Why does this lighting code only work for single lights and break when I add multiple lights?

Question

I'm trying to create a simple 2D game engine using lwjgl and java. But I got stuck during the lighting programming, because the code I wrote only works when I have one light in the scene and not when I add multiple. I just can't figure out why so i decided to ask here.

This is the fragment shader with the lighting calculation:

#version 330 core

layout (location = 0) out vec4 color;
layout (origin_upper_left, pixel_center_integer) in vec4 gl_FragCoord;

in DATA {
    vec2 tc;
} fs_in;

uniform sampler2D tex;

float map(float value, float min1, float max1, float min2, float max2) {
  return min2 + (value - min1) * (max2 - min2) / (max1 - min1);
}

void main() {
    vec3 lightColor = vec3(1, 1, 1);
    float range = 700;
    float x = 200;
    float y = 200;
    float ambient = 0.1;
    float intensity = 0.8;
    float alpha = 1.0;

    vec3 totalDiffuse = vec3(0.0);

    for(int i=0;i<1;i++){
        alpha = 1-map(distance(gl_FragCoord.xy, vec2(x*i, y*i)), 0.0, range, 0.0, 1.0);
        totalDiffuse += alpha*lightColor;
    }

    totalDiffuse = max(totalDiffuse, ambient);

    color = vec4(totalDiffuse, 1.0) * texture(tex, fs_in.tc);
}

If I run this code with only one light in the scene, i.e. setting the times the for loop runs to 1, then it works just fine and creates something like this:

But when I change it to loop for example 3 times, you would expect it to create 3 different lights, but actually it just increases the light intensity of the first light like this:

Anybody know why?

Answer 1

The result of the term

alpha = 1-map(distance(gl_FragCoord.xy, vec2(x*i, y*i)), 0.0, range, 0.0, 1.0);

can be negative.

This will cause that the totalDiffuse is decreased.

Clamp alpha to a minimum of 0:

//totalDiffuse += alpha*lightColor;
totalDiffuse += max(0.0, alpha) * lightColor;

Note, the distance between the light sources is length(vec2(200, 200)), but the illumination range (radius) of each light source is 700. So anyway the light sources are overlapping.

---

I recommend to use the glsl function smoothstep, which performs a Hermite interpolation between two values.

Try the following:

for(int i=0; i<3; i++ )
{
    vec2 pos = vec2(x, y) * float(i);
    float dist = distance(gl_FragCoord.xy, pos);

    alpha = smoothstep(50.0, 100.0, dist);
    totalDiffuse += clamp(1.0-alpha, 0.0, 1.0) * lightColor;
}

in this case the 2nd parameter (100.0) to smoothstep is the maximum radius of the light source and the 1st parameter (50.0) is the radius which is full lit.

---

See the WebGL example, where I used smoothstep:

(function loadscene() {    

var canvas, gl, vp_size, prog, bufObj = {};

function initScene() {

    canvas = document.getElementById( "ogl-canvas");
    gl = canvas.getContext( "experimental-webgl" );
    if ( !gl )
      return;

    progDraw = gl.createProgram();
    for (let i = 0; i < 2; ++i) {
        let source = document.getElementById(i==0 ? "draw-shader-vs" : "draw-shader-fs").text;
        let shaderObj = gl.createShader(i==0 ? gl.VERTEX_SHADER : gl.FRAGMENT_SHADER);
        gl.shaderSource(shaderObj, source);
        gl.compileShader(shaderObj);
        let status = gl.getShaderParameter(shaderObj, gl.COMPILE_STATUS);
        if (!status) alert(gl.getShaderInfoLog(shaderObj));
        gl.attachShader(progDraw, shaderObj);
        gl.linkProgram(progDraw);
    }
    status = gl.getProgramParameter(progDraw, gl.LINK_STATUS);
    if ( !status ) alert(gl.getProgramInfoLog(progDraw));
    progDraw.inPos = gl.getAttribLocation(progDraw, "inPos");
    progDraw.u_time = gl.getUniformLocation(progDraw, "u_time");
    progDraw.u_resolution = gl.getUniformLocation(progDraw, "u_resolution");
    gl.useProgram(progDraw);

    var pos = [ -1, -1, 1, -1, 1, 1, -1, 1 ];
    var inx = [ 0, 1, 2, 0, 2, 3 ];
    bufObj.pos = gl.createBuffer();
    gl.bindBuffer( gl.ARRAY_BUFFER, bufObj.pos );
    gl.bufferData( gl.ARRAY_BUFFER, new Float32Array( pos ), gl.STATIC_DRAW );
    bufObj.inx = gl.createBuffer();
    bufObj.inx.len = inx.length;
    gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, bufObj.inx );
    gl.bufferData( gl.ELEMENT_ARRAY_BUFFER, new Uint16Array( inx ), gl.STATIC_DRAW );
    gl.enableVertexAttribArray( progDraw.inPos );
    gl.vertexAttribPointer( progDraw.inPos, 2, gl.FLOAT, false, 0, 0 ); 
    
    gl.enable( gl.DEPTH_TEST );
    gl.clearColor( 0.0, 0.0, 0.0, 1.0 );

    window.onresize = resize;
    resize();
    requestAnimationFrame(render);
}

function resize() {
    //vp_size = [gl.drawingBufferWidth, gl.drawingBufferHeight];
    vp_size = [window.innerWidth, window.innerHeight];
    //vp_size = [256, 256]
    canvas.width = vp_size[0];
    canvas.height = vp_size[1];
}

function render(deltaMS) {

    gl.viewport( 0, 0, canvas.width, canvas.height );
    gl.clear( gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT );
   
    gl.uniform1f(progDraw.u_time, deltaMS/2000.0);
    gl.uniform2f(progDraw.u_resolution, canvas.width, canvas.height);
    gl.drawElements( gl.TRIANGLES, bufObj.inx.len, gl.UNSIGNED_SHORT, 0 );
    
    requestAnimationFrame(render);
}  

initScene();

})();

<script id="draw-shader-vs" type="x-shader/x-vertex">
precision mediump float;
attribute vec2 inPos;

void main() {
    gl_Position = vec4( inPos.xy, 0.0, 1.0 );
}
</script>

<script id="draw-shader-fs" type="x-shader/x-fragment">
precision mediump float;
uniform float u_time;
uniform vec2 u_resolution;

vec3 HUEtoRGB(in float H)
{
    float R = abs(H * 6.0 - 3.0) - 1.0;
    float G = 2.0 - abs(H * 6.0 - 2.0);
    float B = 2.0 - abs(H * 6.0 - 4.0);
    return clamp( vec3(R,G,B), 0.0, 1.0 );
}

void main() {
    vec2 uv = gl_FragCoord.xy / u_resolution;

    vec3 lightColor = vec3(1, 1, 1);
    float range = length(u_resolution) / 4.0;
    float x = u_resolution.x / 4.0;
    float y = u_resolution.y / 4.0;
    float ambient = 0.1;
    float intensity = 0.8;
    float alpha = 1.0;

    vec3 totalDiffuse = vec3(0.0);

    for(int i=0;i<3;i++)
    {    
        vec2 pos = vec2(x, y) * float(i+1);
        float dist = distance(gl_FragCoord.xy, pos);
        
        alpha = smoothstep(range/4.0, range/2.0, dist);
        totalDiffuse += clamp(1.0-alpha, 0.0, 1.0) * lightColor;
    }
    totalDiffuse = max(totalDiffuse, ambient);

    vec4 texcol = vec4( 1.0-uv.x, 1.0-uv.y, uv.x*uv.y, 1.0 );
    gl_FragColor = vec4(totalDiffuse, 1.0) * texcol;
}
</script>

<canvas id="ogl-canvas" style="border: none"></canvas>