How to get OpenGL-ES working on Raspberry Pi with SDL2? - raspberry-pi

Question

I am trying to get OpenGL-ES working on a Raspberry Pi, but so far no luck. I compiled SDL 2.0.3 from source with this, as the version in Rasbian is missing Raspberry Pi support:
./configure --prefix=/home/pi/run/SDL2-2.0.3/ \
--disable-video-x11 \
--disable-pulseaudio \
--disable-esd \
--disable-video-opengl
The code below should create a OpenGL context and clear the screen to red. When I run the code, the Raspberry Pi is switching video modes, but the screen is turning black instead of red and the calls to glGetString(GL_VERSION) and Co. return NULL which would indicate that something is wrong with the GL context creation.
#include <SDL.h>
#include <SDL_opengles2.h>
#include <iostream>
void print_gl_string(GLenum name)
{
const GLubyte* ret = glGetString(name);
if (ret == 0)
{
std::cerr << "error getting string: " << name << std::endl;
}
else
{
std::cerr << name << ": " << ret << std::endl;
}
}
void set_gl_attribute(SDL_GLattr attr, int value)
{
if (SDL_GL_SetAttribute(attr, value) != 0)
{
std::cerr << "SDL_GL_SetAttribute(" << attr << ", " << value << ") failed: " << SDL_GetError() << std::endl;
}
}
int main()
{
if (SDL_Init(SDL_INIT_VIDEO) != 0)
{
std::cerr << "SDL_Init() failed: " << SDL_GetError() << std::endl;
exit(EXIT_FAILURE);
}
SDL_DisplayMode videomode;
if (SDL_GetCurrentDisplayMode (0, &videomode) != 0)
{
std::cerr << "Error getting current display mode: " << SDL_GetError() << std::endl;
exit(EXIT_FAILURE);
}
std::cout << "Current screen mode: " << videomode.w << "x" << videomode.h << std::endl;
set_gl_attribute(SDL_GL_RED_SIZE, 5);
set_gl_attribute(SDL_GL_GREEN_SIZE, 6);
set_gl_attribute(SDL_GL_BLUE_SIZE, 5);
//set_gl_attribute(SDL_GL_DEPTH_SIZE, 8);
set_gl_attribute(SDL_GL_DOUBLEBUFFER, 1);
set_gl_attribute(SDL_GL_CONTEXT_MAJOR_VERSION, 2);
set_gl_attribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);
set_gl_attribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES);
SDL_Window* window = SDL_CreateWindow("Minimal SDL2 Example",
SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
720, 576,
SDL_WINDOW_OPENGL);
if (!window)
{
std::cerr << "Could not create window: " << SDL_GetError() << std::endl;
exit(EXIT_FAILURE);
}
SDL_GLContext gl_context = SDL_GL_CreateContext(window);
print_gl_string(GL_RENDERER);
print_gl_string(GL_SHADING_LANGUAGE_VERSION);
print_gl_string(GL_VERSION);
print_gl_string(GL_EXTENSIONS);
glClearColor(1.0, 0.0, 0.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT);
SDL_GL_SwapWindow(window);
SDL_Delay(5000);
SDL_GL_DeleteContext(gl_context);
SDL_DestroyWindow(window);
SDL_Quit();
return 0;
}

Answer 1

The problem turned out to not be in the code, but in the library path. A simple -L/opt/vc/lib/ added to the compile command line fixed it. Without that the compiler would pick:
/usr/lib/arm-linux-gnueabihf/libGLESv2.so.2
While the right one would be (use ldd to check):
/opt/vc/lib/libGLESv2.so