Possible to glTexImage2d an NPOT image onto a Pow-2 texture without extra allocation?

Question

I have discovered that there are still a fair number of drivers out there that don't support NPOT textures so I'm trying to retro-fit my 2D engine (based on OpenTK, which is in turn based on OpenGL) with Texture2D support instead of relying on GL_ARB_texture_rectangle. As part of this I am forcing all NPOTS texture bitmaps to allocate extra space up to the next power-of-2 size so they won't cause errors on these drivers. My question is, do I really have to resize the real bitmap and texture and allocate all that extra memory, or is there a way to tell OpenGL that I want a power-of-2 size texture, but I'm only going to use a portion of it in the upper left?

Right now my call looks like this:

GL.TexImage2D(texTarget, 0, PixelInternalFormat.Rgba8, bmpUse.Width, bmpUse.Height, 0, PixelFormat.Bgra, PixelType.UnsignedByte, bits.Scan0);

This is after I have made bmpUse be a copy of my real texture bitmap with extra space on the right and bottom.

Superheterodyne

Answer 1

Use glTexImage2D with empty data to initialize the texture and glTexSubImage2D to fill a portion of it with data. Technically OpenGL allows the data parameter given to glTexImage{1,2,3}D to be a null pointer, indicating that the texture object is just to be initializd. It depends on the language binding, if that feature remains supported in the target language – just test what happens if you pass a null pointer.

Semblance

Is any extension/feature required in order to use this? Am I better off just making a larger texture or using glTexSubImage2D if I'm concerned about compatibility with a wide range of drivers? – Superheterodyne

No, it's standard behaviour, part of OpenGL since ancient ages. But you must initialize the texture with glTexImage before filling it with content using glTexSubImage. – Semblance

For the record, 'NULL' as an acceptable data pointer to glTexImage2D dates back to OpenGL 1.1, which is already a requirement if you're using glGenTextures and is so old that even Microsoft's default GL driver should get it right. – Smaragdine

Answer 2

datenwolf is right on how to initialize the texture with just a partial image, but there are 2 issues with this you need to be aware of:

• you need to remap the texture coordinates of your mesh, as the [0-1] texture range of the full texture now also contains uninitialized data, as opposed to your full texture. The useful range is now [0-orig_width/padded_width]
• wrapping of your texture will only wrap the whole texture, not your sub-part.

Haarlem