Why glDrawElments() is working without using any shader? - c++

Question

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();
}

Answer 1

SDL_GL_SetAttribute() only affects the next SDL_CreateWindow() call.
From the doc wiki:
Use this function to set an OpenGL window attribute before window creation.
So right now you're most likely getting a Compatibility context where shader-less draws are perfectly valid. You can check the value of GL_VERSION to see what you're getting.
If you want a Core context make those SDL_GL_SetAttribute() calls before your SDL_CreateWindow():
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);

Answer 2

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:
What are the Attribute locations for fixed function pipeline in OpenGL 4.0++ core profile?
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:
complete GL+GLSL+VAO/VBO C++ example
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