A way of reading a bitmap as a 2D array of colors in Game Maker Studio. Works with Game Maker 8.1.

read_bmp_ds_grid.gml

///read_bmp(filename)
// Reads 24 or 32 bit bitmap files
//returns a ds_grid of colors encoded as rgb hex
var handle;
handle = file_bin_open(argument0, 0);
var offset, width, height, bits, output;
offset = 29;
width = 0;
height = 0;
bits = 0;

var i;
for (i = 0; i < offset; i += 1) {
  if (i == 10)
    offset = file_bin_read_byte(handle);
  else
  if (i == 11)
    offset += file_bin_read_byte(handle) << 8;
  else
  if (i == 12)
    offset += file_bin_read_byte(handle) << 16;
  else
  if (i == 13)
    offset += file_bin_read_byte(handle) << 24;
  else
  if (i == 18)
    width = file_bin_read_byte(handle);
  else
  if (i == 19)
    width += file_bin_read_byte(handle) << 8;
  else
  if (i == 20)
    width += file_bin_read_byte(handle) << 16;
  else
  if (i == 21)
    width += file_bin_read_byte(handle) << 24;
  else
  if (i == 22)
    height = file_bin_read_byte(handle);
  else
  if (i == 19)
    height += file_bin_read_byte(handle) << 8;
  else
  if (i == 20)
    height += file_bin_read_byte(handle) << 16;
  else
  if (i == 21)
    height += file_bin_read_byte(handle) << 24;
  else
  if (i == 28)
    bits = file_bin_read_byte(handle);
  else
    file_bin_read_byte(handle); // Skip
}

var output;
output = ds_grid_create(width, height);

var j;
for (j = 0; j < height; j += 1) {
  for (i = 0; i < width; i += 1) {

    var r, g, b, value;
    r = file_bin_read_byte(handle);
    g = file_bin_read_byte(handle) << 8;
    b = file_bin_read_byte(handle) << 16;
    if (bits == 32)
      file_bin_read_byte(handle);

    value = r + g + b;

    ds_grid_set(output, i, (height-1)-j, value);
  }
  if (bits == 24) {
    // Padding
    for (h = 0; h < (width * 3) mod 4; h += 1) {
      file_bin_read_byte(handle);
    }
  }
}

file_bin_close(handle);

return output;

Hello,
Was reading through this and noticed there was some duplicate if statements in there (19, 20, 21). Not that this will functionally change anything but I made a more condensed version of that section if you'd like to use it. Anyways, thank you for sharing this, it was super helpful!

for (i = 0; i < offset; i += 1) {
switch (i) {
case 10: offset = file_bin_read_byte(handle); break;
case 11: offset += file_bin_read_byte(handle) << 8; break;
case 12: offset += file_bin_read_byte(handle) << 16; break;
case 13: offset += file_bin_read_byte(handle) << 24; break;
case 18: width = file_bin_read_byte(handle); break;
case 19: width += file_bin_read_byte(handle) << 8; break;
case 20: width += file_bin_read_byte(handle) << 16; break;
case 21: width += file_bin_read_byte(handle) << 24; break;
case 22: height = file_bin_read_byte(handle); break;
case 28: bits = file_bin_read_byte(handle); break;
default: file_bin_read_byte(handle); break; // Skip
}
}

(not sure why it's not adding in the spaces/tabs :/)