//
// Outline shader
//
varying vec2 v_vTexcoord;
varying vec4 v_vColour;
uniform float pixelW;
uniform float pixelH;
uniform float opacity;
void main()
{
  // Compute the textel offsets
  vec2 offsetX;
  offsetX.x = pixelW;
  vec2 offsetY;
  offsetY.y = pixelH;

  float originAlpha = sign(texture2D(gm_BaseTexture, v_vTexcoord).a);
  float alpha = originAlpha;
  // Combine the alpha from all surrounding textels.
  alpha += ceil(texture2D(gm_BaseTexture, v_vTexcoord + offsetX).a);
  alpha += ceil(texture2D(gm_BaseTexture, v_vTexcoord - offsetX).a);
  alpha += ceil(texture2D(gm_BaseTexture, v_vTexcoord + offsetY).a);
  alpha += ceil(texture2D(gm_BaseTexture, v_vTexcoord - offsetY).a);

  // Only blend with the image_blend factor if the original alpha was 0.
  // That means the image_blend parameter is the outline color.
  gl_FragColor = (v_vColour * (1.0 - originAlpha)) +
                 texture2D(gm_BaseTexture, v_vTexcoord);

  // Use the computed alpha
  gl_FragColor.a = alpha;
}