Anti-Aliased Lines

[PickleWorld]

So I have an application with a wireframe type draw and I was wondering if anyone has any useful tips/hints on doing really good line Anti-Aliasing with basic OpenGL?

Currently I...

1) Enable GL_MULTISAMPLE.
2) Enable GL_LINE_SMOOTH.
3) Set glLineWidth() to 0.5
4) Enable GL_BLEND (with standard blend function)
5) Set glDepthMask to FALSE
6) Set glHint to GL_NICEST

That is the basic stuff you can find in the RedBook. Anyone else have any suggestions for improving things?

[overlay]

If you enable GL_MULTISAMPLE (and you created a context such that when you query the value of GL_SAMPLE_BUFFERS, it returns 1),
GL_LINE_SMOOTH is ignored and multisampling rasterization is used.

ref: spec 2.1 page 107, section 3.4.4 Line Multisample Rasterization.

http://www.opengl.org/registry/doc/g...1.20061201.pdf

so 2) and 4) are useless in this case.

For multisampling, I'm not sure that you want to set depth mask to FALSE.

[PickleWorld]

If you enable GL_MULTISAMPLE (and you created a context such that when you query the value of GL_SAMPLE_BUFFERS, it returns 1),
GL_LINE_SMOOTH is ignored and multisampling rasterization is used.

Did not know that, thanks for the reply.

[remdul]

2) Enable GL_LINE_SMOOTH.
3) Set glLineWidth() to 0.5
4) Enable GL_BLEND (with standard blend function)
5) Set glDepthMask to FALSE
6) Set glHint to GL_NICEST

...is be enough for just anti-aliased lines.

GL_MULTISAMPLE is for full anti-aliasing including solid triangles, and you must set up a multisampled framebuffer before it works.

[PickleWorld]

...is be enough for just anti-aliased lines.

Yes and no. The line are anti-aliased but the quality is not all that good, especially when you start drawing lines over other lines (you get bleeding and bluring).

GL_MULTISAMPLE is for full anti-aliasing including solid triangles, and you must set up a multisampled framebuffer before it works.

Yep, I have the framebuffer set up. I have it working, I was just wondering if there were things I could do to improve the quality.

Right now, it looks "ok" but not "great"

[Ilian Dinev]

Draw wide lines (2px or 4px) with a shader, that uses gl_FragCoord.xy and writes an opacity value in gl_FragColor.a

[PickleWorld]

Draw wide lines (2px or 4px) with a shader, that uses gl_FragCoord.xy and writes an opacity value in gl_FragColor.a

How do I use gl_FragCoord.xy?

How does the x,y window relative coordinates of a given fragment help me determine if I am near the edge of a line (which I assume I nned to know to set the alpha value?)?

[Ilian Dinev]

search this board for it. There are two implementations, one by me that uses gl_FragCoord, and one that uses geometry shaders.

[Hampel]

but note, that there are still issues with ATI and gl_FragCoord and FBOs...

[PickleWorld]

So I found the following vertex shader and fragment shader on the forums but they don't seem to work?

varying vec4 posix;
void main(){
	posix = ftransform();
	gl_Position=posix + vec4(0,0,0,0.001);
}

varying vec4 posix;
void main(){
	vec4 FinalColor = vec4(0.0,0.0,0.0,1.0);
 
	vec2 ScreenSizeHalf = vec2(1680/2,1050/2);
	vec2 posix1= ((posix.xy/posix.w)+1.0)*ScreenSizeHalf;
	vec2 fpos = gl_FragCoord.xy;
	float dist = distance(fpos,posix1);
	FinalColor.w = 1.0-dist;
	gl_FragColor = FinalColor;
}

I messed around with the code a little and it appears that the distance is always larger than 2.0? (I switched the second last line to "FinalColor.r = dist-2.0;" and the lines were still always red).

What am I doing wrong?

p.s. I am running a Nvidia Quadro FX570.
p.p.s My monitor is a Dell widescreen running at 1680x1050.