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Pixel Perfect Sprites


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#1 reaperrar   Members   -  Reputation: 134

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Posted 21 January 2014 - 12:15 AM

In a texture I have a square sprite (though it does not take up the whole texture, just a small portion).

 

I set up my UV coordinates to target just the sprite.

 

Sampler code:

    oSamplerDesc.Filter = D3D11_FILTER_MIN_MAG_MIP_POINT;
    oSamplerDesc.AddressU = D3D11_TEXTURE_ADDRESS_WRAP;
    oSamplerDesc.AddressV = D3D11_TEXTURE_ADDRESS_WRAP;
    oSamplerDesc.AddressW = D3D11_TEXTURE_ADDRESS_WRAP;
    oSamplerDesc.MipLODBias = 0.0f;
    oSamplerDesc.MaxAnisotropy = 1;
    oSamplerDesc.ComparisonFunc = D3D11_COMPARISON_ALWAYS;
    oSamplerDesc.BorderColor[0] = 0;
    oSamplerDesc.BorderColor[1] = 0;
    oSamplerDesc.BorderColor[2] = 0;
    oSamplerDesc.BorderColor[3] = 0;
    oSamplerDesc.MinLOD = 0;
    oSamplerDesc.MaxLOD = D3D11_FLOAT32_MAX;

When rendering the sprite to the screen using an orthographic projection with no rotating it renders perfectly.

 

However, if I scale the sprite non-uniformly (as it is being use as a frame that stretches) then the parts of the texture surrounding the sprite seem to be sampled.

 

I drew a coloured border around my square sprite and can see the red bleeding in to the edges of the texture in the above scenario.

 

A more detailed explanation...

Here are some frame pieces for a frame, a bar and a corner. The bar is to be stretched to accommodate the frame dimensions whereas the corner is not.

DpAW13X.png

Here is the frame rendering in-game. Notice the coloured borders are ignored, they were not included using the UV coordinates I set.

zVkAtDt.png

 

Now here is the exact same example as the above except I've changed the overall scale for the frame to reduce its size. (I've re-shaped the frame as that is part of its functionality)

K6WvxIc.png

 

I get the positions for my texture sampling like this...

The squares represent pixels, the blue squares are pixels in a sprite, the red circles are the positions I use.

O7uyAcA.png

 

Why is there sampling around the sprite when the frame is scaled smaller?


Edited by reaperrar, 21 January 2014 - 12:16 AM.


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#2 Phobon   Members   -  Reputation: 247

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Posted 21 January 2014 - 10:34 AM

Do you happen to have Mip-mapping active for this texture?

If so, DirectX will use an automatically downsampled version of your original texture if it is displayed smaller than originally, so lower mip-levels are used.

The default downsampling algorithm does produce color bleeding like you describe.

A solution would be to disable mip-mapping for minification in this case or do the downsampling yourself (eg. in Photoshop).

 

Edit: Just saw your sampler code: Try 

MaxLod=0;

to disable mip-mapping in the sampler.


Edited by Phobon, 21 January 2014 - 10:43 AM.


#3 Ravyne   GDNet+   -  Reputation: 8154

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Posted 21 January 2014 - 11:27 AM

Also, although I think it can depend on a number of things, usually you want to set your UV to the center of the source pixel, not its corners, unless it is otherwise compensated for. When you set UV, think of it as defining the center of the sampling area, so if you sample one pixel coverage at 1-1 ratio, you're sampling a 'virtual' pixel that covers the area in (U-0.5, V-0.5)-(U+0.5, V+0.5). This can also cause or contribute to sampling problems like you have.

 

I also agree with Phobon that mip-mapping could also cause or contribute to the problem you see, and in general you most-likely want to avoid mip-mapping entirely for UI elements.






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