Porting to GLES from GL

Foreword

This guide is based on my limited knowledge (I do have barely the requierement) on the subject. I'm open to get edits on it, in fact it is encouraged :) --Sebt3 17:05, 24 February 2011 (MET)

Requirements

• a working toolchain
• average C/C++ knowledge
• basic knowledge of how GL or GLES works
• read OpenGL_ES_1.1_Tutorial

You might also want to have a look at others' work in this matter :

• http://pickle.gp2x.de/source/kenlabes-src.zip
• http://pickle.gp2x.de/gish_src.zip
• http://sebt3.openpandora.org/src/briquolo-0.5.7-pandora.tar.gz
• http://sebt3.openpandora.org/src/zaz-1.0.0-pandora.tar.gz
• http://paeryn.myby.co.uk/pandora/sdl-ball.tar.bz2
• ...

For more references, have a look to :

• opengles references or opengles references 2
• opengl references

Before starting

Choose a simple GL game for your first experience. Working with a large and complex codebase is not what you want to do at first.

To have a good view of what you'll need to do, here is a simple (and incomplete) bash script that will list you all the code lines you'll need to work with :

#!/bin/sh

search() {
find . -name "*.h" -o -name "*.c" -o -name "*.cpp" -exec grep -Hn "$*" {} \;
}

listIssues() {
search SDL_WM_GrabInput
search SDL_WarpMouse
search SDL_GL
search GL_CLAMP
search glColor|grep -v glColor4f
search glPushAttrib
search glPolygonMode
search GL_QUADS |grep glDrawArrays
search SDL_Init
search SDL_OPENGL
search SDL_ShowCursor
search glClearDepth
search SDL_Quit
search gluBuild2DMipmaps
search glTexImage2D
search glBegin
}

listIssues|sort -u

Run that from the source directory. The fewer lines displayed, the better.

If your project uses some GL libraries, get that to build first. Here are some libraries ported to GLES :

• GLU : http://code.google.com/p/glues/ (you might want to grab a binary version with headers in zaz or briquolo pnd)
• FTGL : http://code.google.com/p/ftgles/ (I do have a fixed version here; if you prefer a binary version, get that in the zaz PND)

Getting it to build

Your first goal is to have it built. You'll probably need to hack the build system.

• If it's just Makefiles, replace -lGL with -lGLES_CM -lEGL -lX11.
• If it's an autotools project, have a look at how it's done in briquolo for references. (take a look at configure.ac, and follow the enable_gles bits ;) Dont forget the Makefile.am too. Once your edits are in, autoreconf is your friend.)

Make sure your build system (or your toolchain) will also set defines some marker to know if it needs to build GL or GLES code. Having one too for pandora specific hack might show itself useful too. I'm using "-DPANDORA -DHAVE_GLES" myself.

Now try to build it. You'll have errors in the build process. To fix them, for now, just #ifdef out the offending lines. Here is what to do with the include instructions :

#if !defined(HAVE_GLES)
#include <GL/gl.h>
#else
#include <GLES/gl.h>
#endif

Other offending code, should be #ifdef out like this :

#if !defined(HAVE_GLES)
        glClearDepth(1.0f);
#endif

At the end of this process you'll end up with a broken pandora binary. Let's now fix it :D

Adding the EGL context

Grab the eglport package Pickle has built here. You will need to add a call to its 4 functions in the sources (make sure, also, that eglport.c is built and linked in the build process). Here is how your sources should look at the end :

#if !defined(HAVE_GLES)
#include <GL/gl.h>
#else
#include <GLES/gl.h>
#include "eglport.h"
#endif

int main( void )
{
	// other stuff here

        SDL_Init( SDL_INIT_VIDEO );
#if defined(HAVE_GLES)
       	if (!EGL_Open())
		exit(1);
#endif

	// some more initialisations

#if !defined(HAVE_GLES)
        screen = SDL_SetVideoMode( 800, 600, 0, SDL_HWSURFACE | SDL_OPENGL | SDL_FULLSCREEN);
#else
        screen = SDL_SetVideoMode( 800, 480, 0, SDL_SWSURFACE | SDL_FULLSCREEN );
	EGL_Init();
#endif

        // the event loop
        while( quit == 0 ) {
                // management of the even and the game


#if !defined(HAVE_GLES)
        	SDL_GL_SwapBuffers();
#else
		EGL_SwapBuffers();
#endif
        }

	// probably some more clean-ups here

#if defined(HAVE_GLES)
	EGL_Destroy();
#endif
        SDL_Quit();

        return 0;
}

Please note that :

• EGL_Open has to be called just after the SDL initialization of the VIDEO layer
• SDL_SetVideoMode arguments have to be those given here (aka good resolution, and no SDL_OPENGL flag)
• EGL_Init goes just after SDL_SetVideoMode
• All SDL_GL_SwapBuffers calls have to be converted to EGL_SwapBuffers calls.
• EGL_Close is called before quitting SDL (or closing the video if that's separated in your original source).

When built, your source should run on the Pandora cleanly (ok, you'll have a black screen, but that's expected as we haven't converted the drawing yet).

Converting the easy stuff

This part could have been done while getting the game built, but I preferred splitting the 2 processes for readability. Feel free to work as you please :D. Remember that all you have #ifdef out in the building part has to be converted; don't forget anything.

GLES only supports float; double (aka GLdouble) doesn't work. So :

• any GLdouble should be converted to GLfloat
• all GL function calls ending with a "d" should have the d replaced with f.

Here are other know conversions :

#define GLdouble     GLfloat
#define GL_CLAMP     GL_CLAMP_TO_EDGE
#define glClearDepth glClearDepthf
#define glOrtho      glOrthof

Colors

GLES only knows glColor4f as the function to create a color; all the glColor3f, glColor3d, glColor4d or any other color conventions have to be converted to glColor4f. If you only have 3 arguments, the last have to be 1.0f (as no transparency).

Converting the immediate mode

If you have read OpenGL_ES_1.1_Tutorial, you already have a good idea of what needs to be changed here. Here are a few other examples to help you in your process :

1) a simple quad with no texture or color :

#if !defined(HAVE_GLES)
  glBegin(GL_QUADS);
  glVertex2f(-10,-10);
  glVertex2f(10,-10);
  glVertex2f(10,10);
  glVertex2f(-10,10);
  glEnd();
#else
    GLfloat q3[] = {
        -10,-10,
        10,-10,
        10,10,
        -10,10
    };

    glEnableClientState(GL_VERTEX_ARRAY);
    glVertexPointer(2, GL_FLOAT, 0, q3);
    glDrawArrays(GL_TRIANGLE_FAN,0,4);
    glDisableClientState(GL_VERTEX_ARRAY);
#endif

2) a textured quad :

      glBindTexture(GL_TEXTURE_2D, carac->TextureName);
#if !defined(HAVE_GLES)
      glBegin(GL_QUADS);

      glTexCoord2f(0,0);
      glVertex3f(pos[0]-tailleX/2, pos[1]-tailleY/2, 0);
      glTexCoord2f(1,0);
      glVertex3f(pos[0]+tailleX/2, pos[1]-tailleY/2, 0);
      glTexCoord2f(1,1);
      glVertex3f(pos[0]+tailleX/2, pos[1]+tailleY/2, 0);
      glTexCoord2f(0,1);
      glVertex3f(pos[0]-tailleX/2, pos[1]+tailleY/2, 0);

      glEnd();
#else
      GLfloat vtx1[] = {
        pos[0]-tailleX/2, pos[1]-tailleY/2, 0,
        pos[0]+tailleX/2, pos[1]-tailleY/2, 0,
        pos[0]+tailleX/2, pos[1]+tailleY/2, 0,
        pos[0]-tailleX/2, pos[1]+tailleY/2, 0
      };
      GLfloat tex1[] = {
        0,0,
        1,0,
        1,1,
        0,1
      };

      glEnableClientState(GL_VERTEX_ARRAY);
      glEnableClientState(GL_TEXTURE_COORD_ARRAY);

      glVertexPointer(3, GL_FLOAT, 0, vtx1);
      glTexCoordPointer(2, GL_FLOAT, 0, tex1);
      glDrawArrays(GL_TRIANGLE_FAN,0,4);

      glDisableClientState(GL_VERTEX_ARRAY);
      glDisableClientState(GL_TEXTURE_COORD_ARRAY);
#endif

3) a colored quad (note that, as GL is a state machine, the color is only showed once in the GL code, while it is copied for every point in gles) :

#if !defined(HAVE_GLES)
    glBegin(GL_QUADS);
    glColor3d( .1, .1, .7);
    glVertex3d(0, 0, 0);
    glVertex3d(0, -pbarheight, 0);
    glColor3d( 0, 0, 0.5);
    glVertex3d((startupProgress / startupProgressSteps) * pbarwidth, -pbarheight, 0);
    glVertex3d((startupProgress / startupProgressSteps) * pbarwidth, 0, 0);
    glEnd();
#else
    GLfloat vtx1[] = {
        0, 0, 0,
        0, -pbarheight, 0,
        (startupProgress / startupProgressSteps) * pbarwidth, -pbarheight, 0,
        (startupProgress / startupProgressSteps) * pbarwidth, 0, 0
    };
    GLfloat col1[] = {
        .1, .1, .7, 1.0f,
        .1, .1, .7, 1.0f,
        0, 0, 0.5, 1.0f,
        0, 0, 0.5, 1.0f
    };

    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);

    glVertexPointer(3, GL_FLOAT, 0, vtx1);
    glColorPointer(4, GL_FLOAT, 0, col1);
    glDrawArrays(GL_TRIANGLE_FAN,0,4);

    glDisableClientState(GL_COLOR_ARRAY);
    glDisableClientState(GL_VERTEX_ARRAY);
#endif

Note that the last argument of glDrawArrays is the number of points. In these examples, that's always 4 because these all have 4 points. You might want to change that if your object has more (or less) than these 4 points.

Here is a list of conversions between GL to GLES draw modes :

• GL_POINTS -> GL_POINTS
• GL_TRIANGLES -> GL_TRIANGLES (or GL_LINE_LOOP if glPolygonMode is set to GL_LINE)
• GL_QUADS -> GL_TRIANGLE_FAN
• GL_LINE_STRIP -> GL_LINE_STRIP
• GL_LINE -> GL_LINE
• GL_POLYGON -> GL_TRIANGLE_FAN

Textures

All textures have to be loaded with with dimensions in powers of 2. See GLES2D sources and GLES2D_p2 function for an easy method of working around that. Briquolo had a more advanced method to do so thanks to Paeryn (it also actually scaled the texture to that given size).

When loading textures with glTexImage2D, the only supported formats are GL_ALPHA, GL_RGB, GL_RGBA, GL_LUMINANCE, or GL_LUMINANCE_ALPHA. Arguments for internal format and format(3 and 7) must be the same. Note: one common situation is where the original program uses 3 or 4 (which is valid for opengl) for internal format and GL_RGB or GL_RGBA for format. GLES requires the 3 to be GL_RGB or 4 to be GL_RGBA. GLES Example:

glTexImage2D( target, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, pixels);

Display Lists

GL Functions: glGenLists, glCallList, glCallLists, glDeleteLists, glNewList

This is a feature provided by GL to allow the application to assign a group of GL commands to a display list. This list is remembered by the graphics card and the application only has to call the list once to get the gpu to rerun all of the assigned commands. GLES does not support this feature and must rerun the set of commands any time the list would have been called. Info on Display Lists

Known issues

• glPushAttrib, glPopAttrib and their client versions don't exist. You'll have to manage these states yourself. Another good way is to have all disabled at all times and only enable what is needed at the right time.
• SDL_WM_GrabInput and SDL_WarpMouse might cause soft freeze of your game. If it does, #ifndef PANDORA them :)