There's a couple of issues with your shader:

• First you are not using brackets in your if statement, so the alpha is set for every pixel and the if is effectively doing nothing.
• Second you are using front.x and front.y, that is the same as front.r and front.g respectively, and those returns color values and not positions. What you meant to use is vTex.x and vTex.y that are the coordinates.
• Just a minor thing, but you can multiply the alpha value directly (front.a *= fA) instead of repeating the alpha then multiplying (front.a = front.a*fA).

So your code should look like this:

lowp vec4 front = texture2D(samplerFront, vTex);

if( vTex.x<fL || vTex.x>1.0-fR || vTex.y<fT || vTex.y>1.0-fB )
{ front.a *= fA; }

gl_FragColor = front;[/code:2pirw3zx]

Sure, if it can provide access to both parameters then it should cover every possible case.

I just think that maybe calling "scroll" to the parameter that returns the scroll position would be less ambiguous, since "viewOrigin" indeed may suggests the top-left corner.

Kaisirak I'm almost sure that these new parameters "viewOrigin" and "layerAngle" where added to account for scrolling and rotation inside shaders, and for this purpose returning the center of the screen is a lot more useful.

If ones want to account only for scrolling than the top left corner is fine, but if there's layer rotation included than the center of the screen is what you need. Of course you can derive the center by the method you described, but doing these operations in the shader for every pixel is very costly and unnecessary when you can pass the proper values to the shader once per tick.

Ashley also mentions in the changelog for r160 that "viewOrigin" should return the layer scroll position, and at least in the editor the scroll position is the center of the screen.

I was previously passing the scroll position to some shaders through events, but from what I could observe there seems to be a one frame delay in doing so that cause small jitters. So an internal parameter that returns the center of the screen in layout coordinates is much needed for me, if not "viewOrigin" then a new "scrollPosition" parameter.