How do I properly achieve subtractive blending in C#, XNA?

Question

I'm working on some kind of mod for Terraria (written in C# and using XNA), in which I need to use some blend modes. I didn't have any troubles getting additive blending to work, but subtractive one causes me some problems.
I managed to display stuff with subtractive blending, but it doesn't really want to return to the standard mode. SpriteBatch.End and Begin doesn't help at all.

This is my custom BlendState:

public readonly static BlendState
    bsSubtract = new BlendState{
        ColorSourceBlend = Blend.SourceAlpha,
        ColorDestinationBlend = Blend.One,
        ColorBlendFunction = BlendFunction.ReverseSubtract,
        AlphaSourceBlend = Blend.SourceAlpha,
        AlphaDestinationBlend = Blend.One,
        AlphaBlendFunction = BlendFunction.ReverseSubtract
    },

Drawing code:

sb.End();
sb.Begin(SpriteSortMode.Immediate,bsSubtract);
(...drawing drawing blah...)
sb.End();
sb.Begin(SpriteSortMode.Immediate,BlendState.Additive);

The problem is, everything that is drawn after this code seems to still use some old options (half-transparent, bland). What am I doing wrong?

I even tried calling just sb.End() and sb.Begin() before setting the blend state back, or using another custom blend state which was a standard additive one, just with BlendFunctions set to Add, to no avail.

EDIT: Seems like setting ANY custom BlendState makes it do that...

EDIT2: Seems like the problem was me splitting the drawing to 3 separate places: one for item slots, one for tiles and one for world in general. And in one of these (items) I forgot to set the SpriteBatch before using and reset it afterwards. I should have spent more time looking at my code. Still, thanks for trying to help!
(can't close the question just yet, gonna close it after StackOverflow lets me do it)

score 0 · Answer 1 · answered Jun 17 '13 at 13:18

The default blending mode is BlendState.AlphaBlend.

Try replacing BlendState.Additive with BlendState.AlphaBlend in your code. Or possibly NonPremultiplied, depending on what Terraria is actually using.

Better yet, you could read out exactly the blend state that Terraria was using, as SpriteBatch sets it on the graphics card and simply leaves it there. Here is some untested code that should do exactly that:

sb.End(); // Sets blend state
BlendState previousState = GraphicsDevice.BlendState; // Retrieve it
sb.Begin(SpriteSortMode.Immediate, bsSubtract);
// (...drawing drawing blah...)
sb.End();
sb.Begin(SpriteSortMode.Immediate, previousState); // Re-use it

The thing is, it's in a middle of sb.Begin() that I called myself, with additive blending set. I just need to draw something with subtractive blending and set it back to additive for the rest. Also, read my EDIT. — Shockah, Jun 17 '13 at 13:24

score 0 · Accepted Answer · answered Jun 19 '13 at 08:27

Seems like the problem was me splitting the drawing to 3 separate places: one for item slots, one for tiles and one for world in general. And in one of these (items) I forgot to set the SpriteBatch before using and reset it afterwards. I should have spent more time looking at my code. Still, thanks for trying to help!

How do I properly achieve subtractive blending in C#, XNA?

2 Answers2