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Terrain: Contour Lines using pixel shader

Started by mr_hell Apr 2, 2009 at 4:20 AM 6 replies 39.3k views

Original Post

Apr 2, 2009 at 4:20 AM

I have to draw x, y, z level lines on my relief surface. 2 soluttions: 1. Using of additional geometry (lines). Major disadvantage - i need time to calculate this lines. 2. Using pixel shader.


float4 TestP1(PS_INPUT Input): COLOR0
{
        float4 Color;
	int z = ceil(Input.Position.y);
	int y = ceil(Input.Position.z);
	int x = ceil(Input.Position.x);
	Color = Input.Color;

	if(abs(fmod(z, 100))<1.0)
	{
		Color.b = 1.0f;
		Color.r = Color.g = 0.0f;
	}
	if(abs(fmod(x, 100))<1.0)
	{
		Color.b = 1.0f;
		Color.r = Color.g = 0.0f;
	}
	if(abs(fmod(y, 100))<1.0)
	{
		Color.r = 1.0f;
		Color.b = Color.g = 0.0f;
	}

	return Color;
}

But on this way i have troubles: first far

and second near

Any ideas, how to get "good" level lines in shader? [Edited by - mr_hell on April 2, 2009 6:31:04 AM]

Trurl

Apr 2, 2009 at 10:52 AM

You can accomplish this with screen-space derivatives. Instead of drawing a band of color over a range of contours, you draw a band of color over a range of pixels near the contour value. Here's an example in GLSL, but it should port easily.

This vertex shader simply passes along the position in a varying variable.

varying vec3 k;void main(){    k = gl_Vertex.xyz;    gl_Position = ftransform();}

For reference, here's a GLSL fragment shader that does something similar to your approach. It uses smoothstep to get anti-aliased contour lines.

varying vec3 k;void main(){    vec3 f  = fract (k * 100.0);    vec3 df = fwidth(k * 100.0);    vec3 g = smoothstep(0.05, 0.10, f);    float c = g.x * g.y * g.z;    gl_FragColor = vec4(c, c, c, 1.0);}

Here's what this looks like applied to the head of the famous dragon. It has the same results that you see: lines that are too narrow in some places and too wide in others.

Now here's a fragment shader that uses screen-space derivatives to accomplish the same task. It uses smoothstep to blend between distances of 1 and 2 pixels from the desired contour value.

varying vec3 k;void main(){    vec3 f  = fract (k * 100.0);    vec3 df = fwidth(k * 100.0);    vec3 g = smoothstep(df * 1.0, df * 2.0, f);    float c = g.x * g.y * g.z;    gl_FragColor = vec4(c, c, c, 1.0);}

The result is a set of contour lines of consistent width everywhere.

PolyVox

Apr 2, 2009 at 12:32 PM

I just happened to stumble across this and it's a nice approach! I will try and remember it for the future.

knighty

Apr 2, 2009 at 12:39 PM

Nice!

A little modification to get smoother result :)

uniform float gsize;uniform float gwidth;varying vec3 P;void main(){	vec3 f  = abs(fract (P * gsize)-0.5);	vec3 df = fwidth(P * gsize);	vec3 g = smoothstep(-gwidth*df,gwidth*df , f);	float c = g.x * g.y * g.z; 	gl_FragColor = vec4(c, c, c, 1.0);// * gl_Color;}

Edit:
another little modification 8^)

uniform float gsize;//size of the griduniform float gwidth;//grid lines'width in pixelsvarying vec3 P;void main(){	vec3 f  = abs(fract (P * gsize)-0.5);	vec3 df = fwidth(P * gsize);	float mi=max(0.0,gwidth-1.0), ma=max(1.0,gwidth);//should be uniforms	vec3 g=clamp((f-df*mi)/(df*(ma-mi)),max(0.0,1.0-gwidth),1.0);//max(0.0,1.0-gwidth) should also be sent as uniform	float c = g.x * g.y * g.z;	gl_FragColor = vec4(c, c, c, 1.0);	gl_FragColor = gl_FragColor * gl_Color;}

[Edited by - knighty on April 3, 2009 10:39:31 AM]

Trurl

Apr 2, 2009 at 3:18 PM

Oh yeah I totally screwed up the antialiasing on that. Not sure what I was thinking. It's still aliased on one side. We'd need two smoothsteps to make it perfect. You get the idea, at least.

mr_hell

Apr 2, 2009 at 3:22 PM

Trurl
yessss, it works, thank you.

mr_hell

Apr 3, 2009 at 1:28 AM

How to rewrite this shader for SM 2.0? (does not support ddx ddy operations - i have to emulate gradiants by myself).
its necessary for me!

[Edited by - mr_hell on April 3, 2009 8:28:30 AM]

rob35

Mar 28, 2012 at 9:42 PM

How to rewrite this shader for SM 2.0? (does not support ddx ddy operations - i have to emulate gradiants by myself).
its necessary for me!

[Edited by - mr_hell on April 3, 2009 8:28:30 AM]

Yes, how do we get this to work on HLSL SM 2.0 cards? There's no ddx/ddy functions in any graphics cards but the new super-duper ones that many people don't have. It is so frustrating to think that it is impossible to have real contour lines without a SM 3.0 card! If anyone could figure out how to implement this on SM 2.0, they'd be a genius, but it seems that it is impossible unless you have the fancy ddx/ddy functions.

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