1 uniform sampler2D brush_texture;

     2 uniform vec2 inv_brush_texture_size;

     3 uniform vec3 inv_matrix_m0;

     4 uniform vec3 inv_matrix_m1;

     5 uniform vec3 inv_matrix_m2;

     6 

     7 vec4 brush()

     8 {

     9     mat3 mat;

    10 

    11     mat[0] = inv_matrix_m0;

    12     mat[1] = inv_matrix_m1;

    13     mat[2] = inv_matrix_m2;

    14 

    15     vec3 hcoords = mat * vec3(gl_FragCoord.xy, 1);

    16     vec2 coords = hcoords.xy / hcoords.z;

    17 

    18     coords *= inv_brush_texture_size;

    19 

    20     float alpha = 1.0 - texture2D(brush_texture, coords).r;

    21 

    22     return gl_Color * alpha;

    23 }