OpenGL saving objects for later drawing

Question

I'm making a program using openGL with transparent objects in it, so obviously I have to paint those last. Unfortunately I was unaware of this requirement when beginning this project and now it would be a real pain to reorder it painting those at last.

I'm drawing objects by calling my drawing functions after translating and rotating the scene. There can be multiple translations and rotations before an actual drawing (e.g. first I draw the ground, then translate, then call the drawing of the house, which repeatedly translates and rotates, then calls the drawing of the walls and so on).

So my idea was saving the current modelview matrices in a list instead of painting the transparent objects when I normally would, then when I'm done with the opaque stuff, I iterate through my list and load each matrix and paint each object (a window, to be precise).

I do this for saving a matrix:

GLdouble * modelMatrix = (GLdouble *)malloc(16 * sizeof(GLdouble));
glGetDoublev(GL_MODELVIEW, modelMatrix);
addWindow(modelMatrix); // save it for later painting

And this is the "transparent stuff management" part:

/***************************************************************************
 *** TRANSPARENT STUFF MANAGEMENT ******************************************
**************************************************************************/

typedef struct wndLst {
    GLdouble * modelMatrix;
    struct wndLst * next;
} windowList;

windowList * windows = NULL;
windowList * pWindow;

void addWindow(GLdouble * windowModelMatrix) {
    pWindow = (windowList *)malloc(sizeof(windowList));
    pWindow->modelMatrix = windowModelMatrix;
    pWindow->next = windows;
    windows = pWindow;
}

void clearWindows() {
    while(windows != NULL) {
        pWindow = windows->next;
        free(windows->modelMatrix);
        free(windows);
        windows = pWindow;
    }
}

void paintWindows() {

    glPushMatrix(); // I've tried putting this and the pop inside the loop, but it didn't help either
        pWindow = windows;

        while(pWindow != NULL) {
            glLoadMatrixd(pWindow->modelMatrix);

            Size s;

            s.height = 69;
            s.width = 49;
            s.length = 0.1;

            glEnable(GL_BLEND);
            glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
            glDepthMask(GL_FALSE);

            glColor4f(COLOR_GLASS, windowAlpha);
            drawCuboid(s);

            glDepthMask(GL_TRUE);
            glDisable(GL_BLEND);

            pWindow = pWindow->next;
        }
    glPopMatrix();
}

/* INTERFACE
 * paint all the components, that are not yet painted,
 * then clean up.
 */
void flushComponents() {
    paintWindows();
    clearWindows();
}

/**************************************************************************/

I call flushComponents(); at the very end of my drawings.

The problem is, that the windows don't get in their place, instead I get weird-shaped blue objects randomly appearing and disappearing in my scene.

Am I doing something wrong? Or such matrix manipulations cannot even be used like this? Then what other method could I use for doing this?

Here is the full code if you need it: farm.zip Matrix-saving is at components.c line 1548, management is at line 142. It might not work on Windows without some minor hacking with the includes, which should probably be done in global.h.

Edit: I can only use C code and the glut library to write this program.

Edit 2: The problem is glGetDoublev not returning anything for some reason, it leaves the modelMatrix array intact. Though I still have no idea what causes this, I could make a workaround using bernie's idea.

Answer 1

OpenGL is not a math library. You should not use it for doing matrix calculations. In fact that part has been completely removed from OpenGL-3. Instead you should rely on a specialized matrix math library. That allows you to calculate the matrices for each object with ease, without jumping through the hoops of OpenGL's glGet… API (which was never meat for this kind of abuse). For a good replacement look at GLM: http://glm.g-truc.net/

Answer 2

Try adding glMatrixMode(GL_MODELVIEW) before your paintWindows() method. Perhaps you are not modifying the correct matrix.

The basic idea of your method is fine and is very similar to what I used for transparent objects. I would however advise for performance reasons against reading back the matrices from OpenGL. Instead, you can keep a CPU version of the current modelview matrix and just copy that to your window array.

As for your comment about push and pop matrix, you can safely put it outside the loop like you did.

edit

Strictly speaking, your method of rendering transparent objects misses one step: before rendering the list of windows, you should sort them back to front. This allows for overlapping windows to have the correct final color. In general, for 2 windows and a blending function:

blend( blend(scene_color, window0_color, window0_alpha), window1_color, window1_alpha )
!=
blend( blend(scene_color, window1_color, window1_alpha), window0_color, window0_alpha )

However, if all windows consist of the exact same uniform color (e.g. plain texture or no texture) and alpha value, the above equation is true (window0_color==window1_color and window1_alpha==window0_alpha) so you don't need to sort your windows. Also, if it's not possible to have overlapping windows, don't worry about sorting.

edit #2

Now you found something interesting with the erroneous matrix readback. Try it out with the following instead (you certainly don't need double precision):

GLfloat* modelMatrix = (GLfloat*)malloc(16 * sizeof(GLfloat)); // glass
glGetFloatv(GL_MODELVIEW, modelMatrix);
addWindow(modelMatrix); // save it for later painting

If that still doesn't work, you could directly store references to your houses in your transparent object list. During the transparent rendering pass, re-render each house, but only issue actual OpenGL draw calls for the transparent parts. In your code, putWallStdWith would take another boolean argument specifying whether to render the transparent or the opaque geometry. This way, your succession of OpenGL matrix manipulation calls would be redone for the transparent parts instead of read using glGetxxx(GL_MODEL_VIEW).

The correct way to do it however is to do matrix computation on the CPU and simply load complete matrices in OpenGL. That allows you to reuse matrices, control the operation precision, see the actual matrices easily, etc.