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# Sunday, 04 December 2005
As part of an ongoing OpenGL implementation project, I was tasked with implementing the texture environment functionality into the pipeline (particularly the per-fragment functions with GL_COMBINE). To that end I had to read a few sections of the OpenGL 1.5 specification document, and let me tell you: having never used OpenGL, getting into the "GL state of mind" is no easy task to begin with, but made far worse when the specifications are horribly written.

To begin with, there is absolutely no reason why in 2005 a standard that is still widely used should be published solely as a document intended for printing. Acrobat's shortcomings aside, trying to use (let alone implement!) a non-hyperlinked specification of an API is the worst thing in terms of usability I've had to endure in recent years (and yes, I was indeed around when you could hardly get any API spec let alone in print. I don't think it matters). Even Borland knew the importance of hyperlinked help with their excellent IDEs of the early '90s. Why is it that so late into the game I still run across things like the spec for TexEnv2D, which describes only some of the function's arguments and then proceeds to tell you that "the other parameters match the corresponding parameters of TexImage3D"? I can only imagine the consternation of someone trying to figured out the spec from a book having to flip all the way to the index at the end to see that the TexEnv3D function appears on pages 91, 126, 128, 131-133, 137, 140, 151, 155, 210 and 217 (the function definition is at page 126, by the way.)

Hey, SGI! I have a four-letter acronym for you. H-T-M-L. It's been here for a while and is just as portable as PDF (and certainly more lightweight.)

Technical issues aside, the spec itself is simply incomplete; there is no obvious place which tells you where, for example, you have to set the OPERANDn_RGB properties; in fact, if you followed the spec to the letter, the people using your implementation would not be able to set these properties at all because the spec for the TexEnv functions explicitly specifies the possible properties, and those properties are not among them. You have to make educated guesses (it does makes sense that you'd set the texture unit state from TexEnv), do "reverse-lookup" in the state tables at the very end and try to figure out from the get function which set function is responsible for the property or dig up the MESA sources and try to figure out how they did it (because it's most likely to be the right way of doing things. I would go as far as to say that MESA serves as an unofficial reference for the OpenGL API). Ideally you would do a combination of the above just to make sure that you are compliant, but the spec will not be there to help you.

Finally, the spec makes problematic assumptions about your background. I think it's a little naïve to assume that anyone who reads the OpenGL background has a strong background in OpenGL programming, or in developing 3D engines. I actually do have some background in 3D engine development and it still took me a few hours going over the chapter again and again (and trying to infer meanings from code examples on newsgroups) to understand precisely what "Cp and Ap are the components resulting from the previous texture environment" means. In retrospect it seems obvious, particularly in light of some clues littered about, that they were referring to the result of the function on the previous texture unit, however that would only be really obvious to someone in the "GL state of mind," which at the time I was not. Tricky bits of logic should be better explained, examples (and particularly visual aids such as screenshots!) should be given. But the OpenGL spec has absolutely none of that.

So bottom line, specs are absolutely necessary for APIs; make sure you have people who proofread them, and put the developers that are most customer-savvy on the job. It would be the single worst mistake you can make to put a brilliant, productive and solitary developer on the task because you would frustrate them beyond belief and the end-result would be of considerably inferiour quality.

Sunday, 04 December 2005 19:56:09 (Jerusalem Standard Time, UTC+02:00)  #    -
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