Rendering 2D shadows in shader

Question

Update:

I am getting slightly better results by modifying the shader so that it sets a flag on entering a obscured region, and then only sets the fragment to black if the ray casting exits the obscured region. The bordering light is a little less noticeable this way. Additionally objects behind other objects will now be correctly obscured as long as they are not touching.

However as you can see in the image, I have noticed a new problem. The shadow becomes very choppy at the back due to the way I am doing raycasting.

Original:

I am currently trying to render shadows in a 2d game. I use the following shader to do so:

#version 120
varying vec2 vTexCoord;

uniform vec3 u_lightColor;
uniform sampler2D u_texture;


float drawShadow(){
    vec2 loc;
    vec2 norm = normalize(vTexCoord - vec2(.5,.5)); // Pointing from center to point
    vec4 smp;

    float dst = 0.0;

    // For now draw all casters
    if(texture2D(u_texture, vTexCoord - dst * norm).a > 0)
        return 1.0;

    for(int i = 0; i < 256; i++){
        loc = vTexCoord - dst * norm;

        if (length(loc-vec2(.5,.5)) < 1.0/256.0)
            break;

        smp = texture2D(u_texture, loc);

        if(smp.a > 0){
            return 0.0;
        }

        dst += 1.0/256.0;
    }

    return 1.0;
}


void main() {
    float dist = length(vec2(.5,.5) - vTexCoord.xy);

    float shad = drawShadow();

    float intensity = 1-dist*2.0;

    gl_FragColor = vec4(u_lightColor, 1.0)*vec4(shad*vec3(intensity), shad);
}

The light is being rendered at the center of the screen, and the texture being drawn is one where any shadow caster is drawn as a non transparent sprite.

The trouble is because of the way I am sending out the rays I end up with the following:

Where the shadows don't hug the edges of the sprites. Also, I would like to somehow obscure shadow casters if they are in the shadow of another shadow caster, but not if they are the first caster the light hits.

Answer 1

A completely different approach to 2D shadows would be by making a dynamic mesh on the CPU or maybe in a geometry shader and using it with a stencil mask to do shadows.
I'd recommend checking out this website: https://www.redblobgames.com/articles/visibility/ for 2D visibility from a mesh.

I also tried implementing your solution in shadertoy: And I had to crank the steps up to 32768 before it would drop below 60fps. So maybe you can get away with increasing the number of steps to reduce your aliasing problems.

Update while writing this answer I found out a way to smooth your shadows in the shader. It's kinda fake but it might work:
https://www.shadertoy.com/view/XlByDm