glViewport size not changing

Question

I have two viewports that are currently display 4 points in each of their corners. One viewport (left side) has gray corners, the other (right) has red corners. The two viewports are placed next to each other, with the red viewport intentionally made smaller. The viewport with red corners is also drawing a GL_TRIANGLES.

You can see them here:

The points in the triangle should be in the bottom left corner, top left corner and bottom right corner of the red viewport, but the bottom right corner is significantly short.

I think the second (red) glViewport is still using measurements from a previous object, the other viewport probably.

Am I setting up the second viewport correctly? I want to have the second viewport 100 pixels across, not 900 pixels, squeezed into 100 pixels.

Here is my code. I am using the latest version of glut and currently still learning how to use OpenGL/glut.

// windows/pannels
int winWidth = 900; // window width
int winHeight = 600; // window height

int pannelToolsWidth = 100;
int pannelToolsHeight = winHeight;
int pannelToolsX = winWidth - pannelToolsWidth;
int pannelToolsY = 0;

int drawSpaceWidth = winWidth - pannelToolsWidth;
int drawSpaceHeight = winHeight;

// drawing
float red = 0.0f;
float green = 0.0f;
float blue = 0.0f;
float lineSize = 1.0f;
float pointSize = 1.0f;

// ...

// draws a pannel. Make sure to define a colour before calling.
void drawPannel(int x, int y, int width, int height)
{   
    glBegin(GL_TRIANGLES);
        glVertex2f(float(x), float(y));
        glVertex2f(float(x), float(height));
        glVertex2f(float(width), 0.0);
glEnd();
    //glBegin(GL_TRIANGLES);
    //  glVertex2f(, );
    //  glVertex2f(, );
    //  glVertex2f(, );
    //glEnd();
}

// Starts a new glViewport and visualises the bounderies with coloured points.
// scene: id of the viewport (viewports draw different things)
// x/y: viewports draw from the bottom left corner
void drawViewport(int x, int y, int width, int height, float red, float green, float blue, int scene)
{
    // draw viewport (colours used to visualise limits)
    glViewport(x, y, width, height);
    glColor3f(red, green, blue);
    glBegin(GL_POINTS);
        glVertex2f(0.0f, float(winHeight));
        glVertex2f(float(winWidth), float(winHeight));
        glVertex2f(0.0f, 0.0f);
        glVertex2f(float(winWidth), 0.0f);
    glEnd();

    // draw viewport content. text methods should be called last because the glRasterPos2i() effects the viewport
    switch (scene)
    {
        // add more cases for other viewports here.
    case 0:
        // colour
        // drawing
        // text
        break;
    case 1:
        glColor3f(0.8, 0.8, 0.7);
        drawPannel(0, 0, pannelToolsWidth, pannelToolsHeight);
        break;
    default:
        // do nothing 
        break;
    }
}

void init()
{
    glClearColor(1.0f, 1.0f, 1.0f, 1.0f); //set clear background colour

    glPointSize(10.0);                     //set point size
    glColor3f(1.0f, 1.0f, 1.0f);          //set draw colour
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0, winWidth, 0, winHeight);           //setup 2D projection
}

void collectScene()
{
    glClear(GL_COLOR_BUFFER_BIT);

    // viewport(s)
    drawViewport(0, 0, drawSpaceWidth, drawSpaceHeight, 0.5, 0.5, 0.5, 0); // paint area
    drawViewport(pannelToolsX, pannelToolsY, pannelToolsWidth, pannelToolsHeight, 1, 0, 0, 1); // GUI area

    glFlush();
    glutSwapBuffers();     //swap front and back buffers
}

//called when no event in queue
void idle()
{
    glutPostRedisplay();
}

int main()
{
    // create the window
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA); // double buffer with RGBA
    glutInitWindowSize(winWidth, winHeight);
    glutInitWindowPosition(100, 100); // from top left
    glutCreateWindow("Paint");
    init();
    glutDisplayFunc(collectScene);

    glutIdleFunc(idle);
    glutMainLoop();
    return 0;
}

Thank you

Answer 1

Projection matrix is relative to the viewport, indeed it is usually set after glViewport call. In your case it doesn't change.