OpenGL ES: Texture2D and the Power of Two

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Introduction

As most of you have figured out, OpenGL ES doesn't like textures to be non-power of two, and I really didn't want to have to force all of my textures to be powers of two before loading them into my game. This presented me with a problem that had to be solved in-code.

To really understand texture mapping in OpenGL, I had to do a lot of research. To be fair, I still don't fully understand it, although there are a few articles that do a really great job at explaining things. One of those articles is titled "OpenGL ES from the ground up" and can be located at http://iphonedevelopment.blogspot.com/2009/05/opengl-es-from-ground-up-table-of.html. Look up chapter 6 on Texture Mapping.

The problem that I was faced with, is that only powers of two can be used inside OpenGL ES, so how do you convert our texture to a power of two if it was loaded in as a 40 x 40 texture (for example)? There are several ways of doing just this, but I settled with the way Apple did it for their Texture objects, and moulded that to fit my own code. They came up with the following to convert the width and height:

//Adjust the width and height to be a power of two.
if(( _width != 1) && ( _width & (_width - 1) )
{
    i = 1;
    while( (sizeToFit ? 2 * i : i) < _width)
        i *= 2;
    _width = i;
} 

if(( _height != 1) && ( _height & (_height - 1) )
{
    i = 1;
    while( (sizeToFit ? 2 * i : i) < _height)
        i *= 2;
    _height = i; 
}

The first portion of the code will convert the width to a power of two, then the height. Since the texture will be stretched, and never shrunk, Apple had to figure out a way to shrink the texture if it became longer than the max texture size defined by kMaxTextureSize = 1024. The following code will do just that:

//      scale down an image greater than the max texture size
while((_width > kMaxTextureSize) || (_height > kMaxTextureSize))
{
    _width /= 2;
    _height /= 2;
    transform = CGAffineTransformScale(transform, 0.5, 0.5);
    imageSize.x *= 0.5;
    imageSize.y *= 0.5; 
}

The "transform" is nothing more than a CGAffineTransform (Core Graphics). So, now that you've glanced over the code, you remember in the article where Jeff was talking about S and T coordinates? We'll take care of those like this:

_maxS = imageSize.x / (float)_width;
_maxT = imageSize.y / (float)_height;

This will give us a number between 0 and 1 which can be used in our textureCoordinates[] for drawing. This should only render the portion of the texture that has the data we wish to make it to the screen, and not the padding we added to ensure it was a power of two.

While I am still learning OpenGL, I figured that a lot of people would be interested in the following solution... especially if you had done what I had, and thrown out the Apples Texture2D class while working on iPhone OpenGL applications. The full source code to the Texture2D, TextureManager, and any other class used by my C++ iPhone Engine can be found at http://code.google.com/p/djinnengine.

Hope this was informative! If anyone has any further information that would be useful, feel free to comment!

Happy coding everyone!

History

• 19^th July, 2009: Initial post