Each thread has its area to search. At the end, when the last thread has finished, the results are merged into a single bitmap. The best way to see it is as 4

Each thread has its area to search. At the end, when the last thread has finished, the results are merged into a single bitmap. The best way to see it is as 4 strips of tracing paper which have the exact same text on, and the strips are searched and the text traced in, then all 4 (clear) strips go in a pile to make the final image. [Edit] To clarify, if you search at position 200,268, if you hit a row of text in the centre, it scans. If you search the same bitmap at 200,350, what you find might not be clear. So each thread searches some of the same areas, but the end result is that all text is found, and heuristics reject certain rows as valid text.