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c++openglshadersdl

Why glDrawElments() is working without using any shader?

I'm trying to debug some shaders but I can't change the one currently loaded. I tried to run without loading any shader, or linking any program and it still working.

I already tried deleting completely the shaders from my HDD. I tried to just call glUseProgram (with any random number including 0) just before calling glDrawElements and it still work. And even if I load any shader it just doesn't make any effect. It still show linking and compile error if I make mistakes in the files but when run the executable it just ignores what is in the shaders.

I draw the vertex with this

void Renderer::renderFrame() {
    vao.bind();
    glUseProgram(0);
    glDrawElements(GL_LINE_LOOP, 3, GL_UNSIGNED_INT, nullptr);
}

and this are my window hints

void App::start() {
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 4);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 4);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);

    window = SDL_CreateWindow(title.c_str(), SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 500,500, SDL_WINDOW_RESIZABLE|SDL_WINDOW_OPENGL);
    this->context = SDL_GL_CreateContext(window);
    glewInit();
    glClearColor(0.5,1.0,1.0,1.0);

     renderer.init();
}
Solution

  • In case no valid shader is binded the default fixed function is usually used (you know GL 1.0 backward compatibility even on core profile sometimes depending on vendor/driver).

    So in case your attribute locations matches the used fixed function ones your CPU side code still renders image see:

    however the locations are not defined by any standard so it is different for any vendor (and can change with time/driver version). Only nVidia defined it and still using it after years...

    So its a good idea to check the GLSL compiler/linker log for any shader in development to avoid confusion ... For more info on how to obtain them see:

    btw some gfx drivers support logging and if enabled it will save the GLSL logs into a file on its own... tha can be done for example with nVidia drivers and NVEmulate utility