I would like to do a simple thing : render a 2d texture to a 2d render target with alpha chanel blending.
So i create my render targer :
renderTarget = new RenderTarget2D(m_graphicsDevice, 200, 200);
then I create my texture :
texture = new Texture2D(_device, 1, 1, false, SurfaceFormat.Color);
Color clr = Color.Red;
// set half transparency on my texture
clr.A = 128;
Color[] bitmap = new Color[1] {clr};
texture.SetData(bitmap);
then I clear my rendertarget white, and draw my texture twice in the render target, with a good scale, and with a multiplicative operation on color and alpha chanel.
graphicsDevice.SetRenderTarget(renderTarget);
graphicsDevice.Clear(Color.White);
BlendState myState = new BlendState();
// multiplicative blending on color
myState.ColorSourceBlend = Blend.Zero;
myState.ColorDestinationBlend = Blend.SourceColor;
// multiplicative blending on alpha
myState.AlphaSourceBlend = Blend.Zero;
myState.AlphaDestinationBlend = Blend.SourceAlpha;
spriteBatch.Begin(SpriteSortMode.Immediate, myState);
// draw my texture twice, with an overlaping part
spriteBatch.Draw(texture, new Vector2(0, 0), null, Color.White, 0, Vector2.Zero, 100, SpriteEffects.None, 0);
spriteBatch.Draw(texture, new Vector2(50, 50), null, Color.White, 0, Vector2.Zero, 100, SpriteEffects.None, 0);
spriteBatch.End();
graphicsDevice.SetRenderTarget(null);
and I draw this render target on screen, with a green background, and a nonPremultiplied renderstate so that alpha value in the texture is applied as transparency.
graphicsDevice.Clear(Color.Green);
spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.NonPremultiplied);
spriteBatch.Draw(renderTarget, Vector2.zero, Color.White);
spriteBatch.End();
the result is as expected :
in render target, for pixels belonging to one texture only :
- red chanel = texture red chanel (1) * render target red chanel (1) = 1
- green and blue chanel = texture chanel (0) * render target chanel = 0
- alpha chanel = texture alpha (0.5) * render target alpha (1) = 0.5
- and this texture drawn above the green background gives a brown color thanks to alpha transparency set to 0.5 (1/2 red + 1/2 green)
in render target, for overlaping pixels (so above computation applied twice)
- red chanel = texture red chanel (1) * texture red chanel (1) * render target red chanel (1) = 1
- green and blue chanel = texture chanel (0) * texture chanel (0) * render target chanel = 0
- alpha chanel = texture alpha (0.5) * texture alpha (0.5) * render target alpha (1) = 0.25
- and this texture drawn above the green background gives a nearly green color thanks to alpha value 0.25 (1/4 red + 3/4 green)
now if I change the renderstate to just let the alpha chanel of the texture be applied, and not letting multiplication on colors :
BlendState myState = new BlendState();
// texture color is ignored, dest color remain unchanged
myState.ColorSourceBlend = Blend.Zero;
myState.ColorDestinationBlend = Blend.One;
// alpha chanel are still multiplied
myState.AlphaSourceBlend = Blend.Zero;
myState.AlphaDestinationBlend = Blend.SourceAlpha;
what I should get is :
in render target, for pixels belonging to one texture only :
- red chanel = 0 * texture red chanel (1) + 1 * render target red chanel (1) = 1
- green and blue chanel = 0 * texture chanel (0) + 1 * render target chanel (1) = 1
- alpha chanel = texture alpha (0.5) * render target alpha (1) = 0.5
- and this texture drawn above the green background should give light green color (1/2 white + 1/2 green)
in render target, for overlaping pixels (so above computation applied twice)
- red chanel = 1
- green and blue chanel = 1
- alpha chanel = texture alpha (0.5) * texture alpha (0.5) * render target alpha (1) = 0.25
- and this texture drawn above the green background should give a more intense green (0.25 * white + 0.75 * green)
and what I get is a plain white texture...
why does the alpha blending computation in my render target stops working when I don't blend colors ? My goal is to only have computation on alpha chanel, without getting the texture color. How can I do that ?
thanks
