使用libpng读取PNG图片像素数据

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哦。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-1.2.3.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！

libpng 的详细介绍：请点这里
libpng 的下载地址：请点这里

下面到重点内容了。如何用pnglib实现对文件的读写？ 现在让我们来试着写一段代码来读取PNG像素数据(from File or Stream)，并把读取的数据存储到一个数组rgba中,用来生成OpenGl纹理

typedef struct {
    u8* pixelData;
    int imageWidth;
    int imageHeight;
}ImageInfo; 

typedef struct {
    u8* data;
    int size;
    int offset;
}ImageSource;

 

//—————————————————————————————————————-
static void pngReaderCallback(png_structp png_ptr, png_bytep data, png_size_t length)
{
    ImageSource* isource = (ImageSource*)png_get_io_ptr(png_ptr);

    if(isource->offset + length <= isource->size)
    {
      memcpy(data, isource->data + isource->offset, length);
      isource->offset += length;
    }
    else
    {
        png_error(png_ptr,”pngReaderCallback failed”);
    }
}

//—————————————————————————————————————-

ImageInfo*  decodePNGFromStream(const u8* pixelData, const u32& dataSize)
{
    png_structp png_ptr;
    png_infop info_ptr;
    int width, height, rowBytes;
    png_byte color_type;  //可以是PNG_COLOR_TYPE_RGB,PNG_COLOR_TYPE_PALETTE…….等
    png_byte bit_depth;
    png_colorp palette;

    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL,NULL,NULL);
    if (!png_ptr)
    {
        png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
        TFC_DEBUG(“ReadPngFile: Failed to create png_ptr”);
    }
    info_ptr = png_create_info_struct(png_ptr);
    if (!info_ptr)
    {
        png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
        TFC_DEBUG(“ReadPngFile: Failed to create info_ptr”);
    }
    if (setjmp(png_jmpbuf(png_ptr)))
    {
        png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
        TFC_DEBUG(“ReadPngFile: Failed to read the PNG file”);
    }

    ImageSource imgsource;
    imgsource.data = (u8*)pixelData;
    imgsource.size = dataSize;
    imgsource.offset = 0;
    //define our own callback function for I/O instead of reading from a file
    png_set_read_fn(png_ptr,&imgsource, pngReaderCallback );

    /* **************************************************
    * The low-level read interface in libpng (http://www.libpng.org/pub/png/libpng-1.2.5-manual.html)
    * **************************************************
    */
    png_read_info(png_ptr, info_ptr);
    width = info_ptr->width;
    height = info_ptr->height;
    color_type = info_ptr->color_type;
    bit_depth = info_ptr->bit_depth;
    rowBytes = info_ptr->rowbytes;
    palette= info_ptr->palette;

    // Convert stuff to appropriate formats!
    if(color_type==PNG_COLOR_TYPE_PALETTE)
    {
        png_set_packing(png_ptr);
        png_set_palette_to_rgb(png_ptr); //Expand data to 24-bit RGB or 32-bit RGBA if alpha available.
    }
    if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
        png_set_gray_1_2_4_to_8(png_ptr);
    if (color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
        png_set_gray_to_rgb(png_ptr);
    if (bit_depth == 16)
        png_set_strip_16(png_ptr);

    //Expand paletted or RGB images with transparency to full alpha channels so the data will be available as RGBA quartets.
    if(png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
    {
        png_set_tRNS_to_alpha(png_ptr);
    }

    //png_read_update_info(png_ptr, info_ptr);
    u8* rgba = new u8[width * height * 4];  //each pixel(RGBA) has 4 bytes
    png_bytep * row_pointers;
    row_pointers = (png_bytep*)png_malloc(sizeof(png_bytep) * height);
    for (int y = 0; y < height; y++)
    {
        row_pointers[y] = (png_bytep)png_malloc(width<<2); //each pixel(RGBA) has 4 bytes
    }
    png_read_image(png_ptr, row_pointers);

    //unlike store the pixel data from top-left corner, store them from bottom-left corner for OGLES Texture drawing…
    int pos = (width * height * 4) – (4 * width);
    for(int row = 0; row < height; row++)
    {
      for(int col = 0; col < (4 * width); col += 4)
      {
          rgba[pos++] = row_pointers[row][col];        // red
          rgba[pos++] = row_pointers[row][col + 1];    // green
          rgba[pos++] = row_pointers[row][col + 2];    // blue
          rgba[pos++] = row_pointers[row][col + 3];    // alpha
      }
      pos=(pos – (width * 4)*2); //move the pointer back two rows
    }

    ImageInfo* imageInfo = (ImageInfo*)kdMalloc(sizeof(ImageInfo));
    imageInfo->pixelData = rgba;
    imageInfo->imageHeight = height;
    imageInfo->imageWidth = width;

    //clean up after the read, and free any memory allocated
    png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
    return imageInfo;
}

//—————————————————————————————————————-

ImageInfo*  decodePNGFromFile(char* fileName)
{
    char png_header[8];
    png_structp png_ptr;
    png_infop info_ptr;
    int width, height, rowBytes;
    png_byte color_type;
    png_byte bit_depth;
    png_colorp palette;

    /* open file and test for it being a png */
    FILE *file = fopen(fileName, “rb”);
    fread(png_header, 1, 8, file);
    if(png_sig_cmp((png_bytep)png_header, 0, 8))
    {
        TFC_DEBUG(“Not a PNG file…”);
        fclose(file);
    }
    /* initialise structures for reading a png file */
    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    info_ptr = png_create_info_struct(png_ptr);
    if (setjmp(png_jmpbuf(png_ptr)))
    {
        png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
        TFC_DEBUG(“ReadPngFile: Failed to read the PNG file”);
        fclose(file);
    }
    //I/O initialisation methods
    png_init_io(png_ptr, file);
    png_set_sig_bytes(png_ptr, 8);  //Required!!!

    /* **************************************************
    * The high-level read interface in libpng (http://www.libpng.org/pub/png/libpng-1.2.5-manual.html)
    * **************************************************
    */
      png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_EXPAND, 0);
      width = info_ptr->width;
      height = info_ptr->height;
      unsigned char* rgba = new unsigned char[width * height * 4];  //each pixel(RGBA) has 4 bytes
      png_bytep* row_pointers = png_get_rows(png_ptr, info_ptr);

      //Original PNG pixel data stored from top-left corner, BUT OGLES Texture drawing is from bottom-left corner
      //int pos = 0;
      //for(int row = 0; row < height; row++)
      //{
          //for(int col = 0; col < (4 * width); col += 4)
          //{
              //rgba[pos++] = row_pointers[row][col];    // red
              //rgba[pos++] = row_pointers[row][col + 1]; // green
              //rgba[pos++] = row_pointers[row][col + 2]; // blue
              //rgba[pos++] = row_pointers[row][col + 3]; // alpha
          //}
      //}

      //unlike store the pixel data from top-left corner, store them from bottom-left corner for OGLES Texture drawing…
      int pos = (width * height * 4) – (4 * width);
      for(int row = 0; row < height; row++)
      {
          for(int col = 0; col < (4 * width); col += 4)
          {
              rgba[pos++] = row_pointers[row][col];        // red
              rgba[pos++] = row_pointers[row][col + 1]; // green
              rgba[pos++] = row_pointers[row][col + 2]; // blue
              rgba[pos++] = row_pointers[row][col + 3]; // alpha
          }
          pos=(pos – (width * 4)*2); //move the pointer back two rows
      }

    ImageInfo* imageInfo = (ImageInfo*)kdMalloc(sizeof(ImageInfo));
    imageInfo->pixelData = rgba;
    imageInfo->imageHeight = height;
    imageInfo->imageWidth = width;

    //clean up after the read, and free any memory allocated
    png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
    fclose(file);
    return imageInfo;
}

//—————————————————————————————————————-

 

int createPNGTextureFromStream(const u8* pixelData, const u32& dataSize)
{
        GLuint textureID;
        glEnable(GL_TEXTURE_2D);
        glGenTextures(1,&textureID);
        glBindTexture(GL_TEXTURE_2D,textureID);
        ImageInfo* imageInfo = decodePNGFromStream(pixelData, dataSize)；

        glTexImage2D(GL_TEXTURE_2D,0,GL_RGBA,imageInfo->imageWidth,imageInfo->imageHeight,0,

                                GL_RGBA,GL_UNSIGNED_BYTE,imageInfo->pixelData);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);

        delete[] imageInfo->pixelData;

        delete imageInfo;
        return textureID;

}

//—————————————————————————————————————-

 

void main()

{

      //Testcase1: decoding png data from a raw png bufferstream and create the corresponding Texture

      //假设我们从某个地方可以拿到一个unsigned char* 的PNG数据源pixelData.

      int texId = createPNGTextureFromStream(pixelData, dataSize);

 

      //Testcase2: decoding png data from a given png file and and create the corresponding Texture

      char* fileName = “example.png”;

      int texId = createPNGTextureFromFile(fileName);

 

      //现在我们就可以用创建出来的textureID来绘制纹理了。。。。

}

接下来请看第2页精彩内容： http://www.linuxidc.com/Linux/2013-08/88651p2.htm

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利用libpng中的函数读写PNG文件 http://www.linuxidc.com/Linux/20