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-   -   To understand compressed picture (https://forums.steves-digicams.com/general-discussion-11/understand-compressed-picture-6878/)

koruvs Feb 6, 2003 7:37 PM

Hi BillDrew,

I have never really used RAW format before and when I did I didn't pay attention to the files sizes so I was some what surprised to see the file sizes being the same. Coming from a JPG world only it made me curious. Thanks for the brief explanation though, it cleared things up for me :)

NHL Feb 6, 2003 9:09 PM

Koruvs

Didn't we just went through this already for using raw files to measure flash speed and not jpegs? http://www.stevesforums.com/phpBB2/v...ghlight=#29457

IMO zipping wouldn't make a difference, since one has to unzip them first back to the original format before the program (ie NikonView or a Photoshop plug-in) can decode them... Some pro dSLRs do compress their raw file , but at the expense of a longer extra step for a smaller file size... either way the original raw data monochrome reading from the CCD has to be decompressed again or preserved to complete the interpolation to a full color pixel. For example adjacent pixels are usually not in-line when data is shifted out from the CCD, but are on the N-1, and the N+1 line, and the consecutive pixels are usually of a different color R, G, or B making any compression not very effective...

koruvs Feb 6, 2003 9:39 PM

NHL, oh, way back then...I was young and unexperienced compared to now :lol: :shock: :lol:

voxmagna Feb 7, 2003 3:39 AM

Quote:

Would be interesting to know what compression factor you would get by ZIPing them. That likely would change with the amount of detail in the picture.
Just a comment: Winzip is a compression algorithm looking for regular pattern and redundancy in data. Photos contain a lot of random data so that's why we use JPEG/MPEG.

NHL Feb 7, 2003 5:40 AM

FYI everyone

An another example why the above test is flawed:
Quote:

Plain black-and-white (two level) images should never be converted to JPEG; they violate all of the conditions given above. You need at least about 16 gray levels before JPEG is useful for gray-scale images. It should also be noted that GIF is lossless for gray-scale images of up to 256 levels, while JPEG is not.
http://www.faqs.org/faqs/jpeg-faq/part1/


Vox

I believe that's what Bill was talking about ie zip the raw file for smaller size before transfering then unzip back to the original format before using on the PC. Jpeg raw was never an option since this compression is performed on blocks of pixels eight on a side!

BillDrew Feb 7, 2003 8:46 AM

My comment about ZIPing the RAW files was purely a matter of curiousity - not sure that it would be worthwhile. With a photo that was purely single valued , there would be a regular patern of byte values. Would ZIP pick up the compression possibility in a patern of similar valued bytes in regions like a flat sky? As NHL pointed out, the patern would not be simple, but there would be a patern Likely he is right about not getting much compression from ZIP, but I am a strong believer in experimenting.


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