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Author Topic: Isolated in TIFF, but not in JPEG  (Read 2981 times)

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« on: December 11, 2007, 19:56 »
I have an image rejected in IS, and it's this one I have in StockXpert:

Most of the image is isolated, except next to the flame, where there is a gradation from the flame's brightness.  I checked the TIFF with a RGB tolerance of 0, so I only get one solid color, except also for the contour around the candle (carefully selected and isolated with a feather of 2 pix).  When saving to JPEG however (at minimum compression, I tried it in PSP7, my regular editor, and also in Paint and in Illustrator), some black pixels next to the candle cease to be (0,0,0) and become (1,1,1), (0,0,2) or (1,0,0), so they consider this a flaw in isolation.

Mind you, it's not just the edge from background to the candle, which due to feathering would not indeed be 100% black, but some spots around it and located in the darker area far from the flame.  I attach below a sample, where I deleted all (0,0,0), so you can see what I mean.  The left portion is the TIFF file, the right portion the JPEG.

Any way to overcome this?


« Reply #1 on: December 11, 2007, 21:16 »
The same occurs with over whites, and to be honest, I never checked that before. Below is the edge of a small red Chili pepper over white (RGB 255,255,255) in a food shot at 200%, in Photoshop CS3, the white selected by magic wand tolerance 0, no anti-aliasing, not contiguous.

The left clip is the 16-bit TIFF, the right clip is the 8-bit jpg saved in top jpg quality 12, and converted to 8-bit right before saving: I do all my image processing in PS at 16-bit, and I start from RAW. Clips are 200%.

Part of the jpg compression algorithm uses dithering around edges, and that is what we might see here. Good work from the reviewers, I never thought about that. I think it's simply unavoidable using the jpg compression algorithm.

Pretty obnoxious for designers though if they have to redo the isolation. Maybe include the clipping path in the images? I never understood how to do it, but it's more laziness since there seem to be a few tutorials online.

To photogs that use clipping paths: where can a good tutorial be found, and does the clipping path also contains this kind of compression artifacts?
« Last Edit: December 11, 2007, 21:25 by FlemishDreams »

« Reply #2 on: December 12, 2007, 20:04 »

I hadn't tried in white, interesting to know it also happens.  Then, if it's a common issue, why reject an image for this?  As discussed in another thread, clipping paths are not required by the sites, nor are they passed to al reduced sizes.



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