The problem with the horizontal and diagonal lines is because you are using the same texture as input and output for the filter. You should use two different textures with a technique called ping-pong to switch their roles alternatively:

1. ...
2. GLTexture[] pmedian;
3. int read = 0;
4. int write = 0;
5. ...
6. void textureSetup() {
7.   ...
8.   pmedian = new GLTexture[2];
9.   pmedian[0] = new GLTexture(this, width, height, params);
10.   pmedian[1] = new GLTexture(this, width, height, params);
11.   
12.   pmedian[0].putImage(img);
13.   read = 0;
14.   write = 1;
15. }
16. 
    
17. void drawTexture() {
18.   ...
20.   feedBack.apply(pmedian[read], pmedian[write]);
21. 
    
22.   image(pmedian[write], 0, 0);
23.    
24.   int temp = write;
25.   write = read;
26.   read = temp;
27. }

That's cool, I knew an expert like you will help me find the issue in no time..

Thanks a million¡¡¡¡

And again, this library you've made... absolutely awesome, 

and invaluable help for no hardcore coders to get into opengl/glsl world.. 

Best

Spector