I would not use Hough Lines for this purpose. But rather would follow of these two strategies:
Strategy 1:
- Blur your image a lot, so that you lose those black spaces between letters.
- Once your letters become "white united pixels", go for contour detection and capture them in a bounding box.
- Once you have your blob in a box, you can then play with it as much as you want. This may guide you through.
Strategy 2:
- Use a feature detector, which may work with this, like in the given example.
- Once you detect the bill and put a bounding box around it, again, it is a matter of geometry to figure out how much it is tilted.
