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
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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
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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。。。。
}
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附录 让我们打开pnglib下面的一个example.c
/* Read a PNG file. You may want to return an error code if the read
* fails (depending upon the failure). There are two "prototypes" given
* here - one where we are given the filename, and we need to open the
* file, and the other where we are given an open file (possibly with
* some or all of the magic bytes read - see comments above).
*/
#ifdef open_file /* prototype 1 */
void read_png(char *file_name) /* We need to open the file */
{
定义两个很重要的pngwen文件指针。png_info里面放的是pngchuck和png的数据。
png_structp png_ptr;
png_infop info_ptr;
这些是读出图像需要的参数，图像高。宽、深度、图像类型、交错类型。
unsigned int sig_read = 0;
png_uint_32 width, height;
int bit_depth, color_type, interlace_type;
FILE *fp;
打开文件，把文件指针给png_prt
if ((fp = fopen(file_name, "rb")) == NULL)
return (ERROR);
#else no_open_file /* prototype 2 */
void read_png(FILE *fp, unsigned int sig_read) /* file is already open */
{
png_structp png_ptr;
png_infop info_ptr;
png_uint_32 width, height;
int bit_depth, color_type, interlace_type;
#endif no_open_file /* only use one prototype! */
/* Create and initialize the png_struct with the desired error handler
* functions. If you want to use the default stderr and longjump method,
* you can supply NULL for the last three parameters. We also supply the
* the compiler header file version, so that we know if the application
* was compiled with a compatible version of the library. REQUIRED
*/
png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
png_voidp user_error_ptr, user_error_fn, user_warning_fn);
if (png_ptr == NULL)
{
fclose(fp);
return (ERROR);
}
/* Allocate/initialize the memory for image information. REQUIRED. */
info_ptr = png_create_info_struct(png_ptr);
if (info_ptr == NULL)
{
fclose(fp);
png_destroy_read_struct(&png_ptr, png_infopp_NULL, png_infopp_NULL);
return (ERROR);
}
/* Set error handling if you are using the setjmp/longjmp method (this is
* the normal method of doing things with libpng). REQUIRED unless you
* set up your own error handlers in the png_create_read_struct() earlier.
*/
if (setjmp(png_jmpbuf(png_ptr)))
{
/* Free all of the memory associated with the png_ptr and info_ptr */
png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
fclose(fp);
/* If we get here, we had a problem reading the file */
return (ERROR);
}
/* One of the following I/O initialization methods is REQUIRED */
#ifdef streams /* PNG file I/O method 1 */
/* Set up the input control if you are using standard C streams */
png_init_io(png_ptr, fp);
#else no_streams /* PNG file I/O method 2 */
/* If you are using replacement read functions, instead of calling
* png_init_io() here you would call:
*/
png_set_read_fn(png_ptr, (void *)user_io_ptr, user_read_fn);
/* where user_io_ptr is a structure you want available to the callbacks */
#endif no_streams /* Use only one I/O method! */
/* If we have already read some of the signature */
png_set_sig_bytes(png_ptr, sig_read);
#ifdef hilevel
/*
* If you have enough memory to read in the entire image at once,
* and you need to specify only transforms that can be controlled
* with one of the PNG_TRANSFORM_* bits (this presently excludes
* dithering, filling, setting background, and doing gamma
* adjustment), then you can read the entire image (including
* pixels) into the info structure with this call:
*/
png_read_png(png_ptr, info_ptr, png_transforms, png_voidp_NULL);
#else
/* OK, you're doing it the hard way, with the lower-level functions */
/* The call to png_read_info() gives us all of the information from the
* PNG file before the first IDAT (image data chunk). REQUIRED
*/
png_read_info(png_ptr, info_ptr);
png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
&interlace_type, int_p_NULL, int_p_NULL);
/* Set up the data transformations you want. Note that these are all
* optional. Only call them if you want/need them. Many of the
* transformations only work on specific types of images, and many
* are mutually exclusive.
*/
/* tell libpng to strip 16 bit/color files down to 8 bits/color */
png_set_strip_16(png_ptr);
/* Strip alpha bytes from the input data without combining with the
* background (not recommended).
*/
png_set_strip_alpha(png_ptr);
/* Extract multiple pixels with bit depths of 1, 2, and 4 from a single
* byte into separate bytes (useful for paletted and grayscale images).
*/
png_set_packing(png_ptr);
/* Change the order of packed pixels to least significant bit first
* (not useful if you are using png_set_packing). */
png_set_packswap(png_ptr);
/* Expand paletted colors into true RGB triplets */
if (color_type == PNG_COLOR_TYPE_PALETTE)
png_set_palette_rgb(png_ptr);
/* Expand grayscale images to the full 8 bits from 1, 2, or 4 bits/pixel */
if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
png_set_gray_1_2_4_to_8(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);
/* Set the background color to draw transparent and alpha images over.
* It is possible to set the red, green, and blue components directly
* for paletted images instead of supplying a palette index. Note that
* even if the PNG file supplies a background, you are not required to
* use it - you should use the (solid) application background if it has one.
*/
png_color_16 my_background, *image_background;
if (png_get_bKGD(png_ptr, info_ptr, &image_background))
png_set_background(png_ptr, image_background,
PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
else
png_set_background(png_ptr, &my_background,
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
/* Some suggestions as to how to get a screen gamma value */
/* Note that screen gamma is the display_exponent, which includes
* the CRT_exponent and any correction for viewing conditions */
if (/* We have a user-defined screen gamma value */)
{
screen_gamma = user-defined screen_gamma;
}
/* This is one way that applications share the same screen gamma value */
else if ((gamma_str = getenv("SCREEN_GAMMA")) != NULL)
{
screen_gamma = atof(gamma_str);
}
/* If we don't have another value */
else
{
screen_gamma = 2.2; /* A good guess for a PC monitors in a dimly
lit room */
screen_gamma = 1.7 or 1.0; /* A good guess for Mac systems */
}
/* Tell libpng to handle the gamma conversion for you. The final call
* is a good guess for PC generated images, but it should be configurable
* by the user at run time by the user. It is strongly suggested that
* your application support gamma correction.
*/
int intent;
if (png_get_sRGB(png_ptr, info_ptr, &intent))
png_set_gamma(png_ptr, screen_gamma, 0.45455);
else
{
double image_gamma;
if (png_get_gAMA(png_ptr, info_ptr, &image_gamma))
png_set_gamma(png_ptr, screen_gamma, image_gamma);
else
png_set_gamma(png_ptr, screen_gamma, 0.45455);
}
/* Dither RGB files down to 8 bit palette or reduce palettes
* to the number of colors available on your screen.
*/
if (color_type & PNG_COLOR_MASK_COLOR)
{
int num_palette;
png_colorp palette;
/* This reduces the image to the application supplied palette */
if (/* we have our own palette */)
{
/* An array of colors to which the image should be dithered */
png_color std_color_cube[MAX_SCREEN_COLORS];
png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS,
MAX_SCREEN_COLORS, png_uint_16p_NULL, 0);
}
/* This reduces the image to the palette supplied in the file */
else if (png_get_PLTE(png_ptr, info_ptr, &palette, &num_palette))
{
png_uint_16p histogram = NULL;
png_get_hIST(png_ptr, info_ptr, &histogram);
png_set_dither(png_ptr, palette, num_palette,
max_screen_colors, histogram, 0);
}
}
/* invert monochrome files to have 0 as white and 1 as black */
png_set_invert_mono(png_ptr);
/* If you want to shift the pixel values from the range [0,255] or
* [0,65535] to the original [0,7] or [0,31], or whatever range the
* colors were originally in:
*/
if (png_get_valid(png_ptr, info_ptr, PNG_INFO_sBIT))
{
png_color_8p sig_bit;
png_get_sBIT(png_ptr, info_ptr, &sig_bit);
png_set_shift(png_ptr, sig_bit);
}
/* flip the RGB pixels to BGR (or RGBA to BGRA) */
if (color_type & PNG_COLOR_MASK_COLOR)
png_set_bgr(png_ptr);
/* swap the RGBA or GA data to ARGB or AG (or BGRA to ABGR) */
png_set_swap_alpha(png_ptr);
/* swap bytes of 16 bit files to least significant byte first */
png_set_swap(png_ptr);
/* Add filler (or alpha) byte (before/after each RGB triplet) */
png_set_filler(png_ptr, 0xff, PNG_FILLER_AFTER);
/* Turn on interlace handling. REQUIRED if you are not using
* png_read_image(). To see how to handle interlacing passes,
* see the png_read_row() method below:
*/
number_passes = png_set_interlace_handling(png_ptr);
/* Optional call to gamma correct and add the background to the palette
* and update info structure. REQUIRED if you are expecting libpng to
* update the palette for you (ie you selected such a transform above).
*/
png_read_update_info(png_ptr, info_ptr);
/* Allocate the memory to hold the image using the fields of info_ptr. */
/* The easiest way to read the image: */
png_bytep row_pointers[height];
for (row = 0; row < height; row++)
{
row_pointers[row] = png_malloc(png_ptr, png_get_rowbytes(png_ptr,
info_ptr));
}
/* Now it's time to read the image. One of these methods is REQUIRED */
#ifdef entire /* Read the entire image in one go */
png_read_image(png_ptr, row_pointers);
#else no_entire /* Read the image one or more scanlines at a time */
/* The other way to read images - deal with interlacing: */
for (pass = 0; pass < number_passes; pass++)
{
#ifdef single /* Read the image a single row at a time */
for (y = 0; y < height; y++)
{
png_read_rows(png_ptr, &
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