System for fast 90-degree rotation of bi-level images

In a 32-bit operating system, a method of rotating a bi-level image by 90.degree., comprising the computer implemented steps of:

• a) selecting an unrotated 8 by 8 pixel matrix from the image data;
• b) generating an intermediate matrix having 8 rows by 8 columns of pixel data bits from the pixel data bits of the block, wherein the block is divided into four row pairs and the intermediate matrix is divided into four nibble column pairs, each said row pair corresponding to one of the nibble column pairs, each row of each row pair being divided into two halves, one of said halves of one row of each row pair being rotated 90.degree. and translated to form a top half of one nibble column of the corresponding nibble column pair, the other one of said halves of said one row also being rotated 90.degree. and translated to form a top half of the other nibble column of the corresponding nibble column pair, one of said halves of the other row of each row pair being rotated 90.degree. and translated to form a bottom half of said one nibble column, the other one of said halves of said other row being rotated 90.degree. and translated to form a bottom half of the other nibble column of the corresponding nibble column pair;
• c) saving the rows of the intermediate matrix sequentially into first and second 32-bit processor registers;
• d) copying the first 32-bit register into a third 32-bit processor register and the second 32-bit register into a fourth 32-bit processor register;
• e) shifting each of the first and second registers by one bit in opposite directions;
• and
• f) overlaying alternate bits from the fourth register onto the first register and from the third register onto the second register;
• and
• g) repeating steps a) through f) until the image has been rotated.

A 32-bit system for operating a block of bi-level image data having 8 rows by 8 columns of pixel data bits, comprising:

• means for generating an intermediate matrix having 8 rows by 8 columns of pixel data bits from the pixel data bits of the block, wherein the block is divided into four row pairs and the intermediate matrix is divided into four nibble column pairs, each said row pair corresponding to one of the nibble column pairs, each row of each row pair being divided into two halves, one of said halves of one row of each row pair being rotated 90.degree. and translated to form a top half of one nibble column of the corresponding nibble column pair, the other one of said halves of said one row also being rotated 90.degree. and translated to form a top half of the other nibble column of the corresponding nibble column pair, one of said halves of the other row of each row pair being rotated 90.degree. and translated to form a bottom half of said one nibble column, the other one of said halves of said other row being rotated 90.degree. and translated to form a bottom half of the other nibble column of the corresponding nibble column pair;
• at least first, second, third, and fourth 32-bit processor registers, the first and third registers for saving, in duplicate, the first four rows of the intermediate matrix and the second and fourth registers for saving, in duplicate, the remaining four rows of the intermediate matrix;
• means for shifting each of the first and second registers by one bit in opposite directions;
• and
• means for overlaying alternate bits from the fourth register onto the first register and from the third register onto the second register.