Opengl: 2d HUD over 3D

Question

I have looked at some questions posted here on the matter and still cant work out why my 2d HUD appears but makes my 3d Rendered world disappear.

EDIT: It seems that the 2d scene is taking control of the entire screen so every now and then I can see the 3d scene glitching through the 2d scene. So even though I its only ment to be rendering a quad thats 10 x 10 pixels it renders this then blanks out the rest of the screen.

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(40.0,(GLdouble)x/(GLdouble)y,0.5,20.0);
glMatrixMode(GL_MODELVIEW);
glViewport(0,0,x,y);
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glTranslatef(0.0,-0.5,-6.0);

glPushMatrix();

..Draw some 3d stuff...

glPopMatrix();
// Start 2d
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, SCREEN_WIDTH, SCREEN_HEIGHT, 0, -1, 1);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glColor3f(0.0f, 255.0f, 1.0f);
glBegin(GL_QUADS);
    glVertex2f(0.0, 0.0);
    glVertex2f(10.0, 0.0);
    glVertex2f(10.0, 10.0);
    glVertex2f(0.0, 10.0);
glEnd();

Then I swap buffers

Here is the order of my code. Its like it makes the 3d space then makes the 2d space which in turn cancels out the 3d space.

What's the difference between `SCREEN_WIDTH` and `x` (used in `glViewport`)? If you comment out the last six lines, but leave the 2-D setup in place, does the 3-D rendering still disappear? Can you narrow it down to one line or command that interferes with the 3-D part? — Ben Voigt, Mar 29 '11 at 03:51
@Ben Voigt: If i comment out the last 6 lines(starting at glbegin()) the 3d rendering still doesn't appear. SCREEN_WIDTH and X are the same, I made two vars without realizing and haven't tidied it up yet. — Elgoog, Mar 29 '11 at 03:58
Ok, keep commenting 2-D setup code out until the 3-D world comes back. Either a line at a time or use a binary search to divide and conquer. Probably you're changing OpenGL state that the 3-D code assumed was left at its default, like texturing or a shader or something like that. — Ben Voigt, Mar 29 '11 at 03:59
@Ben Voigt: If I comment out from the 2nd `glMatrixMode(GL_PROJECTION);` the 3d render displays — Elgoog, Mar 29 '11 at 04:27

Elgoog · Accepted Answer · 2011-03-29T07:35:19.427

37

Took a little while to figure it out, so just in case others have the same issues:

    ...After Drawing 3d Stuff...

glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(0.0, SCREEN_WIDTH, SCREEN_HEIGHT, 0.0, -1.0, 10.0);
glMatrixMode(GL_MODELVIEW);
//glPushMatrix();        ----Not sure if I need this
glLoadIdentity();
glDisable(GL_CULL_FACE);

glClear(GL_DEPTH_BUFFER_BIT);

glBegin(GL_QUADS);
    glColor3f(1.0f, 0.0f, 0.0);
    glVertex2f(0.0, 0.0);
    glVertex2f(10.0, 0.0);
    glVertex2f(10.0, 10.0);
    glVertex2f(0.0, 10.0);
glEnd();

// Making sure we can render 3d again
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
//glPopMatrix();        ----and this?

...Then swap buffers...

:)

edited Mar 29 '11 at 07:35

answered Mar 29 '11 at 06:50

Elgoog

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Please `pop` in inverse `push`-order. Probably those matrix stacks are separated, but it can't hurt anyway. Also I think `pushMatrix` is unnecessary in this case. – Jonas Bötel Mar 29 '11 at 07:28
@LumpN: Im not sure what you ment by `pop` in inverse `push`-order – Elgoog Mar 29 '11 at 07:35
I think he means that the commented-out `glPopMatrix()` should go before the project matrix reset. – geofftnz Mar 29 '11 at 21:40
Might be better to use gluOrtho2D() instead of glOrtho() for a 2D HUD. – Tobias Reich Dec 13 '14 at 11:07
Does someone know how to also go from lighting in 3D to colors in 2D? – Bitterblue Dec 08 '16 at 06:52

score 7 · Answer 2 · answered Mar 29 '11 at 03:38

If you're overlaying a 2D ortho projection over 3D, you generally want to get the depth buffer out of the equation:

glDepthMask(GL_FALSE);  // disable writes to Z-Buffer
glDisable(GL_DEPTH_TEST);  // disable depth-testing

Of course, you'll want to reset these to their original values before doing your next 3D pass.

score 3 · Answer 3 · answered Nov 14 '11 at 09:28

glViewport(0, 0, x, y); //You need to do this only once on viewport resize

//Setup for 3D
glMatrixMode(GL_PROJECTION);
glLoadIdentity;
gluPerspective(40.0, (GLdouble)x/(GLdouble)y, 0.5, 20.0);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity;

glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BIT);

// ... Render 3D ...

//Setup for 2D
glMatrixMode(GL_PROJECTION);
glLoadIdentity;
glOrtho(0, SCREEN_WIDTH, SCREEN_HEIGHT, 0, -1, 1);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity;

glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BIT);

// ... Render 2D ...

SwapBuffers;

Note that there's no need to handle Push/Pop of matrixes if you render 2D completely on top of 3D.

Opengl: 2d HUD over 3D

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