QAID # 9423 Published
Question / Problem:
How can I calculate the size of an uncompressed, and TIFF - Group 4 compressed bitonal scanned image?
Answer / Solution:
All scanned images consist of dots (pixels); in bitonal (black & white) scanning, these dots represent Black or White (On or Off).
When scanning, we use a term called DPI (Dots Per Inch). This equates to how many dots are used to represent the information on the paper.
Think of an A4 size page — A4 size page is 8.5 inches wide and 11 inches tall. In an A4 size page scanning at 200 DPI, there are 8.5 x 200 dots and each dot can be black or white. Also vertically there are 11 x 200 dot Rows.
The following is how you can calculate the image size (Uncompressed - raw):
8.5 x 200 x 11 x 200 dots in the whole page
= 3,740,000 dots or bits
= (3,740,000 / 8) => 467,500 Bytes
= (467,500 Bytes / 1024) => 456.54 KBytes
= approx. half a megabyte (MB)
So if you scan a A4 size document with 200 DPI, the original file size of the image is 456 KBytes. But, in reality, we compress these images to smaller files.
Typically, Group 4 compresses an uncompressed image to 20 times smaller, in 20:1 ratio. Based on this compression ratio, the above image will be the size of (approximately) 23 KBytes.