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How to change transparency dynamically?

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Hi, Another question. Is there a way to change the level of transparency (alpha blending) of a texture via DirectX during runtime, instead of setting it in stone in the alpha channel of the image file? The reason why I want to do this is so that I can make a texture slowly fade into visibility on top of a background picture. Thx ! any and all help appreciated Regards, Ed Zehoo

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You can use the diffuse component of your vertices to alter the color and/or transparency of your texture. Here's a quick copy of some of my code that does this sort of thing:

mpDevice->SetTextureStageState(0, D3DTSS_COLOROP, D3DTOP_MODULATE );
mpDevice->SetTextureStageState(0, D3DTSS_COLORARG1, D3DTA_TEXTURE );
mpDevice->SetTextureStageState(0, D3DTSS_COLORARG2, D3DTA_DIFFUSE );
mpDevice->SetTextureStageState(0, D3DTSS_ALPHAOP, D3DTOP_MODULATE );
mpDevice->SetTextureStageState(0, D3DTSS_ALPHAARG1, D3DTA_TEXTURE );
mpDevice->SetTextureStageState(0, D3DTSS_ALPHAARG2, D3DTA_DIFFUSE );
mpDevice->SetTextureStageState(1, D3DTSS_COLOROP, D3DTOP_DISABLE );
mpDevice->SetTextureStageState(1, D3DTSS_ALPHAOP, D3DTOP_DISABLE );

Basically, all color values are multiplied by the color values of the diffuse component of the vertices. Setting the diffuse to white causes no changes to take place. Similarly, the final alpha value is the multiplication of the alpha component of the vertex's diffuse color, and the alpha component of the texture itself. Setting the vertex's diffuse alpha to 255 means fully opaque, and setting it to 0 means fully transparent.

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There are several ways. You can lock the vertex buffer, update the vertex information with an alpha value for each vertex diffuse color you want the alpha to apply to. Or, more simply you can set the texture's stage for alpha operation to a Constant. You then supply the constant that will be multiplied by every vertex used in the draw call. The latter way of course is best for drawing an entire mesh with a specific transparency. If you needed to make the arms and legs transparent while leaving the rest opaque, I think it would be better to lock the vertex buffer once and update vertices then call draw once than to set the texture stage twice and have 2 draw primitive calls. That might depend on how optimized your balance is between CPU and GPU utilization. If your GPU is busy drawing the last mesh you called it to draw, then you might be better off using the CPU to update the vertices, then lock the buffer, copy new verts in, unlock the buffer. I'm not completely sure about that though.

Chris

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Agony, could your methode be used to make the texture support (the mesh the it is applied to) unaffected by lighing.
the reseon I ask is that I am trying to make my billboard free from lighing.

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Here's how you would implement what I was talking about.



DWORD AlphaValue;
AlphaValue = D3DCOLOR_ARGB(100,255,255,255);

mpDevice->SetTextureStageState(0, D3DTSS_COLOROP, D3DTOP_MODULATE);
mpDevice->SetTextureStageState(0, D3DTSS_COLORARG1, D3DTA_TEXTURE);
mpDevice->SetTextureStageState(0, D3DTSS_COLORARG2, D3DTA_DIFFUSE);
mpDevice->SetTextureStageState(0, D3DTSS_ALPHAOP, D3DTOP_MODULATE);
mpDevice->SetTextureStageState(0, D3DTSS_ALPHAARG1, D3DTA_TEXTURE);

mpDevice->SetTextureStageState(0, D3DTSS_CONSTANT, AlphaValue);
mpDevice->SetTextureStageState(0, D3DTSS_ALPHAARG2, D3DTA_CONSTANT);
mpDevice->SetTextureStageState(1, D3DTSS_COLOROP, D3DTOP_DISABLE);
mpDevice->SetTextureStageState(1, D3DTSS_ALPHAOP, D3DTOP_DISABLE);

pMesh->Draw();



This would allow you to specify all of the following draws to use AlphaValue without having to change one vertex buffer. An example is the main character in URU. When you move the mouse over it, it becomes semi-transparent.

Chris

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Is that D3DTSS_CONSTANT flag only supported in DirectX 9? I looked through my DX 8.1 documentation, and there isn't anything like it. Hopefully I can do something similar?

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Quote:
Original post by Weston
Is that D3DTSS_CONSTANT flag only supported in DirectX 9? I looked through my DX 8.1 documentation, and there isn't anything like it. Hopefully I can do something similar?


Yes, it's a DirectX 9 thing. It's a constant you can set per texture stage.

For older versions of DirectX if you only need a single constant for the whole blend you can use D3DTA_TFACTOR and set the constant value with the D3DRS_TEXTUREFACTOR render state.

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