OpenGL ES 2.0 Shader - 2D Radial Gradient in Polygon

Question

I am trying to paint a radial gradient inside a Polygon. My Stage is 600x320. I am able to draw a gradient but it`s 'distorted/streched'. The goal is a gradient like a light would produce.

u_lightPosition is passed as a relative value: {0.5, 0.5}.
Currently unused lightPositionAbsolute is passed in as absolute value {300.0, 160.0}.

My fragment-shader currently looks like this:

#ifdef GL_ES                            
precision lowp float;                   
#endif                                  
varying vec4 v_fragmentColor;   

uniform vec2 u_lightPosition;
uniform vec2 u_lightPositionAbsolute;
uniform vec4 u_outerColor;
uniform vec4 u_innerColor;
uniform float u_radius;

void main()                             
{
    vec2 resolution = vec2(600,320);
    vec2 position = ( gl_FragCoord.xy / resolution.xy );

    float distanceFromLight = length(position - u_lightPosition);

    gl_FragColor = mix(u_outerColor, u_innerColor, distanceFromLight);
}                                       

With a circle Polygon this results in:

Answer 1

Ok i found the solution which now that it works is quiet obvious. I worked with relative values opposed to absolute ones which resulted in the stretching. The shader now looks like this:

#ifdef GL_ES                            
precision lowp float;                   
#endif                                  
uniform vec2 u_lightPosition;
uniform float u_radius;

void main()                             
{
    float distance  = length( u_lightPosition - gl_FragCoord.xy );
    float intensity = 1.0 - min( distance, u_radius )/u_radius;

    gl_FragColor = vec4(intensity, intensity, intensity, 1.0);
}                                       

Edit: Changed the falloff of the 'light':

#ifdef GL_ES                            
precision lowp float;                   
#endif                                  
uniform vec2 u_lightPosition;
uniform float u_radius;

void main()                             
{
    float distance  = length( u_lightPosition - gl_FragCoord.xy );

    float maxDistance = pow( u_radius, 0.23);
    float quadDistance = pow( distance, 0.23);

    float quadIntensity = 1.0 - min( quadDistance, maxDistance )/maxDistance;

    gl_FragColor = vec4(quadIntensity, quadIntensity, quadIntensity, 1.0);
}                                       

Answer 2

I believe that your shader is just fine, your problem is most likely in the geometry (or in the vertex shader).

I have created an example of your shader in the KickJS GLSL editor, where you can see it works just fine:

http://goo.gl/viDKB

You can change the type of mesh under settings.

Answer 3

You are passing radius as float so x & y are the same but they shouldnt be.

ie. screen : 1280 x 800 , radius 300 = {300.0/1280.0,300.0/800.0 }

"float u_radiusX = (100.0/1280.0);\n"+ 
"float u_radiusY = (100.0/800.0);\n"+ 
"float distanceFromLightX  =  length(v_lightPos.x   - v_Pos.x );\n"+
"float distanceFromLightY  =  length(v_lightPos.y   - v_Pos.y );\n"+
"float quadIntensityX = (1.0-min(distanceFromLightX,u_radiusX)/u_radiusX);\n"+  
"float quadIntensityY = (1.0-min(distanceFromLightY,u_radiusY)/u_radiusY);\n"+  
//finally
"float quadIntensity = quadIntensityX*quadIntensityY;\n"+
"gl_FragColor = vec4(color.r*quadIntensity, color.g*quadIntensity, color.b*quadIntensity, 1.0);\n"

I got this working this way. , where v_Pos is attribute position ( NOT the light position ) * matrix passed via variable from previous shader.