Shadow artifacts in opengl - opengl-es

Question

I am trying to render an object and two lights, one of the lights cast shadows. Everything works ok but I noticed that there are some obvious artifacts, as shown in the below image, some shadows seem to overflow to bright areas.
Below is the shaders to render depth information into a framebuffer
<script id="shadow-shader-vertex" type="x-shader/x-vertex">
attribute vec4 aVertexPosition;
uniform mat4 uObjMVP;
void main() {
gl_Position = uObjMVP * aVertexPosition;
}
</script>
<script id="shadow-shader-fragment" type="x-shader/x-vertex">
precision mediump float;
void main() {
//pack gl_FragCoord.z
const vec4 bitShift = vec4(1.0, 256.0, 256.0 * 256.0, 256.0 * 256.0 * 256.0);
const vec4 bitMask = vec4(1.0/256.0, 1.0/256.0, 1.0/256.0, 0.0);
vec4 rgbaDepth = fract(gl_FragCoord.z * bitShift);
rgbaDepth -= rgbaDepth.gbaa * bitMask;
gl_FragColor = rgbaDepth;
}
</script>
In the above shaders, uObjMVP is the MVP matrix used when looking from the position of the light that cast shadow (the warm light, the cold light does not cast shadow)
And here are the shaders to draw everything:
<script id="shader-vertex" type="x-shader/x-vertex">
//position of a vertex.
attribute vec4 aVertexPosition;
//vertex normal.
attribute vec3 aNormal;
//mvp matrix
uniform mat4 uObjMVP;
uniform mat3 uNormalMV;
//shadow mvp matrix
uniform mat4 uShadowMVP;
//interplate normals
varying vec3 vNormal;
//for shadow calculation
varying vec4 vShadowPositionFromLight;
void main() {
gl_Position = uObjMVP * aVertexPosition;
//convert normal direction from object space to view space
vNormal = uNormalMV * aNormal;
vShadowPositionFromLight = uShadowMVP * aVertexPosition;
}
</script>
<script id="shader-fragment" type="x-shader/x-fragment">
precision mediump float;
uniform sampler2D uShadowMap;
varying vec3 vNormal;
varying vec4 vShadowPositionFromLight;
struct baseColor {
vec3 ambient;
vec3 diffuse;
};
struct directLight {
vec3 direction;
vec3 color;
};
baseColor mysObjBaseColor = baseColor(
vec3(1.0, 1.0, 1.0),
vec3(1.0, 1.0, 1.0)
);
directLight warmLight = directLight(
normalize(vec3(-83.064, -1.99, -173.467)),
vec3(0.831, 0.976, 0.243)
);
directLight coldLight = directLight(
normalize(vec3(37.889, 47.864, -207.187)),
vec3(0.196, 0.361, 0.608)
);
vec3 ambientLightColor = vec3(0.3, 0.3, 0.3);
float unpackDepth(const in vec4 rgbaDepth) {
const vec4 bitShift = vec4(1.0, 1.0/256.0, 1.0/(256.0*256.0), 1.0/(256.0*256.0*256.0));
float depth = dot(rgbaDepth, bitShift);
return depth;
}
float calVisibility() {
vec3 shadowCoord = (vShadowPositionFromLight.xyz/vShadowPositionFromLight.w)/2.0 + 0.5;
float depth = unpackDepth(texture2D(uShadowMap, shadowCoord.xy));
return (shadowCoord.z > depth + 0.005) ? 0.4 : 1.0;
}
vec3 calAmbientLight(){
return ambientLightColor * mysObjBaseColor.ambient;
}
vec3 calDiffuseLight(const in directLight light, const in float visibility){
vec3 inverseLightDir = light.direction * -1.0;
float dot = max(dot(inverseLightDir, normalize(vNormal)), 0.0);
return light.color * mysObjBaseColor.diffuse * dot * visibility;
}
void main() {
vec3 ambientLight = calAmbientLight();
float visibility = calVisibility();
vec3 warmDiffuseLight = calDiffuseLight(warmLight, visibility);
// cold light does not cast shadow and hence visilibility is always 1.0
vec3 coldDiffuseLight = calDiffuseLight(coldLight, 1.0);
gl_FragColor = vec4(coldDiffuseLight + warmDiffuseLight + ambientLight, 1.0);
}
</script>
If I simply draw the depth information out on to the canvas,
void main() {
// vec3 ambientLight = calAmbientLight();
// float visibility = calVisibility();
// vec3 warmDiffuseLight = calDiffuseLight(warmLight, visibility);
// // cold light does not cast shadow and hence visilibility is always 1.0
// vec3 coldDiffuseLight = calDiffuseLight(coldLight, 1.0);
// gl_FragColor = vec4(coldDiffuseLight + warmDiffuseLight + ambientLight, 1.0);
vec3 shadowCoord = (vShadowPositionFromLight.xyz/vShadowPositionFromLight.w)/2.0 + 0.5;
gl_FragColor = vec4(unpackDepth(texture2D(uShadowMap, shadowCoord.xy)), 0.0, 0.0, 1.0);
}
I would get this image
Thanks in advance.