Cross-platform renderer in OpenGL ES

Question

I'm writing an cross-platform renderer. I want to use it on Windows, Linux, Android, iOS.

Do you think that it is a good idea to avoid absolute abstraction and write it directly in OpenGL ES 2.0?

As far as I know I should be able to compile it on PC against standard OpenGL, with only a small changes in code that handles context and connection to windowing system.

Answer 1

Do you think that it is a good idea to avoid absolute abstraction and write it directly in OpenGL ES 2.0?

Your principle difficulties with this will be dealing with those parts of the ES 2.0 specification which are not actually the same as OpenGL 2.1.

For example, you just can't shove ES 2.0 shaders through a desktop GLSL 1.20 compiler. In ES 2.0, you use things like specifying precision; those are illegal constructs in GLSL 1.20.

You can however #define around them, but this requires a bit of manual intervention. You will have to insert a #ifdef into the shader source file. There are shader compilation tricks you can do to make this a bit easier.

Indeed, because GL ES uses a completely different set of extensions (though some are mirrors and subsets of desktop GL extensions), you may want to do this.

Every GLSL shader (desktop or ES) needs to have a "preamble". The first non-comment thing in a shader needs to be a #version declaration. Fortunately for you, the version is the same between desktop GL 2.1 and GL ES 2.0: #version 1.20. The problem is what comes next: the #extension list (if any). This enables extensions needed by the shader.

Since GL ES uses different extensions from desktop GL, you will need to change this extension list. And since odds are good you're going to need more GLSL ES extensions than desktop GL 2.1 extensions, these lists won't just be 1:1 mapping, but completely different lists.

My suggestion is to employ the ability to give GLSL shaders multiple strings. That is, your actual shader files do not have any preamble stuff. They only have the actual definitions and functions. The main body of the shader.

When running on GL ES, you have a global preamble that you will affix to the beginning of the shader. You will have a different global preamble in desktop GL. The code would look like this:

GLuint shader = glCreateShader(/*shader type*/);
const char *shaderList[2];
shaderList[0] = GetGlobalPreambleString(); //Gets preamble for the right platform
shaderList[1] = LoadShaderFile(); //Get the actual shader file
glShaderSource(shader, 2, shaderList, NULL);

The preamble can also include a platform-specific #define. User-defined of course. That way, you can #ifdef code for different platforms.

There are other differences between the two. For example, while valid ES 2.0 texture uploading function calls will work fine in desktop GL 2.1, they will not necessarily be optimal. Things that would upload fine on big-endian machines like all mobile systems will require some bit twiddling from the driver in little-endian desktop machines. So you may want to have a way to specify different pixel transfer parameters on GL ES and desktop GL.

Also, there are different sets of extensions in ES 2.0 and desktop GL 2.1 that you will want to take advantage of. While many of them try to mirror one another (OES_framebuffer_object is a subset of EXT_framebuffer_object), you may run afoul of similar "not quite a subset" issues like those mentioned above.