Why is a fully transparent pixel still rendered?
- by Mr Bell
I am trying to make a pixel shader that achieves an effect similar to this video http://www.youtube.com/watch?v=f1uZvurrhig&feature=related
My basic idea is render the scene to a temp render target then
Render the previously rendered image with a slight fade on to another temp render target
Draw the current scene on top of that
Draw the results on to a render target that persists between draws
Draw the results on to the screen
But I am having problems with the fading portion. If I have my pixel shader return a color with its A component set to 0, shouldn't that basically amount to drawing nothing? (Assuming that sprite batch blend mode is set to AlphaBlend)
To test this I have my pixel shader return a transparent red color. Instead of nothing being drawn, it draws a partially transparent red box.
I hope that my question makes sense, but if it doesnt please ask me to clarify
Here is the drawing code
public override void Draw(GameTime gameTime)
{
GraphicsDevice.SamplerStates[1] = SamplerState.PointWrap;
drawImageOnClearedRenderTarget(presentationTarget, tempRenderTarget, fadeEffect);
drawImageOnRenderTarget(sceneRenderTarget, tempRenderTarget);
drawImageOnClearedRenderTarget(tempRenderTarget, presentationTarget);
GraphicsDevice.SetRenderTarget(null);
drawImage(backgroundTexture);
drawImage(presentationTarget);
base.Draw(gameTime);
}
private void drawImage(Texture2D image, Effect effect = null)
{
spriteBatch.Begin(0, BlendState.AlphaBlend, SamplerState.PointWrap, null, null, effect);
spriteBatch.Draw(image, new Rectangle(0, 0, width, height), Color.White);
spriteBatch.End();
}
private void drawImageOnRenderTarget(Texture2D image, RenderTarget2D target, Effect effect = null)
{
GraphicsDevice.SetRenderTarget(target);
drawImage(image, effect);
}
private void drawImageOnClearedRenderTarget(Texture2D image, RenderTarget2D target, Effect effect = null)
{
GraphicsDevice.SetRenderTarget(target);
GraphicsDevice.Clear(Color.Transparent);
drawImage(image, effect);
}
Here is the fade pixel shader
sampler TextureSampler : register(s0);
float4 PixelShaderFunction(float2 texCoord : TEXCOORD0) : COLOR0
{
float4 c = 0;
c = tex2D(TextureSampler, texCoord);
//c.a = clamp(c.a - 0.05, 0, 1);
c.r = 1;
c.g = 0;
c.b = 0;
c.a = 0;
return c;
}
technique Fade
{
pass Pass1
{
PixelShader = compile ps_2_0 PixelShaderFunction();
}
}
