Composing images of multiple textures

For the curious: The game is developed with SDL using OpenGL for graphics output and will support all three major platforms: Windows, Linux and Mac OS X.

Yesterday, I wrote a class to load arbitrary-sized images into possibly multiple textures none of which is greater than 256*256. I don't know how restrictive modern graphics cards are in what texture sizes they accept, but this way I think we are safe to also support older cards. Everything went quite well, until I tried to render a rotated image composed of multiple textures. The result was something like what can be seen on the first screenshot. Somehow there were visible lines at the texture boundaries.

It took me a while to find out what happend here. The problem was that, when rendering a textured primitive with OpenGL, it repeats the same texture when specifying texture coordinates greater than one. Now, when drawing a rotated quad, OpenGL seems to have accessed such a texture coordinate for interpolation, even though I never gave a number greater than 1.0 to glTexCoord.

Once I understood the problem, it was easy to fix. Simply tell OpenGL not to repeat the texture, but clamp texture coordinates to the range [0,1]. This is done using glTexParameteri, by setting the GL_TEXTURE_WRAP_S and GL_TEXTURE_WRAP_T parameters to GL_CLAMP.

Another solution seems to be to set it to GL_CLAMP_TO_EDGE_EXT, but this requires the GL_EXT_texture_edge_clamp extension, and GL_CLAMP works well enough already.