使用libpng读取PNG图片像素数据

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。

可以到上下载源代码。

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！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 filepng_set_read_fn(png_ptr,&imgsource, pngReaderCallback );/* *************************************************** The low-level read interface in libpng(/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 bytespng_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]; // redrgba[pos++] = row_pointers[row][col + 1]; // greenrgba[pos++] = row_pointers[row][col + 2]; // bluergba[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 allocatedpng_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 methodspng_init_io(png_ptr, file);png_set_sig_bytes(png_ptr, 8); //Required!!!/* *************************************************** The high-level read interface in libpng(/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 bytespng_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]; // redrgba[pos++] = row_pointers[row][col + 1]; // greenrgba[pos++] = row_pointers[row][col + 2]; // bluergba[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 allocatedpng_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->im ageHeight,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 Texturechar* 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。