Writing a 1 bit depth grayscale PNG with libpng results in incorrect image - compression

Question

So I am trying to use libpng to write png's from an array of unsigned char that are a bit depth of 1. Meaning for all bits, there is only black and white, as a gray scale image. I have managed to do this successfully for 8-bit depth gray scale png, but not 1 bit depth. Here is the code i have :
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, 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, 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, info_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, info_ptr, PNG_FREE_ALL, -1);
if (png_ptr != NULL) png_destroy_write_struct(&png_ptr, (png_infopp)NULL);
return code;
}
Then, i have a separate file where i prepare my image
#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");
}
This results in an incorrect image where its not only quite large for a png (about 5mb for only 1080 x 1080), but only the bottom right corner of the image is changed from black.
What am i doing wrong here ? Does libpng require any special steps for writing pngs that are only 1 in bit depth for gray scale i am not doing ?

Answer 1

You are calling png_write_row() way too many times. Your
for (yrow=0 ; yrow<dim_bytes ; yrow++) {
png_write_row(png_ptr, row);
++row;
}
should be something like:
for (yrow=0 ; yrow<dim_bytes ; yrow += dimen / 8) {
png_write_row(png_ptr, row);
row += dimen / 8;
}
in order to write a row of 1080 pixels (135 bytes). You were calling png_write_row() for every byte, as if the image is eight pixels wide. Which it isn't.