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Asked: 2025-04-22T02:14:04+05:30

How can I blend transparent PNG textures in DirectX 10 without introducing black pixels on the edges?

I’m wrestling with a bit of a challenge when it comes to blending transparent PNG textures in DirectX 10. I’m using the following blending settings to try and get everything to blend smoothly, but I’m consistently ending up with those annoying black pixels around the edges of my semi-transparent textures.

Here’s the code fragment I’m using:

“`cpp
D3D10_BLEND_DESC blendDesc = { 0 };
blendDesc.AlphaToCoverageEnable = FALSE;
blendDesc.BlendEnable[0] = TRUE;
blendDesc.BlendOp = D3D10_BLEND_OP_ADD;
blendDesc.SrcBlend = D3D10_BLEND_SRC_ALPHA;
blendDesc.DestBlend = D3D10_BLEND_INV_SRC_ALPHA;
blendDesc.BlendOpAlpha = D3D10_BLEND_OP_ADD;
blendDesc.SrcBlendAlpha = D3D10_BLEND_ONE;
blendDesc.DestBlendAlpha = D3D10_BLEND_ONE;
blendDesc.RenderTargetWriteMask[0] = D3D10_COLOR_WRITE_ENABLE_ALL;
HRESULT hr = pDevice->CreateBlendState(&blendDesc, &pBlendState);

pDevice->OMSetBlendState(pBlendState, nullptr, 0xffffffff);
“`

The problem comes down to how the alpha blending is set up. I get that the blending equation should be something like `final.rgb = src.rgb * src.a + dest.rgb * (1 – src.a)`. The issue seems to be with how this works around the edges where `src.a < 1`. When the alpha value is not fully opaque, it appears to mix the texture with black, thus introducing those undesirable black pixels. I tried changing `blendDesc.DestBlendAlpha` to `D3D10_BLEND_ZERO`, which does eliminate the black artifacts, but it also seems to wipe out any existing content in the destination, rather than blending it as I need. What I want is to maintain the old content while getting rid of those black edges. I’m really at a loss here. It seems like I’m very close to finding the right combination, but I just can’t seem to put my finger on what specifically needs to change to achieve the correct blending without those pesky black pixels. Has anyone faced a similar issue or have any tips on how to configure the blending state correctly for transparent textures? Any guidance or insights would be greatly appreciated!

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    2 Answers

    1. It sounds like you’re encountering a common issue with alpha blending in DirectX. Those black edges typically come from how the alpha values are being processed during blending. Here are a couple of things you might want to experiment with:

      • Change Blend State: Keep `DestBlend` as `D3D10_BLEND_INV_SRC_ALPHA` for your main blend operation. It allows the existing texture color to properly mix with your semi-transparent textures.
      • Alpha Blending Settings: Your current settings for alpha blending seem to be using `D3D10_BLEND_ONE` for `SrcBlendAlpha` and `DestBlendAlpha` which might not be ideal. Try using `D3D10_BLEND_SRC_ALPHA` and `D3D10_BLEND_INV_SRC_ALPHA` respectively for these settings instead. It might help reduce those black edges.

      So your blend desc should look something like this:

          
    D3D10_BLEND_DESC blendDesc = { 0 };
    blendDesc.AlphaToCoverageEnable = FALSE;
    blendDesc.BlendEnable[0] = TRUE;
    blendDesc.SrcBlend = D3D10_BLEND_SRC_ALPHA;
    blendDesc.DestBlend = D3D10_BLEND_INV_SRC_ALPHA;
    blendDesc.BlendOp = D3D10_BLEND_OP_ADD;
    blendDesc.SrcBlendAlpha = D3D10_BLEND_SRC_ALPHA;
    blendDesc.DestBlendAlpha = D3D10_BLEND_INV_SRC_ALPHA;
    blendDesc.BlendOpAlpha = D3D10_BLEND_OP_ADD; 
    blendDesc.RenderTargetWriteMask[0] = D3D10_COLOR_WRITE_ENABLE_ALL;

      Another thing to look into is if your texture’s alpha channel is being handled correctly. Make sure the edge pixels in your PNG aren’t blending to black by ensuring they have appropriate alpha values.

      If you’re still having trouble, sometimes enabling multisampling can help smooth things out, although that’s a bit more advanced.

      Good luck with your graphics project! Keep tinkering with it, and you’ll get those transparent edges looking right in no time!

    2. The issue you’re experiencing isn’t primarily due to incorrect blending settings but rather how your transparent textures’ RGB values are prepared. The infamous black borders around semi-transparent images usually occur when textures are authored against a black or non-matching background, causing blending operations to interpolate darker pixels into the transparent regions. Adjusting your blending state alone (such as changing DestBlendAlpha) won’t fix this entirely, because the underlying problem is related to pre-multiplied alpha handling or lack thereof.

      To completely resolve the black edge artifacts, consider performing premultiplied alpha in your textures—this means multiplying the RGB channels of your images by their alpha channel before loading them into Direct3D. After preparing your textures with premultiplied alpha, you’ll also need to adjust your blending configuration to SrcBlend = D3D10_BLEND_ONE and DestBlend = D3D10_BLEND_INV_SRC_ALPHA. This approach ensures smooth blending without unwanted black outlines, preserving the pristine edges of transparent textures and seamlessly maintaining their transparency integrity in the blending operations.