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OpenGL Poor texture quality in OpenGL w/ alpha channel

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When I load a 32 bit TGA file in OpenGL (with an alpha channel), there is a big loss in the quality of the texture, if I don't read the alpha channel data the texture looks fine. Here's the line that generates the texture: glTexImage2D(GL_TEXTURE_2D, 0, 4, m_dwWidth, m_dwHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, m_pRGBAData); when the third parameter (the 4, for 4 color components) is set to 3 the image appears fine, but when I set it to 4 to read the alpha channel data the image quality is lost. The image data is contained within an array of DWORDs(m_pRGBAData) which holds all four color components. To place the color data in the DWORD I do the following bit shifts: m_pRGBAData = (r)+(g<<8L)+(b<<16L) + (a<<24L); is this correct? BTW, I have a voodoo 3 2000 which does not really support 32 bit rendering, does this affect the image quality if I blend textures with an alpha channel? [i]Digital Radiation Edited by - +AA_970+ on November 10, 2001 5:24:25 PM

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quote:
Original post by +AA_970+
BTW, I have a voodoo 3 2000 which does not really support 32 bit rendering, does this affect the image quality if I blend textures with an alpha channel?


Yes, that will have a huge effect on the number of colors on screen, and 16bits can only hold 65536 colors at once (which equals degradation in quality).

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Guest Anonymous Poster
The previous answer is incorrect. The problem is the additional memory used by 4 component textures vs 3 and the defaults the hardware gives you for the internal format of a 4 component texture. The additional alpha information is causing the driver to restrict the bits available for RGB components. Instead of simply requesting 4 components as the internal format look at the options available to give more precision for color at the expense of alpha, for example try GL_RGB5_A1, you may also be able to improve things by avoiding the default you are given when requesting 4 components if you are more explicit about the bits you want, for example GL_RGBA8 may be slower but produce higher quality results.

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