• CCITT G3 ID scheme is used only for facsimile(fax) machines and low resolution based document imaging systems.

• CCITT G3 standard is good for low resolution Document imaging. It is not good for high resolution document imaging application.

• CCITT G42D is two dimensional coding scheme (2D).

• In this method, the first reference line is an imaginary all white line above the top of image.

• First group of pixels scan-line is encoded utilizing imaginary white line as reference line.

• New coded line becomes reference line for next line.

• K factor in this case is entire page of lines.

• Each successive line is coded with relative previous line. This provides very high level of compression.

• Fig: page structure for encoding

• Flowchart for CCITT G4 2D Algorithm (Working)

Advantages:

• Used for high resolution document images.

• More reliable than CCITT G3 1D compression algorithm.