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Author Topic: Compression Artifacts?? How do I spot them?  (Read 4282 times)

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« on: June 23, 2007, 21:11 »
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Hi All,

I got a rejection on a photo for compression artifacts.  I blew up the image to 200 and 400% but I still don't see the problem. 

Can you see what the issue is? 

Cricket



« Reply #1 on: June 23, 2007, 22:42 »
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See all the dark yellow speckles in the shadows?  Those are artifacts and noise.

« Reply #2 on: June 23, 2007, 23:40 »
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Karimala,

Sorry to be ignorant... but isn't that just the colors of the shadows?

BTW... to help me understand this artifact thing better, could someone show me a crop of a full color image before and after it has been noise filtered?

 I would like to see what the difference would be. 

I am using the noise filter that came with Corel Photo Paint, but I would like to see if some of these plugins like Ninja work any better.

Cricket


« Reply #3 on: June 24, 2007, 03:17 »
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No...the yellows should blend smoothly in with each other.  There also should not be any discolored spots or speckles in the green areas.  I used Neat Image on the after photo, but you can also use the blur tool for more precise smoothing.



Sorry this image is so small, but the file was too large to attach.
« Last Edit: June 24, 2007, 03:18 by Karimala »

« Reply #4 on: June 24, 2007, 07:22 »
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Karimala,

I tried reworking the image again.  Here it is in a before state and then after I used a noise filter on it. 

It might be too soft... I don't know.

Cricket

« Reply #5 on: June 24, 2007, 13:51 »
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I see the spots Karimala pointed out, aren't they noise, though?  Sometimes lightening shadows I get that. 

Can compression cause this type of artifacting too?  I ask that because of the rejection reason Cricket got. 

Regards,
Adelaide

« Reply #6 on: June 24, 2007, 16:31 »
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Best way to understand compression artifacts would be to create some really bad ones deliberately to look at...

Please hold. :)

« Reply #7 on: June 24, 2007, 16:37 »
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Two images for ya..

They are 100% crops out of a stock image of an old Medium Format camera..

The smooth looking one is a high quality jpeg.  The other is at setting 10 out of 100 in the gimp, which I suppose is the same as 1 or 2 in PS..

The smooth area of gradation around the inside of the lens no longer has smooth gradations, a the edges of the lines are effected by 'jaggies'  The smooth gray metal edge has developed a whole lot of 'noise'..

Does that help?

« Reply #8 on: June 24, 2007, 17:17 »
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Chellyar, this was a very interesting example.  I am used to seeing compression as an edge effect or as a problem with gradients, more or less as you explained. 

The type noise you showed, created by compression, do you think it could be mistakenly taken as the problem in Cricket's case?

Regards,
Adelaide

« Reply #9 on: June 24, 2007, 17:30 »
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I was thinking the reviewer got it wrong when they called it "compression" artifacts, because it's clearly noise and regular artifacting that's the problem.  Noise reduction software can't fix true jpeg compression problems.

« Reply #10 on: June 25, 2007, 06:18 »
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Chellyar, this was a very interesting example.  I am used to seeing compression as an edge effect or as a problem with gradients, more or less as you explained. 

The type noise you showed, created by compression, do you think it could be mistakenly taken as the problem in Cricket's case?

Regards,
Adelaide

Possibly...

jpeg works by looking for similar pixel colors nearby, and 'grouping' like pixels together into geometric (square/rectangle) shapes.  The result of that where you have large smooth areas is loss of gradations, which we're all familiar with, you can see it in the lens example in the larger black smooth curved area.

Less obvious is what this does to fine details..  The 'clumps' of like pixels are smaller, so the effect may look like 2 or more pixel 'spots' that are grouped together.

In the example of the flower where you would have subtle detail like the veins of the petal, the result would be similar to the metal edge in my example, you'd get 'noise' appearing between the detail lines..

Remembering that the detail doesn't have to be as obvious as the metal edge for the jpeg compression to mess it up..  The subtle change in tone in the flower texture might as well be the white/silver transition on the lower for all jpeg cares..

I'm sure I can create an example...  Please hold.. :)

« Reply #11 on: June 25, 2007, 06:47 »
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Download and view the attached images at 10x the size.

No tricks here, I created a white jpeg 150x150 pixels, with some gray diagonal lines on it..

1.jpg is at 100% (no compression).  2.jpg is at 50% setting in gimp (6 in PS?).

You'll see when you zoom in the 2.jpg shows 'noise' as well as the detail 'clumps' on the lines.

This is because jpeg replaces like chunks of data with either a solid lump of one color, or a gradient between two colors that were similar.  For the 'blocks' of white in between the gray bits it replaces them gradients approximating the gray to white transition present near the gray line.

That's a very rough idea of what it does of course, as the jpeg algorithm is actually very complex, but you can see the result...  Have a play in PS with your own images..

Anyway, for stock the same old rules apply...  Shoot in best quality jpeg or raw at a low ISO (Let's not start the raw vs jpeg thing all over!) save partially edited files as uncompressed files (native ps or GIMP format, tif if you really must), and save the jpeg at 100% or 12 (???) in PS.


 

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