the program fax.c reads in a jpeg color photo of a line drawing and
prints it out as a full contrast black and white drawing.  This is
useful for using a cell phone or web camera to capture a hand drawn
schematic or block diagram.  Most fax machines had a similar algorithm
to suppress coffee stains, shadows or paper crinkles, hence the name
"fax". 

The algorithm is a 2d baseline tracker with a comparator. 

The program reads an image <file>.jpg, blurs it with a fast X,Y boxcar
convolution, offsets the blurred image by a constant pixel value to
product a clipping threshold, and then compares the original image against
the threshold, outputting black or white appropriately.  The output is
written to <file>.pgm

If given the -g option, <file>.pgm is converted to <file>.gif using the
pamtogif utility.

The algorithm does a great job of ignoring shadows and lighting
irreguluarities, paper wrinkles, stains and smudges and works well for
converting hand drawings or photos into line work. 

You'll find a fast blurring algorithm in the code that convolves a
boxcar in the x-direction, and then in the y-direction handling wrap
around and edge effects appropriately.