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使用libpng讀取PNG圖片像素數據

libpng 是一套免費的、公開源代碼的程序庫，支持對 PNG 圖形文件的創建、讀寫等操作。可以下載源代碼。libpng 使用 zlib 程序庫作為壓縮引擎，zlib 也是著名的 gzip (GNU zip) 所采用的壓縮引擎。zlib 是通用的壓縮庫，提供了一套 in-memory 壓縮和解壓函數，並能檢測解壓出來的數據的完整性(integrity)。zlib 也支持讀寫 gzip (.gz) 格式的文件這些都是免費和公開的哦。在下載了pbglib，並且要下載zlib，如何才能加到代碼中呢？以VC6.為例來說明。其他CBuild等編譯器相信也一樣的。比如我們從網上下載了一個lpng1210.zip和zlib-1.2.3.tar.tar。首先把他們解壓。在lpng的目錄中尋找 projects/visualc6/libpng.dsw。並在zlib中尋找zlib-1.2.3/projects/visualc6 /zlib.dsw。用vc打開libpng工程文件，把zlib的工程文件也添加進去。設置好在setting中設置好的zlib.lib和pbg.lib的輸出路徑，以及include路徑（這個簡單方法相信大家都明白吧）。運行後，就生成了兩個靜態庫了。lib做好以後，就可以將lib加到我們要做的工程中去。在link中設置好，並將png.h和zlib.h都加進去。這樣就可以正式的使用libpng了！

libpng 的詳細介紹：請點這裡
libpng 的下載地址：請點這裡

下面到重點內容了。如何用pnglib實現對文件的讀寫？現在讓我們來試著寫一段代碼來讀取PNG像素數據(from File or Stream)，並把讀取的數據存儲到一個數組rgba中,用來生成OpenGl紋理

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
//}
//}

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

//假設我們從某個地方可以拿到一個unsigned char* 的PNG數據源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);