shaderOutline.gml

//
// FRAGMENT: Sprite outline shader
//
varying vec2 v_vTexcoord;
varying vec4 v_vColour;
uniform float outlineH;
uniform float outlineW;
uniform vec4 outlineColor; 

void main()
{
  vec2 offsetx;
  offsetx.x = outlineW;
  vec2 offsety;
  offsety.y = outlineH;
  
  float alpha = texture2D( gm_BaseTexture, v_vTexcoord ).a;
  vec4 newColor = texture2D( gm_BaseTexture, v_vTexcoord );
  
  if ( alpha < 1.0 ) { newColor = outlineColor; } 

  alpha = max(alpha, texture2D( gm_BaseTexture, v_vTexcoord + offsetx).a);
  alpha = max(alpha, texture2D( gm_BaseTexture, v_vTexcoord - offsetx).a);
  alpha = max(alpha, texture2D( gm_BaseTexture, v_vTexcoord + offsety).a);
  alpha = max(alpha, texture2D( gm_BaseTexture, v_vTexcoord - offsety).a);
    
  
  gl_FragColor = newColor;
  gl_FragColor.a = alpha;
}


// USAGE: in the draw event
shader_set(shaderOutline)

var outlineColor = shader_get_uniform(shaderOutline,"outlineColor")
shader_set_uniform_f(outlineColor, 0.2,0.2,0.2,0 )


var outlineW = shader_get_uniform(shaderOutline,"outlineW")
shader_set_uniform_f(outlineW, 1/sprite_width)

var outlineH = shader_get_uniform(shaderOutline,"outlineH")
shader_set_uniform_f(outlineH, 1/sprite_height)

draw_self()
shader_reset()