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Old May 30, 2004, 10:04 PM   #1
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We keep hearing complaints about the DX6490's 'aggressive jpeg compression'. It's been claimed that this is the camera's most serious flaw. I've been wondering about this, and decided to try some tests, to see if I could come up with an answer. I don't have anything conclusive, but I'd like to share what I've found. Maybe somebody else can go further with this idea.

First of all, on what scale do we measure jpeg compression? I don't think there is a standard. I use Paint Shot Pro 8 (PSP) for editing; it offers a compression scale from 1 (lowest compression, best quality) to 99 (highest compression, lowest quality). However, Irfanview uses a scale that runs in the opposite direction: from 1 (lowest quality) to 100 (best quality).

For that matter, just how much compression does the 6490 use? I've looked at file sizes obtained when saving uncompressed pictures from another four megapixel camera, as well as resaving 6490 pictures at different compression rates. My guess is that the 6490's jpeg compression is about the same as '10' on the PSP scale. Just for simplicity, I'll call that '10% compression'.

I took an uncompressed Tiff file with another four megapixel camera, and opened it in PSP. I saved that image as a jpeg at 10% compression. Then I opened the jpeg in PSP, copied it to the clipboard, and pasted the compressed image into the original (uncompressed) image, as a new layer. I then opened the new layer's property dialog and changed its blend mode to 'difference'.

A difference blend compares each pixel in the difference layer with what's in the layer below it. If the pixels are identical, the difference layer displays a black pixel. If they are not identical, the difference layer displays a pixel with an RGB value that is the arithmetic difference between the two. (That's oversimplified, but sufficient for our purposes.)

Looking at the difference between the uncompressed image and the 10% compressed image, I saw a black screen. Once I zoomed in to nearly 100% and scrolled around, I could detect some faint gray lines in the image. Those gray lines showed where the compressed image did not quite match the original.

I don't have any definite conclusions to draw. I've demonstrated to my satisfaction that there are differences between an uncompressed image and one compressed 'ten per cent'. Not that there was any doubt about that. And I can state that the difference appears to be very small. I find it hard to imagine that the difference will very often be crucial to the final quality of a picture, but it's possible.

I hope somebody else will explore this further. No doubt, somebody could improve on my methodology! If you'd like to take a look at a few samples, you are welcome to look at the site below, but be warned that it's not very informative:


What you see there is an original image, downsized to 640 x 480, purely as a frame of reference. Then there's a 100% crop from the original image (not downsized), almost uncompressed. (I had to use 1% compression to get it onto Photobucket.) Following that, the same 100% crop, but at 20% compression. (Note: that's twenty percent, instead of ten.) And finally, a nearly black frame that displays the difference between the uncompressed original and the 20% compressed version. I used 20% so you could see the difference. At 10% it was too subtle to display well on the web.

From now on, I don't think I'll worry too much about the 6490's excessive compression!

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