I think you need to measure each line. either by splitting your block in lines, or making a string to measure each line... AFAIK textWidth() will count only the longest line in a block

Had an idea about using the pixel array to censor the text and made a quick test. It works pretty well. Here is the proof-of-concept sketch I made that 'censors text' per horizontal line. Of course you can further tweak it from here to work per text line instead of pixel line and only work within the boundaries of the text block etc. In a way this is a measure-by-pixels option.

Proof-Of-Concept Code Example

1. color textColor = color(0);
2.  
3. void setup() {
4.   size(500, 500);
5.   textSize(30);
6. }
7.  
8. void draw() {
9.   background(255);
10.   String s = "The quick brown fox jumped over the lazy dog. But the lazy dog also jumped over the quick brown fox. Bet you didn't see that one coming!";
11.   fill(textColor);
12.   text(s, 50, 50, 300, 300);
13.   if (mousePressed) censorText();
14. }
15.  
16. void censorText() {
17.   loadPixels();
18.   for (int y=0; y<height; y++) {
19.     int end = -1;
20.     for (int x=width-1; x>=0; x--) {
21.       int index = x+y*width;
22.       color c = pixels[index];
23.       if (c == textColor) {
24.         end = x;
25.         break;
26.       }
27.     }
28.     if (end>-1) {
29.       int start = -1;
30.       for (int x=0; x<width; x++) {
31.         int index = x+y*width;
32.         if (start==-1) {
33.           color c = pixels[index];
34.           if (c == textColor) {
35.             if (x<end) { start = x; }
36.             pixels[index] = textColor;
37.           }
38.         } else if (x<=end) {
39.           pixels[index] = textColor;
40.         }
41.       }
42.     }
43.   }
44.   updatePixels();
45. }

amazing :)

Thanks very much guys. Amnon your soultion is supercool, but i needed something for print and quality of that output wasnt that good. For digital its awesome though.

Cool. I'll probably return to slitscanning at some point, but currently it's not on my shortlist. Good to hear my code is still useful though.