Shaders + fog

Graphics and GPU Programming Programming

Started by BradDaBug November 10, 2005 07:09 PM

13 comments, last by BradDaBug 18 years, 5 months ago

913

Author

November 10, 2005 07:09 PM

I've been struggling with this and all the stuff I've found about it hasn't quite helped. I've got a vertex shader in GLSL that looks like this:

void main()
{
	gl_TexCoord[0] = gl_MultiTexCoord0;
	gl_TexCoord[1] = gl_MultiTexCoord0;
		
	gl_Position = ftransform();
	gl_FogFragCoord = gl_FogCoord;
}

And a fragment shader that looks like this:

uniform sampler2D texture0;
uniform sampler2D texture1;
uniform sampler2D texture2;
	
void main()
{
	float fog;
	vec4 color;
	vec4 alphacolor = texture2D(texture2, gl_TexCoord[0].st / 64.0);

	color = texture2D(texture0,gl_TexCoord[0].st) * alphacolor.r + 
			texture2D(texture1,gl_TexCoord[0].st) *	(1.0 - alphacolor.r );	

	fog = (gl_Fog.end - gl_FogFragCoord) * gl_Fog.scale;

	gl_FragColor = color * fog;
}

The texture blending works fine, but I cannot figure out how to get fog to work. Apparently I'm supposed to be setting gl_FogFragCoord to something inside the vertex shader, but what?

I like the DARK layout!

mod42

134

November 11, 2005 01:59 AM

Do you pass fog coordiantes from your OpenGL application? If not and you want to have the standard distance fog you should not write gl_FogCoord to gl_FogFragCoord but the distance from the eye to the current vertex.

mod42

stroma

218

November 11, 2005 02:20 AM

i could not understand what do you do with the shader. do you use a sampler for fog?
you may want to look here: http://www.gamedev.net/community/forums/topic.asp?topic_id=257153

+-+-+-+-+-STR

BradDaBug

913

Author

November 11, 2005 08:33 AM

Quote:Original post by mod42
Do you pass fog coordiantes from your OpenGL application? If not and you want to have the standard distance fog you should not write gl_FogCoord to gl_FogFragCoord but the distance from the eye to the current vertex.

mod42

So I have to calculate that myself? How would I do that? Is the result of ftransform() a vertex in screen space, so that I could just use the z coordinate of gl_Position?

I like the DARK layout!

b3rs3rk

216

November 11, 2005 09:18 AM

in glsl you can do something like this

uniform vec3 camera;void main(void) {   gl_Position = ftransform();		   gl_TexCoord[0] = gl_MultiTexCoord0;    float dist = distance(camera,gl_Vertex.xyz);   .   .   other stuff   .   .}

Gorax

202

November 11, 2005 09:21 AM

Quote:Original post by BradDaBug
Quote:Original post by mod42
Do you pass fog coordiantes from your OpenGL application? If not and you want to have the standard distance fog you should not write gl_FogCoord to gl_FogFragCoord but the distance from the eye to the current vertex.

mod42

So I have to calculate that myself? How would I do that? Is the result of ftransform() a vertex in screen space, so that I could just use the z coordinate of gl_Position?

What mod42 meant was you need fog to be used before you can start using gl_FogCoord. This envolves either using the normal fog, or volumetric fog (glFogCoordfEXT() or glFogCoordPointerEXT() (although I'm not sure if this is the correct name, it's something like this)) to set the fog coordinate for the vertices/fragments. Alternatively, you can set the fog coordinate in the shader code, usually setting the vertex/fragment fog coordinate to some value based upon the distance from the eye (the bottom row of the modelview matrix).

zedzeek

529

November 11, 2005 12:54 PM

for standard depthbased fog u dont need to use fogcoord per se
u can also find a fragments depth value by IIRC gl_FragCoord.z
use this number to add an amount of fog

Captain Courgette, eat more veg

BradDaBug

913

Author

November 11, 2005 01:27 PM

Ok, I changed the line to this in the vertex shader:

gl_FogFragCoord = gl_Position.z;

And the fragment shader looks like this now:

uniform sampler2D texture0;uniform sampler2D texture1;uniform sampler2D texture2;	void main(){	float fog;	vec4 color;	vec4 alphacolor = texture2D(texture2, gl_TexCoord[0].st / 64.0);	color = mix(texture2D(texture1,gl_TexCoord[0].st),texture2D(texture0,gl_TexCoord[0].st), alphacolor.r);	fog = (gl_Fog.end - gl_FogFragCoord) * gl_Fog.scale;	if (fog >= 0.0)		gl_FragColor = mix(gl_Fog.color,color, fog);	else		gl_FragColor = gl_Fog.color;}

It works now! But you can see that I'm having to use a special case to detect whether or not fog > 0. Is that okay, or am I still doing something wrong?

Also, without using this shader on my terrain I'm getting 200 FPS, but with it I'm getting about 115 FPS. Is that normal?

I like the DARK layout!

_the_phantom_

11,263

November 11, 2005 02:23 PM

you shouldnt need that if statement, just doing gl_FragColor = mix(gl_Fog.color,color, fog); should give you the right result.

BradDaBug

913

Author

November 11, 2005 02:40 PM

Without the if the distant terrain looks like it's glowing. Here's a screen shot to show what I mean.

I like the DARK layout!

Shaders + fog

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

Shaders + fog

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

Reticulating splines