Extract Rectangular Image from Scanned Image

I have scanned copies of currency notes from which I need to extract only the rectangular notes. Although the scanned copies have a very blank background, the note itself can be rotated or aligned correctly. I'm using matlab.

Example inputs:

Example output:

I have tried using thresholding and canny/sobel edge detection to no avail.

PS: My primary objective is to determine the denomination of the currency. There are a couple of methods I thought I could use:

1. Color based, since all currency notes have varying primary colors. The advantage of this method is that it's independent of the rotation or scale of the input image.
2. Detect the small black triangle on the lower left corner of the note. This shape is unique for each denomination.
3. Calculating the difference between 2 images. Since this is a small project, all input images will be of the same dpi and resolution and hence, once aligned, the difference between the input and the true images can give a rough estimate.

Which method do you think is the most viable?

Answer 1

[Image Analyst]

Thresholding is your best bet for cropping.

For identification of denomination, you can use the histogram of the color gamut, or identification of the denomination symbol.

[oshawcole]

Hi! How did you finally do it?

[Image Analyst]

DO WHAT? Crop? Denomination determination?

Be aware that this was 4 years ago so he may not answer.