Luminous Landscape Forum

Raw & Post Processing, Printing => Digital Image Processing => Topic started by: hovis on March 01, 2007, 09:56:07 am

Title: pixels and memory
Post by: hovis on March 01, 2007, 09:56:07 am

Hi everyone

I know this is going to seem like a really stupid question to some but I haven't been able to find an exact answer.

Can anyone explain why, when I take two identical sized crops from different areas of the same image I get two different file sizes when I convert those crops to jpegs of the same compression? I was under the impression that pixels had an absolute value unaffected by colour.

Thanks

Title: pixels and memory
Post by: michael on March 01, 2007, 10:13:34 am

The JPG format uses compression based on content, and therefore depending on the image's subject matter file sizes will differ.

Michael

Title: pixels and memory
Post by: hovis on March 01, 2007, 10:55:58 am

Ah ok :-)

Thanks Michael

Title: pixels and memory
Post by: trigeek on March 01, 2007, 12:40:19 pm

Quote

Ah ok :-)

Thanks Michael
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To take it a step further (and I am by no means an expert), I believe file size scales with the complexity of the subject. If, for example, you had an image that was a blank image, the complexity would be very low, and would give the best compression. On the other hand, if you a highly complex image... lots of shapes / colors in non repeating sequences, then compression would be on the low end of the scale (larger file size).

Same goes for other compression engines... such as mp3 for music and whatever they use for satellite TV. I have directTV at home and the image on the screen becomes blockier (when viewed closely) under high action sequences. Compression efficiency goes down and, in the case of DirectTV that has a fixed bandwidth, something has to give.

Title: pixels and memory
Post by: 61Dynamic on March 01, 2007, 05:48:09 pm

All forms of compression will do the same (some more than others). If you saved the images as uncompressed tiffs, then the images would be the same size.