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Asked: 2026-06-04T06:31:01+00:00

I’ve got a textured 4 corner polygon that I’m currently drawing using two triangles

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I’ve got a textured 4 corner polygon that I’m currently drawing using two triangles and GL_TRIANGLE_STRIP. The right side needs to be taller, and this messes things up. There’s a noticeable difference where the triangles intersect in the middle, how do I clear something like this up?

I think I need some sort of affine transform, or to subdivide into even more triangles, but I’m not sure.

screenshot: https://i.stack.imgur.com/ehW84.png

example code:

float x = rect.origin.x;
float y = rect.origin.y;
float w = x + rect.size.width;
float h = y + rect.size.height;
V2fT2f quad[4] = {
    //x  y
    { x, y, 0, 1 },//bottom left
    { w, y-0.2, 1, 1 },//bottom right
    { x, h, 0, 0 },//top left
    { w, h+0.2, 1, 0 },//top right
};
glEnable(GL_TEXTURE_2D);
glEnableClientState(GL_VERTEX_ARRAY);
glBindTexture(GL_TEXTURE_2D,texID);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);glErrorCheck();
glVertexPointer  (2, GL_FLOAT, sizeof(V2fT2f), &quad[0].x);
glTexCoordPointer(2, GL_FLOAT, sizeof(V2fT2f), &quad[0].s);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
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1 Answer

  1. Editorial Team

    In any case to get that effect with a transformation – you will need a perspective projection set up.

    After you’ve done that, here are your vertices (these form a rectangle of width and height)

    1. X: centerX – halfWidth, Y: y – HalfHeight, Tu: 0, Tv: 0
    2. X: centerX – halfWidth, Y: y + HalfHeight, Tu: 0, Tv: 1
    3. X: centerX + halfWidth, Y: y + HalfHeight, Tu: 1, Tv: 1
    4. X: centerX + halfWidth, Y: y – HalfHeight, Tu: 1, Tv: 0

    Enclose this inside a glRotatef(angle, 0, 1, 0) – rotate by angle degrees around the Y axis

    and that should give you what you need.

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