Thank you for your help!

Frank

As for other ways of creating the trails, you just need some kind of buffer that you can use for temporary storage. In essence, we just need somewhere to store a copy of what was on the screen so we can:
- Clear the screen
- Draw what used to be there but with a little less opacity
- Draw the elements of our new frame on top of it

Probably the best way to go about this would be to copy to and from a Frame Buffer Object, but FBOs can be tricky if you’ve not worked with them before (which I haven’t, although I’ll be learning them soon!).

Another alternative might be to NOT clear the screen, but instead draw a full-screen quad with low opacity over the top of everything just before drawing your new frame… I thought this would work, but wasn’t 100% sure – but guess what? It does =D

Here’s the important stuff to make it happen:

// Function to set some initial OpenGL state-machine properties
void initGL()
{
    glfwSwapInterval(1); // Lock to vertical sync of monitor (normally 60Hz, so 60fps)
 
    // ----- Window and Projection Settings -----
 
    // Set the window title
    glfwSetWindowTitle("GLFW Fireworks with Trails");
 
    // Setup our viewport to be the entire size of the window
    glViewport(0, 0, (GLsizei)windowWidth, (GLsizei)windowHeight);
 
    // Change to the projection matrix, reset the matrix and set up orthagonal projection (i.e. 2D)
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0, windowWidth, windowHeight, 0, 0, 1); // Parameters: left, right, bottom, top, near, far
 
    // Disable depth testing (because we're working in 2D!)
    glDisable(GL_DEPTH_TEST);
 
    // Enable blending (we need this to be able to use an alpha component)
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
 
    glEnable(GL_POINT_SMOOTH); // Smooth the points so that they're circular and not square
 
    glPointSize(5.0f);
}
 
void drawBackground()
{
	// The alpha setting for this (the fourth parameter) is what gives us the trails. The lower the value, the longer the trails!
	glColor4f(0.0f, 0.0f, 0.0f, 0.2f);
 
	// Draw the background
	// REMEMBER: Window co-ordinates have the origin at the top left.
	glBegin(GL_QUADS);
            glVertex2i(0,           windowHeight);
            glVertex2i(windowWidth, windowHeight);
            glVertex2i(windowWidth, 0);
            glVertex2i(0,           0);
	glEnd();
}
 
// Function to draw our OpenGL scene
void drawScene()
{
    // Set ModelView matrix mode and reset to the default identity matrix
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
 
    // Displacement trick for exact pixelisation
    glTranslatef(0.375, 0.375, 0);
 
    // Draw a translucent black quad over the entire scene
    drawBackground();
 
    // Draw our new firework frame...
    for (int loop = 0; loop < FIREWORKS; loop++)
    {
        for (int particleLoop = 0; particleLoop < FIREWORK_PARTICLES; particleLoop++)
        {
 
            // Set the point size of the firework particles (this needs to be called BEFORE opening the glBegin(GL_POINTS) section!)
            glPointSize(fw[loop].particleSize);
 
            glBegin(GL_POINTS);
                // Set colour to yellow on the way up, then whatever colour firework should be when exploded
                if (fw[loop].hasExploded == false)
                {
                    glColor4f(1.0f, 1.0f, 0.0f, 1.0f);
                }
                else
                {
                    glColor4f(fw[loop].red, fw[loop].green, fw[loop].blue, fw[loop].alpha);
                }
 
                // Draw the point
                //glColor4f(1.0f, 1.0f, 1.0f, 1.0f);
                glVertex2f(fw[loop].x[particleLoop], fw[loop].y[particleLoop]);
            glEnd();
        }
 
        // Move the firework appropriately depending on its explosion state
        if (fw[loop].hasExploded == false)
        {
            fw[loop].move();
        }
        else
        {
            fw[loop].explode();
        }
    }
 
    // ----- Stop Drawing Stuff! ------
 
    glfwSwapBuffers(); // Swap the buffers to display the scene (so we don't have to watch it being drawn!)
 
} // End of drawScene function

Another thing you might want to try is blurring the entire scene before drawing your new frame, which’ll give you an effect like this: Per-Pixel Collisions (uses blur filter on the stage instead of clearing it), although I’ve not used any blur filters in OpenGL yet, so you’d be on your own there.

Anyways, hope this helps!

I just tried adding the code and I get all zeros.
At work I spoke to our graphics guy and immediately after I mentioned that I have an ATI card he said he knows the problem.
We use nVidia quadro cards at work because they have the best openGL support. According to him even the ATI FirePro cards have rather sub-par openGL support, never mind the consumer ones.
I had a play with an updated card driver version, but it did not change anything.

However, after knowing this I just made an experiment and undusted my old laptop from the medieval era, which has a nVidia geforce go card. It plays the fireworks with about 1fps (tops), but there are nice trails.

So I fear I am stuck without trails, unless there is a way to get these effects without accumulation buffer?

Thanks
Frank

I used the very same executable on all machines (built at home, imagine my surprise when I started it here and it looked different).

However, I think you’re right that I don’t have an accumulation buffer, as when I tried to change
glClearAccum(0.0f, 0.0f, 0.0f, 1.0f);
at home nothing changed, whereas here at work I get a nice colored background.

Can it be that ATI does not support accumulation buffers on consumer cards? (I found http://forums.amd.com/game/messageview.cfm?catid=260&threadid=123808 which seam to have a similar problem).

But I’ll try adding your code snippet just to be sure.

Thanks
Frank

That’s pretty odd – try pasting the following code at the bottom of the setupScreen function to check that you’re getting an accumulation buffer created:

// Check accumulation buffer bit-depths
int AccRed, AccGreen, AccBlue, AccAlpha;
SDL_GL_GetAttribute(SDL_GL_ACCUM_RED_SIZE,   &AccRed);   // Size of the accumulation buffer red component, in bits.
SDL_GL_GetAttribute(SDL_GL_ACCUM_GREEN_SIZE, &AccGreen); // Size of the accumulation buffer green component, in bits.
SDL_GL_GetAttribute(SDL_GL_ACCUM_BLUE_SIZE,  &AccBlue);  // Size of the accumulation buffer blue component, in bits.
SDL_GL_GetAttribute(SDL_GL_ACCUM_ALPHA_SIZE, &AccAlpha); // Size of the accumulation buffer alpha component, in bits.
 
cout << "Accum. Red   : " << AccRed   << endl;
cout << "Accum. Green : " << AccGreen << endl;
cout << "Accum. Blue  : " << AccBlue  << endl;
cout << "Accum. Alpha : " << AccAlpha << endl;

When I add this code I get 16 bits each for Red, Green, Blue and Alpha – so try it on the machines where the trails don’t work to make sure you’re actually getting an accumulation buffer on ‘em.

Also, try the exact same executable that definitely works on your work machine on your home machines to rule out any code issues.

Failing all that, I moved this code over from SDL to GLFW the other day but hadn’t got around to adding that code, but now that I have maybe you’ll have more luck with that =D

Let me know how you get on =D

At home I tried it at three different computer and none had any trails, not even when I just copied your code. However, now I tried it at work and here everything is as it should be, with nice trails behind the rockets.

Is there any setting that controls this that I did not activate at home? The only obvious difference that I can make out between these PCs is the OS (Win7Home vs. Win7Pro) and of course some hardware (ATI Radeon card vs. nVidia Quadro FX).

Thanks
Frank

I’m not in the least bit angry – in fact, quite the opposite! I’m really glad that you found the code useful, and I put the source code up in the first place in the hope that it would be.

Thanks for taking the time to post a comment, and for including me/this-post in your modified source. I hope that when you’ve finished it you’ll come back and post a link to the completed game.

P.S. I used to play Nebulus back on the Spectrum when I was a kid (and then later on the Amiga) – great game! I hope you include the helloiamjmp cheat! =D