On 7 May 2004, at 18:32, Giles Stokoe wrote: Snip
If (with my calculator) I work out 1600 x 2400 x 3 (RGB colour channels) II take it the rest are still lodged inside the calculator!
get 11.5 pixels.
File formats vary in how they store the values they find. A 'bog standard TIFF' reads each and every byte sequentially line by line, so the numbers will correspond, with a proviso that the camera manufacturer was giving you the 'effective' pixels (that is those involved in image capture, as opposed to control pixels).
How can a file occupy fewer megabytes than it has megapixels? Surely each
pixel of each colour channel contains one byte of info.
However, even TIFF can utilize a procedure known as Run Length Encoding (RLE). What happens is that a byte contains a start point and a run length of how many identical values exist along the scan horizontal scan line, then a stop point; thus three bytes can potentially record an entire line of equal value pixels. In practice what this means is that in a landscape taken horizontally with a clear blue sky that goes from rich dark blue at the top to lighter blue lower down, the compression of the data (losslessly) is very efficient, but were you to rotate your camera and take a shot vertically the compression would be far less efficient because the chip's scan lines will still be running along the longer dimension, but the values along the line would be inefficiently compressed since they would change almost continuously.
An RLE-compressed vertical shot would therefore be very close to an uncompressed shot of the same scene, but the former horizontal RLE-compressed shot would be very much smaller.
The kilobyte and megabyte descriptions were introduced for simplification at a time when that seer Bill Gates foretold that no one would ever need more than 640K of RAM! It is now a very foolish way to describe kilobyte memory amounts as 1000 as opposed to 1024, because we now deal in such large chunks that the error is way too great to be meaningful.
The hard drive comments made, fall into marketing speak,- in much the same way as the dimensions given for CRT monitors and scanner resolution. They all tell only part of the story - Hard drive manufacturers mostly describe the unformatted disc space, monitor manufacturers talk of the physical size of the CRT, not the image visible to the user, and scanner manufacturers give interpolated file sizes in large type and the truer optical resolution is buried in some small print on the back page!
Sorry if this has grown longer, but it may help someone else to grasp what is happening.
Rod Rod Wynne-Powell ____________________________________________________ Retoucher, Trainer, Consultant, Author of 'Photoshop Made Simple' Tech Editor for Martin Evening and Deke McLelland's books: "Photoshop 7 for Photographers"/"The Photoshop 7 Bible" SOLUTIONS photographic email: rod(at)solphoto.co.uk 01582-725065____________________ 07836-248126 mobile___
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