, info, PNG_INFO_tRNS)) 
     png_set_tRNS_to_alpha(png); 
    /* These color_type don't have an alpha channel then fill it with 0xff. */ 
    if (color_type == PNG_COLOR_TYPE_RGB || 
      color_type == PNG_COLOR_TYPE_GRAY || 
      color_type == PNG_COLOR_TYPE_PALETTE) 
    png_set_filler(png, 0xFF, PNG_FILLER_AFTER); 
    if (color_type == PNG_COLOR_TYPE_GRAY || 
      color_type == PNG_COLOR_TYPE_GRAY_ALPHA) 
     png_set_gray_to_rgb(png); 
    png_read_update_info(png, info); 
    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,info)); 
    } 
    png_read_image(png, row_pointers); 
    fclose(fp); 
} 

static void write_png_file(char *filename) { 
    unsigned int y; 
    FILE *fp = fopen(filename, "wb"); 
    if (!fp) abort(); 
    png_structp png = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL); 
    if (!png) abort(); 
    png_infop info = png_create_info_struct(png); 
    if (!info) abort(); 
    if (setjmp(png_jmpbuf(png))) abort(); 
    png_init_io(png, fp); 
    png_set_IHDR(
     png, 
     info, 
     width, 
     height, 
     8, 
     PNG_COLOR_TYPE_RGBA, 
     PNG_INTERLACE_NONE, 
     PNG_COMPRESSION_TYPE_DEFAULT, 
     PNG_FILTER_TYPE_DEFAULT 
    ); 
    png_write_info(png, info); 
    /* To remove the alpha channel for PNG_COLOR_TYPE_RGB format, */ 
    /* Use png_set_filler(). */ 
    /*png_set_filler(png, 0, PNG_FILLER_AFTER);*/ 
    png_write_image(png, row_pointers); 
    png_write_end(png, NULL); 
    for (y = 0; y < height; y++) { 
     free(row_pointers[y]); 
    } 
    free(row_pointers); 
    fclose(fp); 
} 

static void process_png(void) { 
    for (unsigned int y = 0; y < height; y++) { 
     png_bytep row = row_pointers[y]; 
     for (unsigned int x = 0; x < width; x++) { 
      png_bytep px = &(row[x * 4]); 
      /*printf("%4d, %4d = RGBA(%3d, %3d, %3d, %3d)\n", x, y, px[0], px[1], px[2], px[3]);*/ 
      png_byte old[4 * sizeof(png_byte)]; 
      memcpy(old, px, sizeof(old)); 
      px[0] = 255 - old[0]; 
      px[1] = 255 - old[1]; 
      px[2] = 255 - old[2]; 
     } 
    } 
} 

int main(int argc, char *argv[]) { 
    char *in; 
    char *out; 
    if (argc > 1) { 
     in = argv[1]; 
    } else { 
     in = "a.png"; 
    } 
    if (argc > 2) { 
     out = argv[2]; 
    } else { 
     out = "b.png"; 
    } 
    read_png_file(in); 
    process_png(); 
    write_png_file(out); 
    return EXIT_SUCCESS; 
} 

Answer 2

您可能最好在寻找一个专门的image library，它会为您解码图像并将其返回到可识别的结构中。如果您想要对图像进行实际操作（保存，显示等），它也可以作为一个更好的平台。