[openMSX-devel] OpenGL: GL ES, extensions, GLEW

[Maarten t. H. <ma...@tr...>]

Hi all,

Currently openMSX supports both software scaling using SDL 1.2 and 
hardware scaling using OpenGL 2.0 + extensions.

The software scaling is something we'd like to drop:
- the code is large, complex and slow to compile
- it doesn't support zoom factors over 4x
- this is a task that GPUs are great at, so why do it on CPU?

Maybe it would be useful to keep the 1x scaling (line stretching) for 
the GCW Zero and other devices with a 320x240 screen and drop only the 
actual upscaling (2x/3x/4x).


However, we also support platforms that don't have full (desktop) OpenGL 
support, for example Android. These platforms typically support OpenGL 
ES. Feature-wise OpenGL ES can do everything we need, but there are some 
practical issues we have to solve.

One thing is acquiring an OpenGL ES context. I think SDL2 can do this 
for us.

Another thing is that currently we use GLEW to manage GL extensions.
But the main GLEW release doesn't support GL ES. Alternatives would be:
- libepoxy (active)
  https://github.com/anholt/libepoxy
- GLEW 1.7 adapted for GL ES (seems inactive, releases from 2011)
  https://launchpad.net/linaro-graphics-misc/
- add our own extension manager for those extensions we use
- switch to a later GL version so we don't need extensions

The last option sounds the most attractive to me, since it would result 
in simpler code and a simpler build. However, we have to check that the 
GL version we would need doesn't disqualify too many users.

I grepped the openMSX source for "EXT" and "ARB" and it seems we only 
use two extensions:
- GL_EXT_framebuffer_object
- ARB_pixel_buffer_object

The functionality offered by both of these extensions is part of GL ES 
2.0, which is also the lowest version we could consider supporting (GL 
ES 1.x does not match modern hardware well at all).

On the desktop, OpenGL 3.0 added support for framebuffer objects and 
OpenGL 2.1 added support for pixel buffer objects. So if we'd require 
OpenGL 3.0 as a minimum version, we wouldn't need GLEW anymore.


Would it be a problem for anyone if we raise the minimum desktop OpenGL 
version requirement from 2.0 to 3.0? If not, I propose to do so and 
remove GLEW as a dependency.

Bye,
		Maarten