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Is there any way to tint a bitmap white?
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

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.

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:

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:

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).

Website. It was
This isn't a game!

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.

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