Question

I am trying to implement some simple object picking in my application, but running into problems from the start. For testing I am trying to use what is given in this tutorial: OpenGL Programming/Object selection

In my code I now do the following:

GLbyte color[4];
GLfloat depth;
GLuint index;
glReadPixels(click->x, WINDOW_HEIGHT - click->y - 1, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, color);
glReadPixels(click->x, WINDOW_HEIGHT - click->y - 1, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &depth);
glReadPixels(click->x, WINDOW_HEIGHT - click->y - 1, 1, 1, GL_STENCIL_INDEX, GL_UNSIGNED_INT, &index);

printf("Clicked on pixel %d, %d, color %02hhx%02hhx%02hhx%02hhx, depth %f, stencil index %u\n", click->x, click->y, color[0], color[1], color[2], color[3], depth, index);

which give me the follwing output:

No values for RGBA or stencil index. I only have 1 object where I have put glStencilFunc(GL_ALWAYS, 99, -1);, so I am not expecting to see a stencil index for most places. For the colors however I did not think anything extra would be required?

For debugging here are some core parts of my OGL initialization code:

glEnable(GL_DEPTH_TEST);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_MULTISAMPLE);

glEnable(GL_STENCIL_TEST);
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);

and SDL initialization code:

   SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
   SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
   SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
   SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);
   SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
   SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
   SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
   SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 4);
   SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8); 
   window = SDL_CreateWindow(windowTitle.c_str(), SDL_WINDOWPOS_CENTERED, 
    SDL_WINDOWPOS_CENTERED, width, height, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN);

From this information is it possible to extract why my glReadPixels is not reporting for example RGBA values?

Answer 1

Found the solution to this problem…

It was swapping the buffer before running the glReadPixels.