Problem with large coordinates

OpenGL OpenGL: Advanced Coding
1

Hello!
I,ve problem with large coordinates! I’m writting simple 2D Cad in Borland C++ Builder, and I have something like this:

 
double startx,starty;
void __fastcall TForm2::FormCreate(TObject *Sender)
{
HDC dc = GetDC(Handle);

    PIXELFORMATDESCRIPTOR pfd = {
    	sizeof(PIXELFORMATDESCRIPTOR),
        1,
        PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL/* | PFD_DOUBLEBUFFER*/,
        PFD_TYPE_RGBA,
        24,
        0,0,0,0,0,0,
        0,0,
        0,0,0,0,0,
        32,
        0,
        0,
        PFD_MAIN_PLANE,
        0,
        0,0,
    };

    PixelFormat = ChoosePixelFormat(dc, &pfd);
    SetPixelFormat(dc, PixelFormat, &pfd);

HGLRC ogl = wglCreateContext(dc);

wglMakeCurrent(dc, ogl);

glMatrixMode(GL_PROJECTION);
startx = 5000000.0;starty = 5000000.0;
//startx = 0000000.0;starty = 0000000.0;
gluOrtho2D( startx, startx + 1.0, starty, starty + 1.0 );
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glViewport(0, 0, ClientWidth, ClientHeight);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
}
//------------------------------------------------
void __fastcall DrawScene(void)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(0.8f, 0.8f, 0.8f);
for( double l = 0.0; l < 1.0; l = l+.02 )
    {
    glBegin(GL_LINES);
        glVertex2d(startx,starty + l);
        glVertex2d(startx + 1.0,starty + l);
    glEnd();

    glBegin(GL_LINES);
        glVertex2d(startx + l,starty);
        glVertex2d(startx + l,starty + 1.0);
    glEnd();
    }
glFlush();
}

When startx = 5000000.0;starty = 5000000.0; DrawScene display
only 1 vertical and 1 horis. line.

When startx = 0.0;starty = 0.0; DrawScene display > 1 vert. lines and 1 > horis. lines and this is what I want.

What is wrong, when I set startx and starty to (5000000,5000000) ?

Thanks for reply, and sorry for my english

2

It appears your program is suffering from numerical inaccuracy.

The float representation does not have enough mantissa bits to represent small differences in large values.

You specify all the coordinates in doubles (which have the necessary accuracy) but these values are always converted to floats in OpenGL.

5000000.2 - 5000000.0 = 0 (32 bit float)
5000000.2 - 5000000.0 = 0.2 (64 bit double)

Greetz,
Nico

3

I understand. Thank you, Nico.