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Erik Rufelt

OpenGL Bilinear backbuffer scaling on Vista

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Hi, For some simple 2D graphics I'm using DirectDraw in windowed mode. I create a backbuffer surface in video memory and fill it with the contents to be displayed each frame. After that I blit it to the primary surface, which is attached to a clipper and a window that can be resized. The window client area is usually much larger than the backbuffer. The backbuffer is currently 320x240 and the output window is 3x to 4x this size usually. I want the scaled output to be bilinear filtered. On Windows XP this works perfectly and the displayed image is scaled with bilinear filtering, but on Vista it's scaled with simple point sampling. On XP point sampling is used when blitting from a system memory surface, but from video memory surfaces it's bilinear. On Vista it's all point sampling in windowed mode. I get the same problem in D3D10, when the backbuffer is smaller than the window. Only in fullscreen exclusive mode is the image bilinear filtered. Can I get bilinear filtering on Vista in windowed mode? I'm thinking of switching to OpenGL and updating a texture with glTexSubImage2d every frame and using a fullscreen quad to display it, but I'd rather use DirectDraw if possible. I want to avoid D3D. I'm using Vista Ultimate 64 bit. Graphics card is Geforce 8800 GT. Comparison images between point sampling and bilinear:

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Doesn't DirectDraw have the facilities to render to a surface and then scale it with linear filtering? I confess I don't know much about it.

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Unfortunately I haven't been able to find any :( I've tried blitting between two video memory surfaces and rescaling in that step but still with point sampling in the result.

EDIT: Google turned up several hits on this, apparently it's just a Vista feature that it doesn't bilinear filter anymore.

[Edited by - Erik Rufelt on August 14, 2008 10:40:39 AM]

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