use x/yres_virtual

[ghost]

`
png_init_io(png_ptr, pngfp);

png_set_IHDR(
    png_ptr,
    info_ptr,
    vinfo.xres_virtual,
    vinfo.yres_virtual,
    8,
    PNG_COLOR_TYPE_RGB,
    PNG_INTERLACE_NONE,
    PNG_COMPRESSION_TYPE_BASE,
    PNG_FILTER_TYPE_BASE);

png_write_info(png_ptr, info_ptr);

png_bytep png_buffer = malloc(vinfo.xres_virtual * 3 * sizeof(png_byte));

if (png_buffer == NULL)
{
    fprintf(stderr, "%s: Unable to allocate buffer\n", program);
    exit(EXIT_FAILURE);
}

//--------------------------------------------------------------------

int r_mask = (1 << vinfo.red.length) - 1;
int g_mask = (1 << vinfo.green.length) - 1;
int b_mask = (1 << vinfo.blue.length) - 1;

int bytes_per_pixel = vinfo.bits_per_pixel / 8;

int y = 0;

for (y = 0; y < vinfo.yres_virtual; y++)
{
    int x;

    for (x = 0; x < vinfo.xres_virtual; x++)
    {
        int pb_offset = 3 * x;

`

[ghost]

Your code only grabs the content of the smallest display which may be the internal graphics, not the bigger screen you use as the main screen. Use _virtual to fix this.