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lordimmortal2

Scissor Test Per Draw

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Recently I've been finishing up clipping for my GUI, which is just now being tested. To clip something, I've been trying to use the scissor test in DX9, and I would need it to be applied per draw call (both for something passed to the sprite interface and for text). While messing around with this, I've deduced that DX9 doesn't apply the scissor test every draw call, instead it applies it when its rendering ends or is told to render everything it has. 

 

So, the only way I've gotten the scissor test to work is by calling flush immediately after I draw the sprite, but this wouldn't work with my text objects (on account of the ID3DXFont interface not allowing you to prematurely flush the text to the screen, unless there's something I'm missing), so this isn't a sufficient solution. 

 

Is there an easier way to get the scissor test to work per-draw and if so how? If not, are there any replacements?

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You're using the ID3DXFont and ID3DXSprite interfaces, so you need to be aware of how these actually work behind the scenes.

 

What you're calling a "draw call" here is not actually a real draw call; all that ID3DXSprite::Draw (in this case) actually does is add data representing the text to draw to it's own backing vertex and index buffers; nothing much else.  When you call ID3DXSprite::Flush - that's when the real draw call (ID3DDevice9::DrawIndexedPrimitive) happens.  So that's consistent with your observed behaviour.

 

If you want to clip text drawing, the documentation for ID3DXFont::DrawText indicates that the pRect parameter will be automatically used for clipping unless DT_NOCLIP is specified in the Format param.  So that's the way to do it (and scissor won't even be needed in this case).

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So, the only way I've gotten the scissor test to work is by calling flush immediately after I draw the sprite, but this wouldn't work with my text objects (on account of the ID3DXFont interface not allowing you to prematurely flush the text to the screen, unless there's something I'm missing), so this isn't a sufficient solution.

 

From what I recall, you can pass in a ID3DXSprite-Pointer to the ID3DXFonts draw method, in which case flushing the text will be drawn in respect to the sprite - you'd want to do that anyway, since it will speed up text rendering, and it should also fix your issue. Unfortunately there really is no other way then to flush whenever you change the clip rect. The only way around it would be to implement your own sprite renderer with a software clip rect, but this tends to be slightly slower than the hardware-equivalent. Yet, if doing so enables you to draw ALL sprites like from your gui in one draw call instead of multiple ones needed in case of the ID3DXSprite, the price might be well worth paying and can actually result in a speedup.

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So, the only way I've gotten the scissor test to work is by calling flush immediately after I draw the sprite, but this wouldn't work with my text objects (on account of the ID3DXFont interface not allowing you to prematurely flush the text to the screen, unless there's something I'm missing), so this isn't a sufficient solution.

 

From what I recall, you can pass in a ID3DXSprite-Pointer to the ID3DXFonts draw method, in which case flushing the text will be drawn in respect to the sprite - you'd want to do that anyway, since it will speed up text rendering, and it should also fix your issue. Unfortunately there really is no other way then to flush whenever you change the clip rect. The only way around it would be to implement your own sprite renderer with a software clip rect, but this tends to be slightly slower than the hardware-equivalent. Yet, if doing so enables you to draw ALL sprites like from your gui in one draw call instead of multiple ones needed in case of the ID3DXSprite, the price might be well worth paying and can actually result in a speedup.

 

Oh wow. I was already passing in a ID3DXSprite pointer to the ID3DXFonts draw. I guess I didn't put two and two together and realize that it was doing that to actually draw the text =P. I guess I'll just flush every time I need to scissor clip these things. Thanks for the answer, it's working now.

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