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MichaelNolan

OpenGL A new too OpenGL

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Hi, Why does OpenGL draw a triangle across 1/4 of the buffer? I just use this code.
  
    glBegin(GL_TRIANGLES);
        glVertex3f(0.0f,0.0f,1.0f);
        glVertex3f(1.0f,0.0f,1.0f);
        glVertex3f(1.0f,1.0f,1.0f);
    glEnd();
  
Best Regards, Michael

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Let me see if I can tackle this...

You are wondering why you see something like this:


.........
....*....
....**...
....***..
.........


when you wanted to see something like this:

.........
....*....
...***...
..*****..
.........


Well, openGL plotted exactly what you gave it.

Change your code from

glBegin(GL_TRIANGLES);
glVertex3f(0.0f,0.0f,1.0f);
glVertex3f(1.0f,0.0f,1.0f);
glVertex3f(1.0f,1.0f,1.0f);
glEnd();


to

glBegin(GL_TRIANGLES);
glVertex3f(-1.0f,0.0f,1.0f);
glVertex3f(1.0f,0.0f,1.0f);
glVertex3f(0.0f,1.0f,1.0f);
glEnd();

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Hey,
I see someting like this.
.....*....
.....***..
.....*****
..........
..........
When i use that code. I want to see a triangle the size of a pixel. I typed:
glBegin(GL_TRIANGLES);
glVertex3f(0.0f,0.0f,1.0f);
glVertex3f(1.0f,0.0f,1.0f);
glVertex3f(1.0f,1.0f,1.0f);
glEnd();
I would expect to see this.
..........
..........
....*.....
..........
..........
And if i did want to see what i did, i would type:
glBegin(GL_TRIANGLES);
glVertex3f(0.0f,0.0f,1.0f);
glVertex3f(width,0.0f,1.0f);
glVertex3f(width,height,1.0f);
glEnd();

I hope this helped to clear-up what i was asking.

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So you want the units of x, y in the vertex position to be one pixel each?

You need to use an orthographic projection instead of a persective one. Use
gluOrtho2D(-windowWidth/2, -windowHeight/2, windowWidth/2, windowHeight/2);

http://users.ox.ac.uk/~univ1234

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