Drawing round points using modern OpenGL

Question

I know how to draw round points using fixed pipeline. However I need to do the same using modern OpenGL. Is it possible, or should I use point sprites and textures?

For the interested.Here is how it is done with fixed pipeline:

        glEnable(GL_ALPHA_TEST);
    glAlphaFunc(GL_NOTEQUAL, 0);
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable( GL_POINT_SMOOTH );
    glPointSize( 8.0 );

    glMatrixMode(GL_PROJECTION);
    glLoadMatrixf(myMatrix);
    glMatrixMode(GL_MODELVIEW);
    glLoadMatrixf(myAnotherMatrix);

    glBegin(GL_POINTS);
    glColor3f(1,1,1);

    glVertex3fv(position);


    glEnd();
    glDisable(GL_POINT_SMOOTH);
    glBlendFunc(GL_NONE, GL_NONE);
    glDisable(GL_BLEND);

How did you draw round points using the fixed pipeline, though, if not using point sprites and textures? Dis you use `glPointSize` together with `GL_POINT_SMOOTH`? I don't think that has ever been a reliable method. — Christian Rau, Jun 24 '13 at 12:00

Christian Rau · Answer 1 · 2013-06-24T12:16:35.737

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One way would be to draw point sprites with a circle-texture and a self-made alpha test in the fragment shader:

uniform sampler2D circle;

void main()
{
    if(texture(circle, gl_PointCoord).r < 0.5)
        discard;
    ...
}

But in fact you don't even need a texture for this, since a circle is a pretty well-defined mathematical concept. So just check the gl_PointCoord only, which says in which part of the [0,1] square representing the whole point your current fragment is:

vec2 coord = gl_PointCoord - vec2(0.5);  //from [0,1] to [-0.5,0.5]
if(length(coord) > 0.5)                  //outside of circle radius?
    discard;

edited Jun 24 '13 at 12:16

answered Jun 24 '13 at 12:08

Christian Rau

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Why do I need self made alpha test?If the texture comes with alpha then hardware alpha blending should suffice. Shouldn't it? – Michael IV Jun 24 '13 at 12:19
@MichaelIV But alpha testing is deprecated and not part of the core profile anymore. Maybe alpha blending suffices for you, but usually alpha testing would be preferred if possible, since it doesn't pollute the depth buffer with "invisible" fragments (and might also be slightly faster, since `discard` is an early out). And like said, the method without a texture would be better anyway. – Christian Rau Jun 24 '13 at 12:32
@MichaelIV Imagine you want to draw many many opaque but round particles (yay, confetti time!). When using alpha testing instead of just alpha blending, then there is absolutely no need to depth-sort your particles. If the particles are transparent anyway, though, you would still need to sort them and use alpha blending, so there might really not be a functionality improvement from the `discard`. – Christian Rau Jun 24 '13 at 12:35
@ChristianRau: Alpha-testing alone (without alpha-blending) will result in jagged edges. – SigTerm Jun 24 '13 at 13:05
@SigTerm Right indeed, but there are solutions for this, too. Anything drawn to a discrete pixel grid will result in jagged edges. The solution for this is using multisampling, and together with alpha-to-coverage that will take care of this problem, too (you still need actual alpha, but no need for alpha blending and depth-sorting). – Christian Rau Jun 24 '13 at 13:12

score 7 · Accepted Answer · edited May 23 '17 at 12:34

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edited May 23 '17 at 12:34

Community

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answered Jun 24 '13 at 11:58

Cfr

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In fact I have asked if it's possible without doing vodoo in the shaders...But +1 for the links :) – Michael IV Jun 24 '13 at 12:08
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In modern OpenGL *nothing* is possible without doing *anything* in the shaders, though. I'd rather call the weird behaviour of `GL_POINT_SMOOTH` voodoo. – Christian Rau Jun 24 '13 at 12:12
"voodoo in the shaders" -- not how you should think about it, and not just with respect to drawing circles, either. Better get with the program, cuz this is how it gets done from here on out. Fixed function is *legacy* – Steven Lu Jun 24 '13 at 16:46
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while this answer can be useful , you should append more explanation in it because the extern links can be expired later and the answer become useless – Hassen Dhia Oct 15 '16 at 11:40

Drawing round points using modern OpenGL

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