Porting DirectX to OpenGL ES (iPhone)

3
votes

I have been asked to investigate porting 10 year old Direct X (v7-9) games to OpenGL ES, initially for the iPhone

I have never undertaken a game port like this before (and will be hiring someone to do it) but I'd like to understand the process.

  1. Are there any resources/books/blogs that will help me in understanding the process?
  2. Are there any projects like Mono that can accomplish this?
2
opengl-esdirectxporting
Are these PC games, for a desktop computer, that you want to port to a mobile device? How intensive are the graphics, I am assuming they are 3D, is the expected refresh rate 30 fps? Are you taking advantage of what DirectX9 offers? – James Black
Yes they are desktop games with 3D but nothing very special they are all 10 years old, and quite simplistic. – Richard Stelling

2 Answers

6
votes

TBH A porting job like this is involved but fairly easy.

First you start by replacing all the DirectX calls with "stubs" (ie empty functions). You do this until you can get the software to compile. Once it has compiled then you start implementing all the stub functions. There will be a number of gotchas along the way but its worth doing.

If you need to port to and support phones before iPhone 3GS you have a more complex task as the hardware only supports GLES 1 which is fixed-function only. You will have to "emulate" these shaders somehow. On mobile platforms I have written, in the past, assembler code that performs "vertex shading" directly on the vertex data. Pixel shading is often more complicated but you can usually provide enough information through the "vertex shading" to get this going. Some graphical features you may just have to drop.

Later versions of the iPhone use GLES 2 so you have access to GLSL ... ATI have written, and Aras P of Unity3D fame has extended, software that will port HLSL code to GLSL.

Once you have done all this you get on to the optimisation stage. You will probably find that your first pass isn't very efficient. This is perfectly normal. At this point you can look at the code from a higher level and see how you can move code around and do things differently to get best performance.

In summary: Your first step will be to get the code to compile without DirectX. Your next step will be the actual porting of DirectX calls to OpenGL ES calls. Finally you will want to refactor the remaining code for best performance.

(P.S: I'd be happy to do the porting work for you. Contact me through my linkedin page in my profile ;)).

0
votes

Not a complete answer, but in the hope of helping a little...

I'm not aware of anything targeting OpenGL ES specifically, but Cadega, Cider and VirtualBox — amongst others — provide translation of DirectX calls to OpenGL calls, and OpenGL ES is, broadly speaking, OpenGL with a lot of very rarely used bits and some slower and redundant parts removed. So it would probably be worth at least investigating those products; at least VirtualBox is open source.

The SGX part in the iPhone 3GS onwards has a fully programmable pipeline, making it equivalent to a DirectX 10 part, so the hardware is there. The older MBX is fixed pipeline with the dot3 extension but no cube maps and only two texture units. It also has the matrix palette extension, so you can do good animation and pretty good lighting if multiple passes is acceptable.