Legacy OpenGL Display Issue

Question

I am using Legacy OpenGL to make a fish tank:

#include <GL/glut.h>
#include <stdio.h>

float fishPosX = 0.0f;
float fishDirection = -1.0f;
float rotationAngle = 0.0f;
int rotating = 0;

void setupGraphics() {
    glEnable(GL_DEPTH_TEST); // Enable depth testing
    glDisable(GL_CULL_FACE); // Disable back-face culling for debugging
    glDisable(GL_LIGHTING); // Disable lighting temporarily for visibility
    glClearColor(0.0, 0.0, 0.0, 1.0); // Black background for contrast
    glLineWidth(3.0); // Increase wireframe visibility
}

void display() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    
    printf("Rendering frame. Fish position: %f, Rotation: %f\n", fishPosX, rotationAngle);

    // Draw large fish
    glPushMatrix();
    glTranslatef(fishPosX, 0.0f, -400.0f);
    glRotatef(rotationAngle, 0.0f, 1.0f, 0.0f);
    glutWireOctahedron();
    glPopMatrix();
    
    glutSwapBuffers();
}

void update(int value) {
    if (!rotating) {
        fishPosX += 5.0f * fishDirection;
        if (fishPosX < -396.0f || fishPosX > 396.0f) {
            rotating = 1;
        }
    } else {
        rotationAngle += 5.0f * fishDirection;
        if (rotationAngle >= 180.0f || rotationAngle <= -180.0f) {
            rotationAngle = 0.0f;
            fishDirection *= -1.0f;
            rotating = 0;
        }
    }
    glutPostRedisplay();
    glutTimerFunc(50, update, 0);
}

void reshape(int w, int h) {
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-400, 400, -400, 400, -900, -100); // Ensure correct projection setup
    glMatrixMode(GL_MODELVIEW);
}

void keyboard(unsigned char key, int x, int y) {
    if (key == 'q' || key == 'Q') {
        printf("Exiting simulation.\n");
        exit(0);
    }
}

int main(int argc, char** argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize(800, 800);
    glutCreateWindow("Fish Tank Simulation");
    setupGraphics();
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutKeyboardFunc(keyboard);
    glutTimerFunc(50, update, 0);
    glutMainLoop();
    return 0;
}

It seems the fish are being created but I just cannot see them. What is causing my fish to not be displayed in the canvas? Maybe it's a camera positioning issue?

I have tried using transforms to move the camera, didn't work. I have tried altering colors, didn't work. I tried gluLookAt, which errors out the program for some reason. I tried displaying a green square to the middle of the screen, and when that didn't work I realized it may be a camera placement issue or something of the sort.

"It seems the fish are being created but I just cannot see them." -- Should be "it" not "them". One fish. Nothing more. No rotation, translation, keyboard input, or the like. Reduce your code to the bare minimum to show something (one thing; maybe your green square instead of the fish). If that works, then add things back one at a time until the fish disappears and focus on the last thing added back. (If the bare minimum does not show your fish, then you've got a much simpler minimal reproducible example.) — JaMiT
– JaMiT, Commented Apr 3 at 3:52

rafix07 · Accepted Answer · 2025-04-03 04:31:34Z

-1

Default viewer position is (0,0,0). It looks with direction (0,0,-1). In glOrtho you specify two distances to near and far clipping planes. When these values are positive, the view volume lies in front of viewer (along the direction vector). If these values are negative, the view cuboid lies behind viewer. In your case the latter happens. You have defined the view volume where clipping planes have Zcoords 900 and 100, but your object has Zcoord -400, so it is not strange that you see nothing.

Stay with the view volume you defined, but then translate object by 400, to place it in the view volume:

glTranslatef(fishPosX, 0.0f, 400.0f);

Or call

glOrtho(-400, 400, -400, 400, 100, 900);

to define the view volume at -100 and -900 Zcoords respectively.

answered Apr 3 at 4:31

rafix07

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