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XNA alpha overlay problem

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I am a beginner in XNA, but have used MDX a short time. I am in the process of moving a MDX project to XNA and got into a strange painting issue with alpha blending. This was working fine in MDX. I am setting RenderState: GraphicsDevice.RenderState.AlphaBlendEnable = true; GraphicsDevice.RenderState.SourceBlend = Blend.SourceAlpha ; GraphicsDevice.RenderState.DestinationBlend = Blend.InverseSourceAlpha; And all works well until I get to overlay 2 TriangleStrips. I would have expected the color to simply get darker when they are overlaid (alpha channel set to 130 for both). A png to illustrate is here: http://www.farmergps.com/download/test4/XNA-Alpha1.png Had some trouble converting from LH to RH, however I don't think that is related. (you can see the grid shining through, although it was drawn first and has the same 0 z value as the violet drawing) Maybe somebody can point me in the right direction? Thanks, Johannes

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After some more experiments I am thinking that my problem may not be alpha channel related but maybe more to the order of triangles. I have seen some triangles disappear to white....

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Found my problem:

GraphicsDevice.RenderState.DepthBufferEnable = true;
GraphicsDevice.RenderState.DepthBufferWriteEnable = false;

This article did the trick:
http://blogs.msdn.com/shawnhar/archive/2009/02/18/depth-sorting-alpha-blended-objects.aspx

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