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DPChallenge Forums >> Hardware and Software >> Chroma Subsampling?
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11/04/2018 11:39:17 AM · #1
Question. Do any of you ever use Chroma subsampling when saving a file to jpg for challenge entries?

I'm desperate in getting detailed images to show anything near the same quality I'm seeing in my post application prior to saving at 700px. How do you guys do it? I've experimented with various settings and my process is usually to choose the optimizer and change the compression setting until the image is at or below 700px. That's without any chroma subsampling. If I mess with some of those the compression doesn't need to be as high (example ... I may need compression of 12 or 13 at regular jpg and only a compression of 7 when chroma is set to 1x2 1x1 1x1).

My software for processing is Corel PSP (current version).

So, so, frustrated at what I know the image should look like versus how it displays on the challenge entry page.
11/04/2018 12:20:18 PM · #2
I don't use it, but I do see how it could be useful. Essentially, you're giving up the resolution of the color data to retain detail in luminosity data. That may be a good trade-off, since the eye perceives detail based more on luminosity than on color.
11/04/2018 02:36:58 PM · #3
I assume you mean 700kb, not 700 pixels ...

Another thing to try is using different software for the final conversion to JPEG -- I always save my final image to TIFF first, then to JEPG at various compression settings to get it under 700k. There are (used to be?) many free graphics conversion programs; I usually use IrfanView when I'm not using Photoshop.
11/04/2018 06:53:06 PM · #4
Originally posted by GeneralE:

I assume you mean 700kb, not 700 pixels ...

Another thing to try is using different software for the final conversion to JPEG -- I always save my final image to TIFF first, then to JEPG at various compression settings to get it under 700k. There are (used to be?) many free graphics conversion programs; I usually use IrfanView when I'm not using Photoshop.

Yeah, meant 700kb. I work with the pspimage format from the Corel PSP program and then convert to JPG. Hadn't considered an interim step. Hmm.
11/04/2018 07:41:53 PM · #5
You can also use a multi-step size reduction strategy, and tailor the algorithm at each step. For instance, reduce first to 4800px, then 2400px, and finally 1200px. At each step you can change the resampling algorithm. Using an algorithm like "bicubic sharper" at all the steps, will probably result in an oversharp final image, while using a more conservative algorithm may result in lost detail. A combination can be used to optimize detail without introducing visible artifacts.
11/04/2018 08:03:04 PM · #6
I also use "intermediate" steps to adjust sharpening ... for example if my final image is slightly over 700k, I'll go back and apply slightly less sharpening and re-save as JPEG, rather than increase the JPEG compression.

My typical entry will (eventually) consist of six files: Original, Photoshop. adjusted TIFF, resized TIFF, resized and sharpened TIFF, and final JPEG -- the last three are relatively tiny so it's no big deal to save them all, and the big TIFF file is what I use for prints.

Message edited by author 2018-11-04 20:03:46.
11/04/2018 08:36:08 PM · #7
Originally posted by kirbic:

You can also use a multi-step size reduction strategy, and tailor the algorithm at each step. For instance, reduce first to 4800px, then 2400px, and finally 1200px. At each step you can change the resampling algorithm. Using an algorithm like "bicubic sharper" at all the steps, will probably result in an oversharp final image, while using a more conservative algorithm may result in lost detail. A combination can be used to optimize detail without introducing visible artifacts.

I usually resize in two steps, first to 2400 (bicubic 100), then a second to 1200 (bicubic 50). After that I may dup a layer and use Nik Output sharpener and reduce the opacity until I'm satisfied with it. The problem really isn't getting good results on my local PC - often I'm very satisfied with how the final image looks. It's the compression and loading to DPC that's bringing the image quality down somewhat.

I'll probably play with a couple different settings and see what shakes out. I do appreciate the feedback - Thanks to both of you.
11/04/2018 08:48:07 PM · #8
Originally posted by glad2badad:

I'll probably play with a couple different settings and see what shakes out. I do appreciate the feedback - Thanks to both of you.

You're welcome -- consider posting some examples (or sections thereof) of your experiments so others can learn from them too ... :-)
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