Achieving 2D Lighting for Terraria-Like Game

Question

I'm trying to achieve the fog-of-war lighting style that Terraria and Starbound have. Terraria's lighting system seems to be more blocky/tile based, but Starbound's is a lot more smooth.

The Desired Effect:

Starbound is using a shader to render its lighting, but how it's going about that is quite a mystery to me.

The idea I had on how to achieve the desired effect is the following: Create a texture that is black and somehow calculate the area that is white, storing this in a light map render texture, finally doing an Additive Blend of both the main texture and the light map texture to achieve a similar effect to Starbound's. (However this does not take into account lights in the world, which I also have absolutely no clue how to include in my calculation to the light map render texture)

I say somehow in italics because I have no clue as to how I can create this "Mask" effect in the light map render texture. How I can do calculations necessary to create the mask is beyond me; I don't even know how I can begin to translate the game world's data, relative to the what the camera sees, and update the texture accordingly.

TLDR: How can I achieve lighting similar to that of either Terraria or Starbound in Unity 2D?

Answer 1

First you draw your color map

Then you draw your light map

And finally you multiply both.

If you want a some sort of secondary lighting in your game, and if your game world uses grid for terrain and/or object placement, then I have a solution for you. This is something like instant radiosity.

First, you find which cells are lit by the light by tracing them from the light source.

Then you place another light sources on lit cells with attenuation of intensity based on a distance from original light. You may also want to blur them a bit (I did). Here's resulting map.

Add original light source there.

Then multiply with your color map.

Here's pseudocode for the rendering pipeline (don't mind the syntax highlight)

for each light on scene:
    if light is on screen:
        add light to the drawing queue

draw frame:
    render frame color map:
        draw each visible tile and object

    for each light in queue:
        trace secondary lights on visible cells and add them to the drawing queue                

    render frame light map:
        set blending to additive
        for each light in queue:
            move light object to the light position
            set light object width and height to the light radius
            set light object color to the light color
            draw light object
        //then we render shadows for solid places
        set blending to subtraction
        for each visible cell:
            if cell is surrounded by other cells from every side:
                move light object to the cell
                set light width and height to the radius slightly bigger than cell
                set light color to white//to subtract any kind of light color
                draw light object

    set blending to normal
    draw color map
    set blending to multiplication
    draw light map