Question on the Curve adjustment layer

[natrev]

Hi EveryOne!,

I have another clarification with the Curve adjustment layer

Please find the screenshot for more reference

The first screenshot shows the value in input - 50 & output - 50 and ColorSampler red value is 153

if I change the Curve output value to 79 then ColorSampler red value changed into  84 

Is there is any formula available PS object model?

Or any suggestion to calculate input and output value?

Could anyone please help me to resolve the issue? 

-yajiv

 

[r-bin]

There is pretty complicated math

// standard curve view
// var p = [[0,0], [127, 54], [255,255]];

// yours curve view
var p = [[0,0], [255-Math.round(50/100*255), 255-Math.round(79/100*255)], [255,255]];

     
var pp = curve_array(p);

//alert(curve_array(p).toSource()); 

//alert(pp[153].toSource()); 

alert(pp[153][1]); 

function curve_array(p) 
    {
    try {
        var sd = second_derivative(p);

        var ret = new Array();

        for (var i = 0; i < p.length-1; i++) 
            {
            var cur   = p[i];
            var next  = p[i+1];

            for (var x = cur[0]; x < next[0]; x++) 
                {
                var t = (x-cur[0])/(next[0]-cur[0]);
 
                var a = 1-t;
                var b = t;
                var h = next[0]-cur[0];
 
                var y= a*cur[1] + b*next[1] + (h*h/6)*( (a*a*a-a)*sd[i]+ (b*b*b-b)*sd[i+1] );
 
                ret.push([x,y]); 
                }
            }

        ret.push(p[i]); 

        return ret;
        } 
    catch(e) { alert(e); }
    }

function second_derivative(points) 
    {
    try {
        var n = points.length;
    
        var matrix = new Array(); for (var i = 0; i < n; i++) matrix.push([0,0,0]);
        var result = new Array(); for (var i = 0; i < n; i++) result.push(0);
    
        matrix[0][1]=1;
    
        for (var i = 1; i < n-1; i++) 
            {
            matrix[i][0] = (points[i][0]-points[i-1][0])/6;
            matrix[i][1] = (points[i+1][0]-points[i-1][0])/3;
            matrix[i][2] = (points[i+1][0]-points[i][0])/6;
    
            result[i] = (points[i+1][1]-points[i][1])/(points[i+1][0]-points[i][0]) - (points[i][1]-points[i-1][1])/(points[i][0]-points[i-1][0]);
            }
    
        matrix[n-1][1]=1;
     
        // solving pass1 (up->down)

        for (var i = 1; i < n; i++) 
            {
            var k = matrix[i][0]/matrix[i-1][1];
            matrix[i][1] -= k*matrix[i-1][2];
            matrix[i][0] = 0;
            result[i] -= k*result[i-1];
            }

        // solving pass2 (down->up)
    
        for(var i = n-2; i >= 0; i--) 
            {
            var k = matrix[i][2]/matrix[i+1][1];
            matrix[i][1] -= k*matrix[i+1][0];
            matrix[i][2] = 0;
            result[i] -= k*result[i+1];
            }

        // return second derivative value for each point
    
        var y2 = new Array(n);
        for (var i=0; i < n; i++) y2[i] = result[i]/matrix[i][1];

        return y2;
        } 
    catch(e) { alert(e); }
    }

[Kukurykus]

What's source of this amazing function you created - Adobe Documents or something else to share? 🙂

[r-bin]

I found this algorithm a long time ago somewhere on the Internet.

[natrev]

Dear r-bin,

Thank you so much for your reply. Definitely this is complicated math.

Actually I have input value 153 and the output value is 84.

Now I need to know If I change which output value to get the ColorSample values is 84 

[r-bin · Accepted solution]

Make the curves look in the usual form (0-255) and not (0-100) like yours.
Set point [inp: 153, out: 84]
And do not bother yourself. )

[natrev]

hi r-bin,

Thank God.

It is working like a charm :). Your timely help much appreciated. 

Thank you once again.