OpenGL primitives in Win32 window distorted according to window size

Question

I've made a simple OpenGL program in C that draws a triangle rotating inside a box. The triangle is supposed to be equilateral but even when the window width and height are the same the triangles vertices contract and extrude when they go along the width and length of the window. I can tell because I made a static equiangular box and the vertices of the triangle go outside of it. Here's a short clip of what it's doing: https://edwardseverinsen1717-gmail.tinytake.com/sf/NDg5NjQ0XzI2MDQzMDM Excuse the lag at some moments.

And here's my code:

#include <windows.h>
#include <gl/gl.h>
#include <gl/glu.h>

LRESULT CALLBACK WindowProc(HWND, UINT, WPARAM, LPARAM);
void EnableOpenGL(HWND hwnd, HDC*, HGLRC*);
void DisableOpenGL(HWND, HDC, HGLRC);
void DrawTriangle(float theta);
void DrawBox(void);

int WINAPI WinMain(HINSTANCE hInstance,
                   HINSTANCE hPrevInstance,
                   LPSTR lpCmdLine,
                   int nCmdShow)
{
    WNDCLASSEX wcex;
    HWND hwnd;
    HDC hDC;
    HGLRC hRC;
    MSG msg;
    BOOL bQuit = FALSE;
    float theta = 0.0f;
    int i;

    /* register window class */
    wcex.cbSize = sizeof(WNDCLASSEX);
    wcex.style = CS_OWNDC;
    wcex.lpfnWndProc = WindowProc;
    wcex.cbClsExtra = 0;
    wcex.cbWndExtra = 0;
    wcex.hInstance = hInstance;
    wcex.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground = (HBRUSH)GetStockObject(BLACK_BRUSH);
    wcex.lpszMenuName = NULL;
    wcex.lpszClassName = "GLSample";
    wcex.hIconSm = LoadIcon(NULL, IDI_APPLICATION);;


    if (!RegisterClassEx(&wcex))
        return 0;

    /* create main window */
    hwnd = CreateWindowEx(0,
                          "GLSample",
                          "OpenGL Sample",
                          WS_OVERLAPPEDWINDOW,
                          CW_USEDEFAULT,
                          CW_USEDEFAULT,
                          500,
                          500,
                          NULL,
                          NULL,
                          hInstance,
                          NULL);

    ShowWindow(hwnd, nCmdShow);

    /* enable OpenGL for the window */
    EnableOpenGL(hwnd, &hDC, &hRC);

    /* program main loop */
    while (!bQuit)
    {
        /* check for messages */
        if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
        {
            /* handle or dispatch messages */
            if (msg.message == WM_QUIT)
            {
                bQuit = TRUE;
            }
            else
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
        }
        else
        {
            /* OpenGL animation code goes here */

            glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
            glClear(GL_COLOR_BUFFER_BIT);

            DrawTriangle(theta);

            DrawBox();

            SwapBuffers(hDC);

            theta += 1.0f;
            Sleep (1);
        }
    }

    /* shutdown OpenGL */
    DisableOpenGL(hwnd, hDC, hRC);

    /* destroy the window explicitly */
    DestroyWindow(hwnd);

    return msg.wParam;
}

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    switch (uMsg)
    {
        case WM_CLOSE:
            PostQuitMessage(0);
        break;

        case WM_DESTROY:
            return 0;

        case WM_KEYDOWN:
        {
            switch (wParam)
            {
                case VK_ESCAPE:
                    PostQuitMessage(0);
                break;
            }
        }
        break;

        default:
            return DefWindowProc(hwnd, uMsg, wParam, lParam);
    }

    return 0;
}

void EnableOpenGL(HWND hwnd, HDC* hDC, HGLRC* hRC)
{
    PIXELFORMATDESCRIPTOR pfd;

    int iFormat;

    /* get the device context (DC) */
    *hDC = GetDC(hwnd);

    /* set the pixel format for the DC */
    ZeroMemory(&pfd, sizeof(pfd));

    pfd.nSize = sizeof(pfd);
    pfd.nVersion = 1;
    pfd.dwFlags = PFD_DRAW_TO_WINDOW |
                  PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
    pfd.iPixelType = PFD_TYPE_RGBA;
    pfd.cColorBits = 24;
    pfd.cDepthBits = 16;
    pfd.iLayerType = PFD_MAIN_PLANE;

    iFormat = ChoosePixelFormat(*hDC, &pfd);

    SetPixelFormat(*hDC, iFormat, &pfd);

    /* create and enable the render context (RC) */
    *hRC = wglCreateContext(*hDC);
    wglMakeCurrent(*hDC, *hRC);
}

void DisableOpenGL (HWND hwnd, HDC hDC, HGLRC hRC)
{
    wglMakeCurrent(NULL, NULL);
    wglDeleteContext(hRC);
    ReleaseDC(hwnd, hDC);
}

void DrawTriangle(float theta)
{
    glPushMatrix();

    if(theta != 0)
    {
        glRotatef(theta, 0.0f, 0.0f, 1.0f);
    }
    if(theta == 1)
    {
        glRotatef(89, 0.0f, 0.0f, 1.0f);
    }

    glBegin(GL_TRIANGLES);

        glColor3f(1.0f, 0.0f, 0.0f);   glVertex2f(0.0f,   0.50f);
        glColor3f(1.0f, 0.0f, 0.0f);   glVertex2f(0.45f,  -0.50f);
        glColor3f(1.0f, 0.0f, 0.0f);   glVertex2f(-0.45f, -0.50f);

    glEnd();

    glPopMatrix();
}

void DrawBox(void)
{
    glPushMatrix();

    glBegin(GL_LINE_LOOP);

        glColor3f(1.0f, 0.0f, 0.0f); glVertex2f(0.45f, 0.5f);      /*I decremented each x value by 0.05 to compensate for the bullshit with the window throwing off scale*/
        glColor3f(1.0f, 0.0f, 0.0f); glVertex2f(0.45f, -0.5f);
        glColor3f(1.0f, 0.0f, 0.0f); glVertex2f(-0.45f, -0.5f);
        glColor3f(1.0f, 0.0f, 0.0f); glVertex2f(-0.45f, 0.5f);

    glEnd();

    glPopMatrix();
}

Your triangle has sides of length 1.096586, 1.096586, and 0.9, so it is not equilateral. At the point where it is oriented straight up, you can see it fits inside the box (at 0:09 in your movie clip). After you rotate it from that position, of course it extends beyond the box! What reason do you have to believe it would remain in the box? — H. Guijt

BDL BDL · Accepted Answer · 2016-02-13T22:08:56

The coordinates you use do not span a equilateral triangle. Equilateral triangles always have three sides with the same length. In your example, the bottom side has a length of 0.9, but the other two have a length of sqrt(0.45^2 + 1.0^2) = 1.097. Even when assuming that you took 0.45 instead of 0.5, this is still not equilateral. (Bottom = 1.0, Other sides = sqrt(0.5^2 + 1.0^2) = 1.12)

As you have already noted, you have to compensate for the aspect ratio of the renderable area (which is not quadratic when passing 500x500 to CreateWindowEx since titlebar and borders are included here). This is normally done by defining a projection matrix with the correct aspect ratio of the client area. Have a look at GetClientRect to query the client area size and glOrtho to specify the projection matrix.

OpenGL primitives in Win32 window distorted according to window size

1 Answers