I'm trying to write (using libpng) an 16-bit grayscale image where each point color equals to sum of its coordinates. The following code should produce a 16-bit PNG, but instead produces 8-bit like this. Why?
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <png.h>
void save_png(FILE* fp, long int size)
{
png_structp png_ptr = NULL;
png_infop info_ptr = NULL;
size_t x, y;
png_bytepp row_pointers;
png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (png_ptr == NULL) {
return ;
}
info_ptr = png_create_info_struct(png_ptr);
if (info_ptr == NULL) {
png_destroy_write_struct(&png_ptr, NULL);
return ;
}
if (setjmp(png_jmpbuf(png_ptr))) {
png_destroy_write_struct(&png_ptr, &info_ptr);
return ;
}
png_set_IHDR(png_ptr, info_ptr,
size, size, // width and height
16, // bit depth
PNG_COLOR_TYPE_GRAY, // color type
PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
/* Initialize rows of PNG. */
row_pointers = (png_bytepp)png_malloc(png_ptr,
size*png_sizeof(png_bytep));
for (int i=0; i<size; i++)
row_pointers[i]=NULL;
for (int i=0; i<size; i++)
row_pointers[i]=png_malloc(png_ptr, size*2);
//set row data
for (y = 0; y < size; ++y) {
png_bytep row = row_pointers[y];
for (x = 0; x < size; ++x) {
short color = x+y;
*row++ = (png_byte)(color & 0xFF);
*row++ = (png_byte)(color >> 8);
}
}
/* 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);
//png_write_image(png_ptr, row_pointers);
/* Cleanup. */
for (y = 0; y < size; y++) {
png_free(png_ptr, row_pointers[y]);
}
png_free(png_ptr, row_pointers);
png_destroy_write_struct(&png_ptr, &info_ptr);
}
int main()
{
FILE* f;
if((f=fopen("test.png", "wb"))!=NULL)
{
save_png(f, 257);
fclose(f);
}
return 0;
}
The linked-to image shows as being "16-bit" in Windows 7's "Properties". I guess you're just seeing various applications falling back to converting down to 8-bit for display, which (I guess) is pretty expected since most display devices don't support 16 bits.