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Texture Atlases – Part 4 – Compressing Them

michelle — Fri, 21 May 2010 16:54:26 +0000

OK, so we have loved them, made them, used them, now I recommend compressing them.

There are four of options for compressions technologies in the apple PVRTC tool, texturetool:
–channel-weighting-linear
–channel-weighting-perceptual
–bits-per-pixel-2
–bits-per-pixel-4

For realistic looking images, they recommend perceptual, but there’s still a lot of artifacting with that technology.

You might try compressing them with the PVRTexTool by Imagination Technologies.

http://www.imgtec.com/powervr/insider/powervr-pvrtextool.asp.

They have a command line version that run on Mac, Windows, etc. There’s a window GUI that is nice too.

There are some nice things about the PVRTexTool, my favorites being that it let’s you resize the image during the compression and works on non-square images.

The resize feature of the PVRTexTool came in handy for my last project. The only way I could get decent looking compressed images was to doubled the texture atlases in Photoshop (using bicubic sampling) and then shrink them back down again during compression. Thankfully, I was able to automate it as part of the build process.

Texture Atlases – Part 3 – Using Them

michelle — Fri, 21 May 2010 03:05:03 +0000

OK, so we have seen how cool they are, we’ve discussed how to make them, now we are going to look at how to read them into your OpenGL application.

First you need to understand how textures map in Open GL. Jeff Lemarche does an amazing job explaining that in his blog, http://iphonedevelopment.blogspot.com/2009/05/opengl-es-from-ground-up-part-6_25.html.

I love that tutorial, thanks Jeff.

So, a texture is bound to a set of vertices via texture coordinates. They are usually passed as arrays of vertices and texture coordinates. A texture is mapped to the vertices based on a values between 0 < 1. (0,0) being (top, left) and (1,1) being the (bottom, right) of the image.

For example, if you wanted the top half of the image to be displayed, you would give the coordinates (0, 0, .5, .5).

Here’s another example. If we have an 200×200 image that is at 20, 300 in our texture atlas and our texture atlas is 1024×1024. We would give it’s coordinages as (top, left, width, height) = (20/1024, 300/1024, 200/1024, 200/1024) = (0.01953…, 0.2929…,0.1953…, 0.1953…).

Texture Atlases – Part 2 – Making Them

michelle — Tue, 13 Apr 2010 05:19:52 +0000

Making texture atlas is either the responsibility of the artist or the developers. In my last project it was the developers

So I created a tool that produced square pngs files with widths and heights a power of two (128×128, 256×256, 512×512, 1024×1024). These texture atlases contained as many smaller images as possible.

I then stored information about the texture atlas filename and the top, left, width and height of original image in texture atlas.

There are alot of “rect packing” algorithms out there, but my favorite was Nate’s blog. He has a great example that is simple and elegant. I was able to easily customize it for my scenario.