Barrel / Distortion effect?

Example of denorm and clamping.

> /////////////////////////////////////////////////////////
// Barrel

//The current foreground texture co-ordinate
varying mediump vec2 vTex;
//The foreground texture sampler, to be sampled at vTex
uniform lowp sampler2D samplerFront;
//The current foreground rectangle being rendered
uniform mediump vec2 srcStart;
uniform mediump vec2 srcEnd;
//The current foreground source rectangle being rendered
uniform mediump vec2 srcOriginStart;
uniform mediump vec2 srcOriginEnd;
//The current foreground source rectangle being rendered, in layout 
uniform mediump vec2 layoutStart;
uniform mediump vec2 layoutEnd;
//The background texture sampler used for background - blending effects
uniform lowp sampler2D samplerBack;
//The current background rectangle being rendered to, in texture co-ordinates, for background-blending effects
uniform mediump vec2 destStart;
uniform mediump vec2 destEnd;
//The time in seconds since the runtime started. This can be used for animated effects
uniform mediump float seconds;
//The size of a texel in the foreground texture in texture co-ordinates
uniform mediump vec2 pixelSize;
//The current layer scale as a factor (i.e. 1 is unscaled)
uniform mediump float layerScale;
//The current layer angle in radians.
uniform mediump float layerAngle;

//effect.fx
precision mediump float;

uniform mediump float scale;

void main(void)
{
mediump vec2 n = (vTex - srcOriginStart) / (srcOriginEnd - srcOriginStart);

vec2 st = n - 0.5;
float theta = atan(st.x, st.y);
float rad = sqrt(dot(st, st));
rad *= 1.0 + scale * pow(rad, 2.0);

n = clamp(vec2( 0.5 + sin(theta) * rad, n.y), vec2(0.), vec2(1.));

gl_FragColor = texture2D(samplerFront, mix(srcOriginStart,srcOriginEnd,n));

}

Thank you for this and all who helped, managed to get this working as planned.

A negative scale caused a pincushion along X and positive scale causes barrel along X.

Thank you