December 14th, 2009, 01:50 PM | #1 | ||||
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Moire and Photoshop...
This thread is coming here after the...ahem..."gentle nudge" from Al Gebra..
I originally posted this observation in my scan set thread... Quote:
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I always scan at 300 DPI and 200% size. This wasn't so much because I was trying to avoid moire but because I would be able to end up with a substantially larger image after downsizing to 72 DPI (I would first downsize to 72 DPI but not change the pixel size and then lower the pixel size to get a larger image) but also because I would color adjust and use Levels on the image before I did any adjusting of the DPI and size as this would result in a more overall uniform histogram with no spikes (the downsizing would kill off the spikes). Quote:
Say I have a scanned image at 2000x1000 with a 300 DPI. Moire exists at that size. Given the size of the original image, in the past using 6.0 my target aspect ratio when all was said and done would be about 500x450 with 72 DPI. And usually the moire would be gone. But now it isn't gone. It's still there. Photoshop preserved the effect. But the twist is if I tried ending up with a different size setting, say 600x550, the moire could be worse or better or gone completely. I find that I have to keep trying different sizes for the final image to find the size that shows the least moire or no moire. We're talking a difference of +/- 5-10 pixels in dimensions - 610x560 produces more moire, 600x550 produces less moire, 590x540 produces a relatively moire free image, 580x530 produces more moire again. And so on. It's cyclical. You have to find the dimension in the cycle that produces the least moire/no moire. And because it is cyclical, there are several possible "sweet spots" you can pick from. This can be good or bad. It can be good because I can now get final "clean" images which are bigger than I could have gotten 10 years ago. Whereas 10 years ago I couldn't get higher than 500x450 for one image, I could now get 600x550 or 700x650, depending on the image. A nice improvement. But it's bad because there is no single formula that works for every single image. It's a crap-shoot and you have to constantly guess a size for a starting point and then work up or down in size to find the correct size that's moire free. Quote:
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December 14th, 2009, 11:35 PM | #2 | |||||
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This makes a huge image of course but it gives me the leeway to size right down usually to less than 50%. After which he evidence of any kind of litho dot pattern is totally gone. Let me just illustrate that because I just uploaded some old scans I did of Cara Lott: Rest of them are here: http://www.vintage-erotica-forum.com...cara-lott.html Post #102. That was not a great scanner in those days and yes it's still big but you don't see the dots, they're gone therefore now I could size that to any size I like and not get any moire whatsoever. As for screen / print dpi changes, I usually don't bother. As you know in PS you can change from 300dpi to 72dpi without resizing the image. You check the box below to not resample. However, if you are not doing that you are using the 72dpi to reduce the size as well and will get whatever size it equates to. I prefer to put the target size in directly, eg. 800 x 600 or whatever. 72dpi is usually for screen and 300dpi for printing generally they can be changed independently of actual size later. Quote:
What I'd do is try the same thing in some other packages to see if you get the same cyclic effect. Perhaps Paintshop pro which is my second choice for doing some quick and dirty things. Let's see if it's an algorythm. I'd also be willing to confirm for you if this is a CS4 bug perhaps. If you'd like to send me a troublesome example I can try this on CS3 to see if this has the same effect. Quote:
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Although there are colour shifts due to my original scanner I am happy with the sharpness of the Cara images and most of the scans I did in those days, I've not tried this on my current Canon Lide 500 but it does seem a reasonable scanner. Each scanner's CCD will also be part of this equation, a subject I'm not that familiar with, eg how they differ now days to then. I'd be interested though in finding out if CS4 has a particular problem or perhaps other later versions after 6.
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December 15th, 2009, 12:47 AM | #3 | ||||
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I find that is still true now though I'm still getting acquainted with what this scanner can do. It now takes me under 45 minutes to get a set scanned in. If I can do an end run around this photoshop moire wierdness by blurring the image at 300 DPI that will speed up the process even more for me. I'm not trying for the "perfect image". Good enough more often than not is acceptable for me. Quote:
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The one area of my scanning which is a continual work in progress as well as a bane of my existence has to do with the touchy subject of color correction. I think I'll bust out a separate topic on this subject to stimulate discussion and feedback. |
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December 15th, 2009, 01:05 AM | #4 | |
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As a photographer I know that problem well too. if you've got a white and a black in the picture it can be very easy but alas this doesn't always happen.
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December 16th, 2009, 05:19 AM | #5 |
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Ok the Gaussian blur really made a huge difference. Despeckle doesn't seem to do much (I do despeckle first). But I'm still using it.
Since the moire problem is no longer a problem I'm not too inclined now to do a comparison set of pictures to show what Photoshop does with moire when you downsize to various sizes. |
December 16th, 2009, 03:30 PM | #6 |
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If blur is acceptable to you then experiment also with the median filter you may find it better at retaining sharpness where it matters but it's a case of suck it and see.
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December 16th, 2009, 05:04 PM | #7 |
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Unsharpen Mask seems to do the trick for retaining sharpness...
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December 23rd, 2009, 06:10 PM | #8 |
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Well, after reading all of that, I guess I must inform you that I scan using my HP Scanjet 3200C. This is a parallel scanner, old timers must have heared of it, it was a very low end scanner at that time, I got this one for free, had PSU problems, I replaced one capasitor in there and it works like a charm!
This one takes about 15 minutes to scan a page at 400 dpi, I use this setting because the default of the scanner is 150 dpi and this produces moire many times. After completing the scan, I save it as JPG using highest quality. Then I use Irfanview to resize the image at maybe 1000 pixels horizontaly using the Lanczos resample filter and adding sharpening after resizing. After all of that, I post the image over here. I guess this must be the reason that I do not post that many images... Last edited by muler; December 23rd, 2009 at 06:15 PM.. |
December 24th, 2009, 09:46 AM | #9 | |||
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Let me illustrate this: 1. your original scan 2. how it looks like on my screen (full of moire) 3. your scan with Descreen ON: (created in Photoshop) <-- click to enlarge I hope you can see the difference. Please observe the file size too (482kB vs 281kB). Quote:
Tuffy |
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December 24th, 2009, 09:06 PM | #10 |
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Thanks for the info.
I tested scanning at 200 dpi and 300 dpi, the aquired image may be very large (for a JPG at 100% quality) but my joke of a scanner does a better job higher than 300 dpi. At 200 dpi there is moire you wouldn't believe. The scanned pic looks even better at 600 dpi but I am too bored to wait that long. Here is my masterpiece of a scanner... http://www2.shopping.com/xPO-Hewlett...-ScanJet-3200C It is correct that I should not save a JPG twice, actually the second time I use 90% quality to end up with a smaller file. No way of using anything like photoshop, my computer cannot handle such an application, it has the age of my joke scanner (if not more). Once I had scanned and saved as TIFF, produced very large files indeed, then made a multipage image file with these. I have to try it again maybe, altough doing that would propably mean that I would have to erase the original TIFF files, due to hard disk space constraints. The image you descreened looks like a bit blurred on my side, so I will check if the blur filter (of Irfanview) does any good in mixing the dots together. Anyway, I can do a lot of tests on a scanned file, but cannot do many scan tests because of the 15 minutes per page, also while using the scanner, some other specific apps on my computer do not work because of some IRQ or (more likely) DMA conflict. Makes me wonder why I bother to scan at all, but it looks right to try to give something back to this great forum. |
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