I'm working on implementing some semi-basic lighting in a fragment shader for a 2D renderer.

At the moment, the light attenuation is calculated using: I / (d/r + 1)2

where I = intensity, d = distance between light source and gl_FragCoord, r = radius of light

The final attenuation for any given fragment is the sum of attenuation values from all other sources.

Here is an example scene - two quads and a background (no textures)

No lighting:

With lighting:

This is with very minimal ambient lighting.

Right now the color of the fragment is computed with: rgb * light_color * attenuation

How do I mix the frag color with light color AND ambient color so that ambient color is applied consistently throughout the frame while the light color only has an effect within the lights' attenuation range?

What other factors should I be putting in for a solid 2D light system (i.e. material/diffuse/etc.)?

Really? You couldn't figure this out? How about lightingCalculation + ambient..............

Oh I'm really sorry for being so stupid and new to GLSL programming..... your contempt is much appreciated.

Do you mean: rgb * ambientColor + lightColor ? or: rgb + lightColor + ambientColor ?

I assure you I am aware of the distributive property :/

rgb * (ambientColor * ambientAmount + lightColor * lightAttenuation)

That seems to work pretty nicely:

What should happen when the object passes "over" the light source in a 2D scene like this? At the moment it looks kind of transparent.

The equation you are using, is almost like a filter, where you filter light sources through "rgb".

I think the source of the problem is that the background rgb is white? If so, this part should really reflect more. Is that what you are going to use, or are there going to be other colors?

The background image is light gray at the moment (RGB 0.75, 0.75, 0.75). I've changed it a few times to test different things.

No that will not necessarily be what I use exclusively in the future. The background could be anything from a solid color to a collection of static textures.

I mainly just want the background to be illuminated to whatever extent there is light in that region of the frame. Any objects rendered on top of it, however, I'm trying to make interact better with the light.

The "rgb" at any given point is just the color of the fragment either sampled from a texture (if one is bound) or otherwise the front color reported by the vertex shader.

I guess one thing to decide would be where exactly the light source actually is in the z direction for 2D lighting? Should it be as though it's shining onto the frame from user space?

Please feel free to offer any improvements to that formula or alternative methods. I'm just trying stuff out.

I think what you have looks about right. Ambient light is typically added at the end of your lightning equation to give your objects' color a uniformly-lit scene.

Ok. I will stick with this model for now. I still find the transparent look a little weird, but I found if you play with the blending settings (combine source/dest alpha channels, etc) you can get different effects with a more opaque/matte look.

I also haven't implemented directional light yet, which in a lot of cases might be more important.