用libpng编写1位深度的灰度PNG会导致图像不正确

问题描述

因此，我尝试使用libpng从unsigned char数组中写入png，这些数组的位深度为1。这意味着对于所有位，只有黑色和白色（灰度级）图片。我已经成功地针对8位深度灰度png而不是1位深度成功做到了这一点。这是我的代码：

extern int write_png_bwsq(const char* filename,int dimen,const unsigned char *buffer,char* title)
{
        
    int yrow;
        int dim_bytes;
    int code = 1;
    FILE *fp = NULL;
    png_structp png_ptr = NULL;
    png_infop info_ptr = NULL;
    png_bytep row = NULL;
        dim_bytes = (dimen * dimen) / 8;

    
    // Open file for writing (binary mode)
    fp = fopen(filename,"wb");
    if (fp == NULL) {
        fprintf(stderr,"PNG ERROR: Could not open file %s for writing\n",filename);
        code = 0;
        goto finalise;
    }

    // Initialize write structure
    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,NULL,NULL);
    if (png_ptr == NULL) {
        fprintf(stderr,"PNG ERROR: Could not allocate PNG write struct\n");
        code = 0;
        goto finalise;
    }

    // Initialize info structure
    info_ptr = png_create_info_struct(png_ptr);
    if (info_ptr == NULL) {
        fprintf(stderr,"PNG ERROR: Could not allocate PNG info struct\n");
        code = 0;
        goto finalise;
    }

    // Setup Exception handling
    if (setjmp(png_jmpbuf(png_ptr))) {
        fprintf(stderr,"PNG Error: Creating the PNG output Failed.\n");
        code = 0;
        goto finalise;
    }

    png_init_io(png_ptr,fp);

    png_set_IHDR(png_ptr,info_ptr,dimen,1,PNG_COLOR_TYPE_GRAY,PNG_INTERLACE_NONE,PNG_COMPRESSION_TYPE_BASE,PNG_FILTER_TYPE_BASE);

    // Sets the title
    if (title != NULL) {
        png_text title_text;
        title_text.compression = PNG_TEXT_COMPRESSION_NONE;
        title_text.key = "Title";
        title_text.text = title;
        png_set_text(png_ptr,&title_text,1);
    }

    png_write_info(png_ptr,info_ptr);

    row = (png_bytep) buffer;

    // Write image data
    for (yrow=0 ; yrow<dim_bytes ; yrow++) {
        png_write_row(png_ptr,row);
        ++row;
    }

    // End write operation
    png_write_end(png_ptr,NULL);

    finalise:
    if (fp != NULL) fclose(fp);
    if (info_ptr != NULL) png_free_data(png_ptr,PNG_FREE_ALL,-1);
    if (png_ptr != NULL) png_destroy_write_struct(&png_ptr,(png_infopp)NULL);

    return code;
}

然后，我有一个准备文件的单独文件

#include "write_pngs.h"
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

#if CHAR_BIT != 8
#  error "CHAR_BIT is not usable on this platform"
#endif

#define PIC_SIZE_DIM 1080
#define PIC_SIZE_BW ((PIC_SIZE_DIM * PIC_SIZE_DIM) / CHAR_BIT) 
static unsigned char PIC_BITS[PIC_SIZE_BW] = {0};

static void __write_bits_pic(void)
{
     size_t i = 1000;
     for(;i < 140000;++i) {
        PIC_BITS[i] = ((i + 76) >> 5) & 0xff;
     }
}


int main(void) {
   __write_bits_pic();
   printf("Writing picture of %d bytes and %d bits\n",PIC_SIZE_BW,PIC_SIZE_BW * CHAR_BIT);
   return !write_png_bwsq("bwpixs.png",PIC_SIZE_DIM,PIC_BITS,"bwpixs");
}

这会导致图像不正确，不仅对于png来说很大（对于1080 x 1080仅约为5mb），但是图像的右下角却从黑色更改了。

我在这里做错了什么？ libpng是否需要采取任何特殊步骤来写我不做的灰度级位深度仅为1的png？

解决方法

您多次致电png_write_row()。您的

for (yrow=0 ; yrow<dim_bytes ; yrow++) {
    png_write_row(png_ptr,row);
    ++row;
}

应该是这样的：

for (yrow=0 ; yrow<dim_bytes ; yrow += dimen / 8) {
    png_write_row(png_ptr,row);
    row += dimen / 8;
}

以便写入1080像素（135字节）的行。您正在为每个字节调用png_write_row()，就好像图像是八像素宽。不是。