Is there any way to tint a bitmap white?

Is there any way to tint a bitmap white?

AleaInfinitus

Member #18,873

December 2020

I know that you can darken a bitmap with al_draw_tinted_bitmap if you turn down all color values equally, but is there any way to do the opposite? Or is my only option to draw a slightly transparent white bitmap on top of the main one?

Edgar Reynaldo

Major Reynaldo

May 2007

You can do that, or draw your bitmap tinted translucent over a white rectangle.

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William Labbett

Member #4,486

March 2004

Presumably you decrease each r, g, b value by some fixed percentage (ratio) to
make an image darker. You could take

255 - r, 255 - g, and 255 - g and add a percentage of each of these to r, g, and b respectively to do the opposite.

dthompson

Member #5,749

April 2005

The most scalable way to do this is with a pixel shader. The setup is a bit long-winded though.

Here's an example with GLSL:

#SelectExpand
  1uniform sampler2D al_tex;
  2uniform bool al_use_tex;
  3uniform vec4 tint;
  4varying vec4 varying_color;
  5varying vec2 varying_texcoord;
  6
  7void main()
  8{
  9  vec4 t;
 10
 11  if(al_use_tex)
 12    t = texture2D(al_tex, varying_texcoord);
 13  else
 14    t = varying_color;
 15
 16  gl_FragColor = t + tint;
 17}

To use this with Allegro 5:

#SelectExpand
  1// firstly, before you create your display, you'll need to add this line:
  2al_set_new_display_flags(ALLEGRO_OPENGL | ALLEGRO_PROGRAMMABLE_PIPELINE);
  3
  4// you'll then need to load the above GLSL code somehow, maybe from a text file.
  5// here, I'm assuming you've loaded it into a string called 'source'.
  6
  7ALLEGRO_SHADER* shader;
  8shader = al_create_shader(ALLEGRO_SHADER_AUTO);
  9
 10al_attach_shader_source(shader, ALLEGRO_VERTEX_SHADER, al_get_default_shader_source(ALLEGRO_SHADER_AUTO, ALLEGRO_VERTEX_SHADER));
 11al_attach_shader_source(shader, ALLEGRO_PIXEL_SHADER, source);
 12al_build_shader(shader);
 13
 14al_use_shader(shader);
 15
 16// set the tint to { 1, 1, 1, 0 }.
 17// this will make every pixel of your bitmap white.
 18al_set_shader_float_vector("tint", 4, (float[]){ 1, 1, 1, 0 }, 1);
 19
 20// now draw your bitmap(s).

Ideally, you'll also want to check the return values of al_create_shader, al_attach_shader_source and al_build_shader (as they can fail).

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Edgar Reynaldo

Major Reynaldo

May 2007

Additive blending is NOT what you want here.

You can use the formula William gave to tint white, or any other color. Just add a percentage of the difference between colors to the original pixel.