Texture mapping a circle made using GL_POLYGON

Question

I am trying to map a texture to a circle using GL_POLYGON using this code:

void drawCircleOutline(Circle c, int textureindex)
{
    float angle, radian, x, y;       // values needed by drawCircleOutline

    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, textureLib[textureindex]);

    glBegin(GL_POLYGON);

    for (angle=0.0; angle<360.0; angle+=2.0)
    {
        radian = angle * (pi/180.0f);

        x = (float)cos(radian) * c.r  + c.pos.x;
        y = (float)sin(radian) * c.r  + c.pos.y;

        glTexCoord2f(x, y);
        glVertex2f(x, y);
    }

    glEnd();
    glDisable(GL_TEXTURE_2D);
}

it looks like this when running.

And should look like this:

x and y you are passing into glTexCoord2f are being scaled by c.r and translated by c.pos, is this intended? Typical values in glTexCoord2f are 0.0->1.0 (although it can wrap, I'm making sure you meant to have potentially large values). — ClickerMonkey, Jan 06 '12 at 18:50
Just a suggestion: Instead of GL_POLYGON I'd use a GL_TRIANGLE_FAN, , the starting vertex being at (0,0) and texture coordinate (0.5, 0.5) – this will give you a (slightly) robuster texture perspective correction. Plus it's not deprecated. Also I suggest switching to vertex arrays. — datenwolf, Jan 07 '12 at 13:24

score 9 · Accepted Answer · edited Jan 06 '12 at 18:59

9

Try:

radian = angle * (pi/180.0f);

xcos = (float)cos(radian);
ysin = (float)sin(radian);
x = xcos * c.r  + c.pos.x;
y = ysin * c.r  + c.pos.y;
tx = xcos * 0.5 + 0.5;
ty = ysin * 0.5 + 0.5;

glTexCoord2f(tx, ty);
glVertex2f(x, y);

edited Jan 06 '12 at 18:59

Christian Rau

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answered Jan 06 '12 at 18:52

ClickerMonkey

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Thank you soo much! :) would never of thought to do that the math slightly caught me out. – Mike Tarrant Jan 06 '12 at 22:45

Texture mapping a circle made using GL_POLYGON

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