如何使用 libpng 将 PNG 编码为缓冲区？
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【中文标题】如何使用 libpng 将 PNG 编码为缓冲区？【英文标题】：How to encode PNG to buffer using libpng? 【发布时间】：2010-12-21 18:20:18 【问题描述】：我目前正在使用以下内容将 PNG 写入文件：
#include <png.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
/* Pixels in this bitmap structure are stored as BGR. */
typedef struct _RGBPixel
uint8_t blue;
uint8_t green;
uint8_t red;
RGBPixel;
/* Structure for containing decompressed bitmaps. */
typedef struct _RGBBitmap
RGBPixel *pixels;
size_t width;
size_t height;
size_t bytewidth;
uint8_t bytes_per_pixel;
RGBBitmap;
/* Returns pixel of bitmap at given point. */
#define RGBPixelAtPoint(image, x, y) \
*(((image)->pixels) + (((image)->bytewidth * (y)) \
+ ((x) * (image)->bytes_per_pixel)))
/* Attempts to save PNG to file; returns 0 on success, non-zero on error. */
int save_png_to_file(RGBBitmap *bitmap, const char *path)
FILE *fp = fopen(path, "wb");
png_structp png_ptr = NULL;
png_infop info_ptr = NULL;
size_t x, y;
png_uint_32 bytes_per_row;
png_byte **row_pointers = NULL;
if (fp == NULL) return -1;
/* Initialize the write struct. */
png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (png_ptr == NULL)
fclose(fp);
return -1;
/* Initialize the info struct. */
info_ptr = png_create_info_struct(png_ptr);
if (info_ptr == NULL)
png_destroy_write_struct(&png_ptr, NULL);
fclose(fp);
return -1;
/* Set up error handling. */
if (setjmp(png_jmpbuf(png_ptr)))
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
return -1;
/* Set image attributes. */
png_set_IHDR(png_ptr,
info_ptr,
bitmap->width,
bitmap->height,
8,
PNG_COLOR_TYPE_RGB,
PNG_INTERLACE_NONE,
PNG_COMPRESSION_TYPE_DEFAULT,
PNG_FILTER_TYPE_DEFAULT);
/* Initialize rows of PNG. */
bytes_per_row = bitmap->width * bitmap->bytes_per_pixel;
row_pointers = png_malloc(png_ptr, bitmap->height * sizeof(png_byte *));
for (y = 0; y < bitmap->height; ++y)
uint8_t *row = png_malloc(png_ptr, sizeof(uint8_t) * bitmap->bytes_per_pixel);
row_pointers[y] = (png_byte *)row;
for (x = 0; x < bitmap->width; ++x)
RGBPixel color = RGBPixelAtPoint(bitmap, x, y);
*row++ = color.red;
*row++ = color.green;
*row++ = color.blue;
/* Actually write the image data. */
png_init_io(png_ptr, fp);
png_set_rows(png_ptr, info_ptr, row_pointers);
png_write_png(png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);
/* Cleanup. */
for (y = 0; y < bitmap->height; y++)
png_free(png_ptr, row_pointers[y]);
png_free(png_ptr, row_pointers);
/* Finish writing. */
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
return 0;
如何编写类似的函数（用 C 语言）将 PNG 编码到内存缓冲区？
原型看起来像这样：
uint8_t *encode_png_to_buffer(RGBBitmap *source);
看来我可能需要使用png_set_write_fn()。
但除此之外，我不确定如何处理这个问题。有没有这样做的例子？当然我不是第一个需要这个功能的人。
【问题讨论】：
首先，尝试编译它。检查save_png_to_file 的定义是否缺少* 即RGBBitmap * bitmap（因为您稍后将位图视为指针。bytes_per_pixel 定义在哪里？（无处！）
“将 PNG 编码到内存缓冲区”到底是什么意思？您的意思是将图像编码为PNG到缓冲区吗？您的意思是将 PNG 解码为缓冲区吗？
啊，错别字抱歉，我已经改正了。对不起，如果我不清楚；我的意思是：我在“RGBBitmap”结构中有一个解压缩的位图，我想将其转换为 PNG。不过，我不想将此 PNG 写入文件，我只想要将要写入的数据的原始缓冲区。
我认为 row_pointers = png_malloc(png_ptr, bitmap->height * sizeof(png_byte ));应该是 png_byte *row = (png_byte)png_malloc(png_ptr, sizeof(uint8_t) * bitmap->bytes_per_pixel * bitmap->width);
请问您是如何安装 libpng 的，因为我无法在 Windows 上使用 #include <png.h>。
【参考方案1】：
根据我的口味，其他答案似乎并不完整。因此，我使用这些答案和其他研究将黑色背景写入缓冲区。然后检查，我将缓冲区写入文件。这是用 gcc 编译的。添加了库标志 -lpng。
#define PNG_SETJMP_NOT_SUPPORTED
#include <stdio.h>
#include <stdlib.h>
#include <png.h>
struct libpng_inmem_write_struct /* This is from png.c */
unsigned char * pngBfr; /* destination memory */
unsigned long pngSiz; /* destination memory size (bytes) */
;
void freeExit_w_msg(char * msg);
void wrtBgPng(png_structp pngWrtPtr, png_bytep data, png_size_t length);
png_structp pngWrtPtr; /* The pointer that points the PNG write structure */
png_infop pngWrtInfoPtr; /* The pointer that points the PNG write information */
struct libpng_inmem_write_struct p_io; /* Holds the encoded PNG data */
FILE * fw; /* The file pointer of the test file that will be wrote. */
void freeExit_w_msg(char * msg)
if (pngWrtPtr) png_destroy_write_struct(&pngWrtPtr, &pngWrtInfoPtr);
if (p_io.pngBfr) free(p_io.pngBfr);
fclose(fw);
printf("%s\n", msg);
exit(0);
int main(int argc, char *argv[])
pngWrtInfoPtr = NULL; /* write_info_ptr */
p_io.pngBfr = NULL;
p_io.pngSiz = 0;
int imgWdth = 2558;
int imgHght = 1438;
fw = fopen (argv[1], "wb"); /* argv[1] is the name of the test file */
if (!fw)
char msg[300];
sprintf(msg, "The file, %s, did not correctly open.\n", argv[1]);
freeExit_w_msg(msg);
pngWrtPtr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL); /* write_ptr */
if (!pngWrtPtr) freeExit_w_msg((char *) "The PNG write memory did not correctly allocate.");
pngWrtInfoPtr = png_create_info_struct(pngWrtPtr);
if (!pngWrtInfoPtr) freeExit_w_msg((char *) "The PNG write information memory did not correctly allocate.");
png_set_IHDR(pngWrtPtr, pngWrtInfoPtr, imgWdth, imgHght, 8, PNG_COLOR_TYPE_RGBA, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
png_byte ** row_pointers = (png_byte **) png_malloc(pngWrtPtr, imgHght * sizeof(png_byte *));
size_t bytesPerRow = imgWdth << 2; /* 4 Bytes per pixel */
unsigned char * imgBfr = (unsigned char *) calloc(1, imgHght * bytesPerRow * sizeof(unsigned char));
for (int rw = 0; rw < imgHght; rw++)
png_byte * rwPtr = row_pointers[rw] = (png_byte *) (imgBfr + rw * bytesPerRow);
for (int pxl = 0, byt = 0; pxl < imgWdth; pxl++) /* Write a black background */
for (int clr = 0; clr < 3; clr++) rwPtr[byt++] = 0;
rwPtr[byt++] = 0xff;
p_io.pngBfr = (unsigned char *) malloc(4); /* Defines final PNG data location */
p_io.pngSiz = 4;
png_init_io(pngWrtPtr, (png_FILE_p) &p_io);
png_set_rows(pngWrtPtr, pngWrtInfoPtr, &row_pointers[0]);
png_set_write_fn(pngWrtPtr, &p_io, wrtBgPng, NULL);
png_write_png(pngWrtPtr, pngWrtInfoPtr, PNG_TRANSFORM_IDENTITY, NULL);
fwrite(p_io.pngBfr + 4, 1, p_io.pngSiz, fw); /* Test file */
freeExit_w_msg((char *) "The exit was normal.");
void wrtBgPng(png_structp pngWrtPtr, png_bytep data, png_size_t length)
struct libpng_inmem_write_struct * p = (struct libpng_inmem_write_struct *) png_get_io_ptr(pngWrtPtr);
p->pngBfr = (unsigned char *) realloc(p->pngBfr, p->pngSiz + length); /* From png.c */
if (!p->pngBfr) freeExit_w_msg((char *) "The PNG write memory did not correctly allocate.");
memmove(p->pngBfr + p->pngSiz, data, length);
p->pngSiz += length;
【讨论】：
【参考方案2】：我找到了这段代码的早期版本，并破解了它来生成并保存一个 16 位灰度 PNG 文件。它运行并生成这个 PNG：
这里是完整的源代码和构建可执行文件的 GCC 命令。它在我的系统上运行，OpenSuse 42/64 和 GCC -> gcc 版本 4.8.5 (SUSE Linux) 和 libpng 1.6.23，但如果有人如此轻率或无耻地实际尝试运行它，它可能会熔化你的通量电容器。 ;)
#include <png.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include "string.h"
/* 8 QBit RGB/24 to 16 QBit Grayscale hack
* based on code found at
* http://www.lemoda.net/c/write-png/ and png.h libpng version 1.6.23
*/
/*
gcc -L/usr/local/static -I/usr/local/static/include -lpng16 /home/photog/bin/png.test.gray16.c -lm -o /home/photog/bin/png.tg16
*/
// =============================================================================
typedef struct
uint8_t red; uint8_t green; uint8_t blue; // A colored pixel
pixel_t;
typedef struct
uint16_t gray; // A GRAY pixel
pixel_gray_16_t;
typedef struct // A picture
pixel_gray_16_t *pixels;
size_t width;
size_t height;
bitmap_t;
// =============================================================================
// Write "bitmap" to a PNG file specified by "path"; returns 0 on
// success, non-zero on error
static int save_png_to_file (bitmap_t *bitmap, const char *path)
FILE * fp;
png_structp png_ptr = NULL;
png_infop info_ptr = NULL;
size_t x, y;
int pidx=0; // Pixel_Index
png_byte **row_pointers = NULL; // KLUDGE!!
/* "status" contains the return value of this function. At first
it is set to a value which means 'failure'. When the routine
has finished its work, it is set to a value which means
'success'. */
int status = -1;
/* The following number is set by trial and error only. I cannot
see where it it is documented in the libpng manual */
int pixel_size = 2; // 3 for RGB/24;
int depth = 16; // 8 for RGB/24;
fp = fopen (path, "wb"); if (! fp) goto fopen_failed;
png_ptr = png_create_write_struct (PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (png_ptr == NULL) goto png_create_write_struct_failed;
info_ptr = png_create_info_struct (png_ptr);
if (info_ptr == NULL) goto png_create_info_struct_failed;
/* Set up error handling. */
if (setjmp (png_jmpbuf (png_ptr))) goto png_failure;
// Set image attributes; # de fine PNG_COLOR_TYPE_GRAY 0
png_set_IHDR (png_ptr, info_ptr, bitmap->width, bitmap->height, depth,
PNG_COLOR_TYPE_GRAY, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT,
PNG_FILTER_TYPE_DEFAULT);
/* Initialize rows of PNG. */
row_pointers=png_malloc(png_ptr, bitmap->height * sizeof(png_uint_16 *));
// Copy system Callocated user data to PNG owned space
for (y=0, pidx=0; y < bitmap->height; ++y)
png_byte *row =
png_malloc(png_ptr, sizeof(uint8_t) * bitmap->width * pixel_size);
row_pointers[y] = row;
memcpy((void *)row, bitmap->pixels+pidx, bitmap->width * 2);
pidx += bitmap->width; // Move to next row
/* Write the image data to "fp". */
png_init_io (png_ptr, fp);
png_set_rows (png_ptr, info_ptr, row_pointers);
png_write_png(png_ptr,info_ptr, PNG_TRANSFORM_SWAP_ENDIAN,NULL);
//png_write_png (png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);
// The routine has successfully written the file, so we set "status" to a
// value which indicates success
status = 0;
for(y=0; y < bitmap->height; y++) png_free (png_ptr, row_pointers[y]);
png_free (png_ptr, row_pointers);
png_failure:
png_create_info_struct_failed:
png_destroy_write_struct (&png_ptr, &info_ptr);
png_create_write_struct_failed:
fclose (fp);
fopen_failed:
return status;
// =============================================================================
// =============================================================================
int main ()
bitmap_t fruit;
const char ofn[]= "fruit.g16.png" ; // Output FileName
int x, y, pidx=0;
uint16_t gray_u16;
float graysf; // Gray Scale factor. 0->0, last_pix -> QMax(16)
fruit.width = 400; // Size the image
fruit.height = 400;
graysf=(65535.0f/fruit.width)/fruit.height; // Last pix => 65535
fruit.pixels=calloc(sizeof(pixel_gray_16_t), fruit.width * fruit.height);
// Create linear black -> white gradient
for(y=0; y < fruit.height; y++)
for(x=0; x < fruit.width; x++)
gray_u16=(uint16_t)lrintf((y*fruit.width+x)*graysf);
fruit.pixels[pidx++].gray = gray_u16;
// Write the image to a file
save_png_to_file (&fruit, ofn);
printf("Wrote gray/16 PNG file %s\n", ofn);
return 0;
this line may not compile
// =============================================================================
groch，我发现上面的精简代码有助于使 16 QBit PNG 代码正常工作。添加此未经测试且具有潜在危险的黑客攻击仅供参考，并附有关于运行它的固有风险的免责声明。添加了编译器错误行，因此有人必须对其进行编辑才能编译，使其成为自己的定制版本。该帖子可能已经为已经很好的线程添加了一些有用的信息。纯粹的哲学思考会增加噪音但没有光...也许您可以在代码中添加一个功能？
注意：第一次运行在 Intel Skylake CPU 上得到了一个奇怪的结果：
你已经被字节序排列了！
不得不改变这一行：
png_write_png(png_ptr,info_ptr, PNG_TRANSFORM_SWAP_ENDIAN,NULL);
【讨论】：
我认为更重要的是思考代码与 OP 有何不同，而不是判断是否运行代码。 这个答案错过了问题的重点——将 png 写入磁盘文件很简单，但问题是如何将它写入 RAM 中的缓冲区，这从根本上改变了您使用 libPNG 的方式，例如，你不能使用 png_init_io，你的 write 回调函数在哪里？ png_set_write_fn() 在哪里？【参考方案3】：#include <png.h>
#include <vector>
#include <iostream>
#include <stdlib.h>
//encode and write PNG to memory (std::vector) with libpng on C++
typedef unsigned char ui8;
#define ASSERT_EX(cond, error_message) do if (!(cond)) std::cerr << error_message; exit(1); while(0)
static void PngWriteCallback(png_structp png_ptr, png_bytep data, png_size_t length)
std::vector<ui8> *p = (std::vector<ui8>*)png_get_io_ptr(png_ptr);
p->insert(p->end(), data, data + length);
struct TPngDestructor
png_struct *p;
TPngDestructor(png_struct *p) : p(p)
~TPngDestructor() if (p) png_destroy_write_struct(&p, NULL);
;
void WritePngToMemory(size_t w, size_t h, const ui8 *dataRGBA, std::vector<ui8> *out)
out->clear();
png_structp p = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
ASSERT_EX(p, "png_create_write_struct() failed");
TPngDestructor destroyPng(p);
png_infop info_ptr = png_create_info_struct(p);
ASSERT_EX(info_ptr, "png_create_info_struct() failed");
ASSERT_EX(0 == setjmp(png_jmpbuf(p)), "setjmp(png_jmpbuf(p) failed");
png_set_IHDR(p, info_ptr, w, h, 8,
PNG_COLOR_TYPE_RGBA,
PNG_INTERLACE_NONE,
PNG_COMPRESSION_TYPE_DEFAULT,
PNG_FILTER_TYPE_DEFAULT);
//png_set_compression_level(p, 1);
std::vector<ui8*> rows(h);
for (size_t y = 0; y < h; ++y)
rows[y] = (ui8*)dataRGBA + y * w * 4;
png_set_rows(p, info_ptr, &rows[0]);
png_set_write_fn(p, out, PngWriteCallback, NULL);
png_write_png(p, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);
【讨论】：
这似乎来自here。 对于 libpng，它是size_t，而不是 png_size_t。见png_rw_ptr的定义。【参考方案4】：
是的，使用 png_set_write_fn 这样的东西 - 未经测试：
更新评论编辑
/* structure to store PNG image bytes */
struct mem_encode
char *buffer;
size_t size;
void
my_png_write_data(png_structp png_ptr, png_bytep data, png_size_t length)
/* with libpng15 next line causes pointer deference error; use libpng12 */
struct mem_encode* p=(struct mem_encode*)png_get_io_ptr(png_ptr); /* was png_ptr->io_ptr */
size_t nsize = p->size + length;
/* allocate or grow buffer */
if(p->buffer)
p->buffer = realloc(p->buffer, nsize);
else
p->buffer = malloc(nsize);
if(!p->buffer)
png_error(png_ptr, "Write Error");
/* copy new bytes to end of buffer */
memcpy(p->buffer + p->size, data, length);
p->size += length;
/* This is optional but included to show how png_set_write_fn() is called */
void
my_png_flush(png_structp png_ptr)
int save_png_to_file(RGBBitmap *bitmap, const char *path)
...
/* static */
struct mem_encode state;
/* initialise - put this before png_write_png() call */
state.buffer = NULL;
state.size = 0;
/* if my_png_flush() is not needed, change the arg to NULL */
png_set_write_fn(png_ptr, &state, my_png_write_data, my_png_flush);
... call png_write_png() ...
/* now state.buffer contains the PNG image of size s.size bytes */
/* cleanup */
if(state.buffer)
free(state.buffer);
【讨论】：
谢谢，这正是我所需要的。但是有一个错字：&(p->buffer + p->size) 应该只是 p->buffer + p->size。另外，我认为不需要空的 my_png_flush() 函数；您可以将 NULL 传递给 png_set_write_fn() 的最后一个参数。 关于 io_ptr 取消引用不起作用的评论，较新版本的 libpng 的正确方法是调用 png_get_io_ptr(png_ptr) - 请参阅 refspecs.linuxbase.org/LSB_3.1.0/LSB-Desktop-generic/… 定义一个空的my_png_flush 函数不是可选的 - 如果你使用 NULL 代替，libpng 会尝试在你的状态结构上使用 flush()。
你能告诉我什么是 libpng16 中 png_get_io_ptr() 的正确替换吗？
这段代码可能存在内存泄漏，而且带有NULL 指针的realloc() 与malloc() 基本相同。所以if 毫无意义。以上是关于如何使用 libpng 将 PNG 编码为缓冲区？的主要内容，如果未能解决你的问题，请参考以下文章
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