basic-concepts.md

Basic Concepts

The very basic object crass operates on is a page with separator lines. First the page will be cropped into segments based on the separator lines and afterwards spliced together to a new image. There must be at least one vertical and one horizontal line that the code runs correctly. In an additionally preprocessing step, crass might detect the rotation of the page and will rotating it to the correct angle. This process is called "deskewing".

Input and Output Image Files

crass can process either one page or a folder containing several pages with the same extension. The output files will have the same extension as the inputs ones.

By default, crass places the single segments and the debug information resp. images into the directory "out/..", e.g. the subdirectory "out/spliced/.." will contain the final spliced images.

File Formats

The image-file format accepted by crass is jpg.

Single Steps to Victory

Deskew

1. Find the top or bottom horizontal line.
2. Compute the deskew angle.
3. Rotate to the correct angle.

Line Analyse

1. Find the top or bottom horizontal line.
2. Finds all vertical lines in the middle (by default) of the page.

Crop

1. Compute the clipping masks.
   • There are 5 types of segments:
     • h = header
     • a = left side separated by a vertical line
     • b = right side separated by a vertical line
     • c = space between header and vertical line or vertical line and another vertical line
     • f = footer
2. Crop the single segments (by default: the footer and header information will also be stored)

Splice

1. Splice the single segments according to a certain order (by default, a then b then a etc. until a c segments forms the end).

Further Information

You can find more detailed information about the single steps and setting options in the image processing documentation.