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Phantom previous texture exists behind updated texture, when sampled


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#1 Slippery Jim   Members   -  Reputation: 103

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Posted 18 July 2012 - 01:02 PM

My inexperience with DirectX is certainly at fault here, but after several days I'm still mystified by this undesired behavior in my relatively simple 2D application:

Each time I render, I loop through several textures and draw them in a back-to-front order by, for each of them:

1) Calling PSSetShaderResources() to update the shader's single Texture2D resource to the next ShaderResourceView
2) Calling DrawIndex() on the appropriate vertex buffer segment.

I have a simple pixel shader that is really just one basic texture sampling line:
ps.color = myOneTexture2D.Sample(samLinear, IN.texcoord0);

I've enabled alpha blending so that the alpha channel controls transparency, and everything is fine with that, except:

The Phantom Lingers.

Which is to say that each sampled texture is for some reason alpha-blended with the previously sampled texture.
So for totally opaque images, it's fine. For the first texture drawn -- such as this opaque double jellyfish I've drawn onto the background -- it's fine:
texture1.jpg

But the second texture (which I've made 50% transparent) drawn onto the scene shows the first texture (the double jellyfish) blended with it. Behind it, you can see a piece of the large background jellyfish previously drawn, which is the only thing that should be showing behind Snidely Whiplash.
texture2.jpg

A third texture showing fog (also set to 50% transparency) has that entire previous texture (Snidely + the first texture) mixed with it. It again shows the background portion behind it (though you can't see it well here), like I want it to, but also the previous texture, like I don't want it to.
texture3.jpg

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So can any kind soul take a stab at telling me what's going on, here?

If I create multiple Texture2D resources in the shader, and have a separate one for each texture, then it works fine. The problem is, I want to be able to draw dozens of textures each time I render the scene, and the shader/my hardware doesn't support having, say, 50 different textures with a long sequence of "if (index == 3) texture[3].Sample..." etc.

I can't use TextureArrays because the textures have different dimensions.

I'm sure I'm missing something fundamental in the way shaders and samplers work, but for the life of me I haven't been able to discover it over days of research and experimentation. I know it's happening before the output merger stage, but I can't figure out why the Texture2D resource is apparently not being completely overriden by PSSetShaderResource(), like one would expect, and is instead somehow alpha-blended with it. Or the previous incarnation lingers, and is blended when Sampled? I can't tell, but perhaps one of you generous individuals can!

Many thanks for any insight you can offer.
James

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#2 Slippery Jim   Members   -  Reputation: 103

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Posted 18 July 2012 - 03:22 PM

Belay that! Ignore this post.

If I'd only waited a couple more hours, I could have avoided embarrassing myself by having to reveal that I had a bug my DrawIndexed() parameter calculation that -- you guessed it -- was causing it to draw to quads it wasn't supposed to.

My shame is more than made up for by my relief at finally figuring out my mistake (braindead though it may have been). On the off chance that someone else is having a similar problem and finds this: be it known that the described behavior is not in any way what directx is supposed to do, and you're probably making a simple mistake somewhere in your scene rendering calls.

James




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