I've searched around for two days now on the internet but cannot find a solution. I've also read up on Pixel Shaders on MSDN with no luck.
I'm trying to apply a simple pixel shader to a sprite batch for alpha masking. It worked fine when I used two separate textures; one for the sprite and a matching one for the shader.
Problem is I don't want to do this. I'd like to use my sprite sheet and source rectangle and use a separate mask from a sprite sheet of masks.
However it doesn't work. The sprite draw perfectly but the mask does not and appears to be using a sprite at a different locating in the mask sheet. It has something to do with getting the coordinates of the mask.
My idea was if the sprite to draw was at 200*200 in the sheet, subtract the correct tex coordinates from it to get the pixel, then add this to the mask.
pos = texCoord - sprite start = offset + mask start = location of mask from current pixel
EG; pos = 200*200 - 200*200 = 0 + maskPos = 440*330 pos = 201*201 - 200*200 = 1 + maskPos = 441*330
Pixel shader:
uniform extern texture ScreenTexture;
sampler screen = sampler_state
{
Texture = <ScreenTexture>;
};
uniform extern texture MaskTexture;
sampler mask = sampler_state
{
Texture = <MaskTexture>;
};
float2 startpos;
float2 maskpos;
float4 PixelShaderFunction(float2 inCoord: TEXCOORD0) : COLOR
{
// Get the matching pixel offset in mask.
float2 pos = inCoord - startpos;
pos = pos + maskpos;
float4 color = tex2D(screen, inCoord);
float4 color2 = tex2D(mask, pos);
if (color2.a > 0) color.rgba = 0;
return color;
}
technique
{
pass P0
{
PixelShader = compile ps_2_0 PixelShaderFunction();
}
}
And here is the draw call:
mask.Parameters("MaskTexture").SetValue(maskSheet) ' Sprite sheet with mask.
mask.Parameters("startpos").SetValue(New Vector2(200, 200))
mask.Parameters("maskpos").SetValue(New Vector2(440, 330)) ' Postion of mask in sheet.
Dim source As New Rectangle(200, 300, 22, 22)
spriteBatch.Draw(sprites, Vector2.Zero, source, Color.White, 0.0F, New Vector2(0, 0), 1.0F, SpriteEffects.None, 1.0F)
