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Simple diffuse light. Weird Behavior.


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#1 OmarShehata   Members   -  Reputation: 205

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Posted 10 October 2012 - 01:49 AM

So I'm trying to make a simple lighting effect. I have my normal map and my object posted below.

Posted ImagePosted Image

And my code in GLSL is written below:

uniform sampler2D normal;
uniform vec2 mouse;
uniform float Za;
	 vec4 effect(vec4 color,sampler2D tex,vec2 tc,vec2 pc)
	 {
	 vec4 img_color = texture2D( tex, tc );
	 vec4 normalColor = texture2D( normal, tc );
	 float X = (mouse.x - pc.x);
	 float Y = (mouse.y - pc.y);
	 float Z = Za;
	 float dotProduct = X * normalColor.r + Y * normalColor.g + Z * normalColor.b;
	 dotProduct /= sqrt(X * X + Y * Y + Z * Z) * sqrt(normalColor.r * normalColor.r + normalColor.g * normalColor.g + normalColor.b * normalColor.b);
	 float factor = dotProduct;
	 img_color.r = img_color.r + factor;
	 img_color.g = img_color.g + factor;
	 img_color.b = img_color.b + factor;
	 return img_color;
	 }


So basically I had a few questions:

Is my math correct?
What I'm trying to do is get the vector of the mouse (the light source for now) to the pixel, and the normal of that pixel, and then get the dot product between them, normalize that, and that is the brightness factor. However the angle seems to be a bit off. I just wanted to double check if my questions were all right because debugging on the GPU doesn't seem to be very easy

For the usual lighting methods, do you multiply by a distance factor?
Naturally when a light is further away the object should be dimmer. But this doesn't seem to be the case with this approach. Should I just multiply by some arbitrary distance factor?

Finally, I'm basically using 3D light for a 2D game. Although it seems pretty simple in the implementation though. I'm told the Phong Reflection Model may be better suited in this case. Any thoughts on that?

Thanks in advance! Any help or tips on anything would be appreciated!

Edited by OmarShehata, 10 October 2012 - 06:49 AM.


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#2 Ashaman73   Crossbones+   -  Reputation: 7492

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Posted 10 October 2012 - 03:58 AM

First off, your glsl haa no line breaks making it incredible hard to read.

Then your left texture, what is it ? The diffuse map, in this case it would contain pseodo light information which will result in a weird look at the edges.

For the usual lighting methods, do you multiply by a distance factor?

Yes, it is even sometimes useful to not only reduce the intensity with distance but to clamp it to a certain area (ie. sphere). A simple,smooth clamping function would be

normalized_distance = min(1.0, dist / max_dist);
final_intensity = (1.0 - normalized_distance * normalized_distance) * intensity;


I'm told the Phong Reflection Model may be better suited in this case.

Better suited compared to what ?
You should choose a diffuse model (ie lambert) and a specular model (ie phong or blinn).


Here you can see the diffuse lighting model demonstrated in blender and here the specular models. Lambert,phong,blinn are the easiest one, the others are more complicated.

Edited by Ashaman73, 10 October 2012 - 04:03 AM.


#3 OmarShehata   Members   -  Reputation: 205

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Posted 10 October 2012 - 07:02 AM

Thanks for the quick reply Ashaman!

First off, your glsl haa no line breaks making it incredible hard to read.


Sorry about that, format messed up when I copied it. It should have line breaks now.

Then your left texture, what is it ? The diffuse map, in this case it would contain pseodo light information which will result in a weird look at the edges.


Well the texture on the left right now is the actual image of the object that I'm applying lighting too (since it's a 2D game). And the normal map has the RGB values that are the XYZ of the normal at each pixel as I understand.


I'm told the Phong Reflection Model may be better suited in this case.

Better suited compared to what ?
You should choose a diffuse model (ie lambert) and a specular model (ie phong or blinn).


Well..compared to what I'm doing right now.

Speaking of which, everyone who looks at my shader code either says that my shading model is weird or incomplete, but it really makes sense to me.

I mean, I'm getting the angle between the light source and the normal, and the brightness is proportional to that, shouldn't that work on its own?

I was planning on implementing a well known light model but thought if I tried doing it myself first I'd be able to understand it better.

#4 CryZe   Members   -  Reputation: 768

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Posted 10 October 2012 - 07:39 AM

Sorry about that, format messed up when I copied it. It should have line breaks now.

The forum can't handle code inside the code-tag right now unfortunately, so it's still a single line :(

Well the texture on the left right now is the actual image of the object that I'm applying lighting too (since it's a 2D game). And the normal map has the RGB values that are the XYZ of the normal at each pixel as I understand.

Like he said, it seems like it has some kind of lighting baked into it (hand painted though), which might look a bit odd when you're lighting it.

I mean, I'm getting the angle between the light source and the normal, and the brightness is proportional to that, shouldn't that work on its own?

The brightness should in the simplest case be proportional to the cosine of the angle between the light source and the normal. Not quite sure what you're doing as the code is unreadable with this forum bug :(
You might want to paste it directly without the code-tag, change the font to Courier New and you might also want to color the syntax a bit, to help the reader :D

#5 OmarShehata   Members   -  Reputation: 205

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Posted 10 October 2012 - 07:51 AM

The brightness should in the simplest case be proportional to the cosine of the angle between the light source and the normal. Not quite sure what you're doing as the code is unreadable with this forum bug Posted Image
You might want to paste it directly without the code-tag, change the font to Courier New and you might also want to color the syntax a bit, to help the reader Posted Image


Yeah that's what I meant. After getting the dot product and normalizing I should have the cosine of the angle.

I put up the code on pastebin:

http://pastebin.com/W4UMH593

#6 CryZe   Members   -  Reputation: 768

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Posted 10 October 2012 - 08:11 AM

Looks ok, as far as I can tell. But you should multiply the factor with the image color instead of adding it. You might want to add a constant "ambient term" to your factor before multiplying it to have some kind of ambient lighting, so that the non-lit area's are not completely black (spherical harmonics or environment mapping are better approaches for ambient lighting though).

Also, the normals in your image are stored in the range (0, 1). But what you actually want are normals in the range (-1, 1). So you have to load them like this:
vec4 normalColor = 2 * texture2D( normal, tc ) - 1;

As a distance attenuation factor you might want to divide the resulting illumination by the light vectors length squared. This would be the most physically correct distance attenuation factor.

Edited by CryZe, 10 October 2012 - 08:19 AM.


#7 Ashaman73   Crossbones+   -  Reputation: 7492

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Posted 10 October 2012 - 11:35 PM

Here's your modified code, including shifting the texture into [-1..1] and an ambient term without specular (only diffuse yet).

	uniform sampler2D normal;
	uniform vec2 mouse;
	uniform float Za;
		 vec4 effect(vec4 color,sampler2D tex,vec2 tc,vec2 pc)
		 {
		 vec4 img_color = texture2D( tex, tc );
		 vec4 normalColor = texture2D( normal, tc );
		 vec3 lightDir = normalize(vec3( (mouse.x - pc.x), (mouse.y - pc.y), Za));
		 vec3 normal = normlaize(normalColor.xyz*vec3(2.0)-vec3(1.0));
		 float dotProduct = dot(lightDir,normal);
		 float ambientFactor = 0.2; 	  
		 img_color *= mix(vec3(1.0) , dotProduct, ambientFactor);

		 return img_color;
		 }

Once this work you can adjust the high of the light, consider the light distance and add a specular term.

Edited by Ashaman73, 10 October 2012 - 11:37 PM.


#8 MrOMGWTF   Members   -  Reputation: 439

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Posted 14 October 2012 - 12:30 AM

(spherical harmonics or environment mapping are better approaches for ambient lighting though).


Here's little code snippet for SH lighting:
varying vec3 Normal;

const float C1 = 0.429043;
const float C2 = 0.511664;
const float C3 = 0.743125;
const float C4 = 0.886227;
const float C5 = 0.247708;

// Constants for Old Town Square lighting
const vec3 L00  = vec3( 0.871297,  0.875222,  0.864470);
const vec3 L1m1 = vec3( 0.175058,  0.245335,  0.312891);
const vec3 L10  = vec3( 0.034675,  0.036107,  0.037362);
const vec3 L11  = vec3(-0.004629, -0.029448, -0.048028);
const vec3 L2m2 = vec3(-0.120535, -0.121160, -0.117507);
const vec3 L2m1 = vec3( 0.003242,  0.003624,  0.007511);
const vec3 L20  = vec3(-0.028667, -0.024926, -0.020998);
const vec3 L21  = vec3(-0.077539, -0.086325, -0.091591);
const vec3 L22  = vec3(-0.161784, -0.191783, -0.219152);

void main()
{
    vec3 AmbientColor =  C1 * L22 * (Normal.x * Normal.x - Normal.y * Normal.y) +
                    C3 * L20 * Normal.z * Normal.z +
                    C4 * L00 -
                    C5 * L20 +
                    2.0 * C1 * L2m2 * Normal.x * Normal.y +
                    2.0 * C1 * L21  * Normal.x * Normal.z +
                    2.0 * C1 * L2m1 * Normal.y * Normal.z +
                    2.0 * C2 * L11  * Normal.x +
                    2.0 * C2 * L1m1 * Normal.y +   
                    2.0 * C2 * L10  * Normal.z;

    gl_FragCoord = AmbientColor;
}

It was taken from some article about sh... I can't find it.

You can use only 2 sh bands and you will still get nice ambient light. Here's code if you want to use two bands:
varying vec3 Normal;

const float C1 = 0.429043;
const float C2 = 0.511664;
const float C3 = 0.743125;
const float C4 = 0.886227;
const float C5 = 0.247708;

// Constants for Old Town Square lighting
const vec3 L00  = vec3( 0.871297,  0.875222,  0.864470);
const vec3 L1m1 = vec3( 0.175058,  0.245335,  0.312891);
const vec3 L10  = vec3( 0.034675,  0.036107,  0.037362);
const vec3 L11  = vec3(-0.004629, -0.029448, -0.048028);

void main()
{

    vec3 AmbientColor =  C4 * L00 -
                    2.0 * C2 * L11  * Normal.x +
                    2.0 * C2 * L1m1 * Normal.y +   
                    2.0 * C2 * L10  * Normal.z;

    gl_FragCoord = AmbientColor;
}

Edited by MrOMGWTF, 14 October 2012 - 12:31 AM.


#9 OmarShehata   Members   -  Reputation: 205

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Posted 14 October 2012 - 09:40 AM

Here's your modified code, including shifting the texture into [-1..1] and an ambient term without specular (only diffuse yet).


Thanks for taking the time to write that Ashaman!

I still have a few questions though.

I'm not sure I understand what's going on in this line:

vec3 normal = normalize(normalColor.xyz*vec3(2.0)-vec3(1.0));

Actually..reading it again, this is the part where you convert it from [0,1] to [-1,-1] right? I'm assuming this affects the angle of the light? Because that seems to be what fixes everything.

Also this line:

img_color *= mix(vec3(1.0) , dotProduct, ambientFactor);

What's being mixed and why? And what's the purpose of that (1.0) ?

Finally, the reason I was adding to the color instead of multiplying is that, when I multiply, the color of the material or the object seems to turn greyish as it darkens. Like it loses its color. Which makes sense. But if I add the color then, with some tweaks, it looks a lot better because the object's color just gets brighter, more towards white, and darker, towards black, without really seeming to lose its color.

Do any lighting models add/subtract to the color instead of multiplying or is that just a bad idea?

Here's little code snippet for SH lighting:


Thanks for this as well! I'll try to implement that and post again if I have any problems.




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