Drawing Colored Geometry in OpenGL using SDL

Question

First off, I will confess I have asked the same question on stackoverflow, but I think this forum might be a better fit.

I am trying to combine 2 things using SDL:

1. Draw a webcam feed via an SDL_Surface (from OpenCV).
2. Draw some plain colored geometry on top, using OpenGL.

The problem I have is that—I think—the geometry is textured by the screen texture that I draw the webcam feed in, even if I call glDisable(GL_TEXTURE_2D) right before drawing the quad. See the screenshot below, the square in the top-left is supposed to be white, but it seems to have the color of the bottom-right texel.

The code in my Display function is as follows:

// screen_surface_ contains a frame from the camera
SDL_UpdateTexture(screen_texture_, NULL, screen_surface_->pixels, screen_surface_->pitch);
SDL_RenderClear(renderer_);
SDL_RenderCopy(renderer_, screen_texture_, NULL, NULL);

glLoadIdentity();
glDisable(GL_TEXTURE_2D);
glColor3f(1.0, 1.0, 1.0);
glBegin( GL_QUADS );
  glVertex3f( 10.0f, 50.0f, 0.0f ); /* Top Left */
  glVertex3f( 50.0f, 50.0f, 0.0f ); /* Top Right */
  glVertex3f( 50.0f, 10.0f, 0.0f ); /* Bottom Right */
  glVertex3f( 10.0f, 10.0f, 0.0f ); /* Bottom Left */
glEnd( );

glColor3f(1.0, 1.0, 1.0);
SDL_RenderPresent(renderer_);

You can view all the code of relevant functions here.

I got a blanket answer saying "Don't mix SDL and OpenGL draw code"; as addressed in this bug report. But that would mean I'm simply stuck waiting for that bug to be fixed, so I'm still looking for a way to disable texturing after SDL_RenderCopy has been called.

Edit: I've confirmed that it's indeed still using the screen_texture_. Setting the texCoord to values between 0-100 shows a part of the webcam feed. (I tried 0-1 first but then I read that texCoords are different for rectangular images).

Answer 1

(Crosspost from stackoverflow)

I have fixed the problem by simply adding

glcontext_ = SDL_GL_CreateContext(window_);

to the SDL_Init code. I think all my calls to openGL functions (like glDisable(GL_TEXTURE_2D) were applied to the wrong context. Explicitly creating a context from SDL sets the right context to be active, I'm guessing.

Something else to look out for: when using textured geometry after using SDL_RenderCopy; I had to make sure to reset

glPixelStorei(GL_UNPACK_ROW_LENGTH, 0);

before calling glTexImage2d, because it uses

GL_UNPACK_ROW_LENGTH if it is greater than 0, the width argument to the pixel routine otherwise

(from the OpenGL docs)