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Author Topic: The order of changing bit depth and color profile  (Read 1899 times)

aderickson

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The order of changing bit depth and color profile
« on: May 30, 2015, 12:20:15 am »

I have read that if one is working in a large color space such as prophoto one should also be working in a high bit depth to avoid possible banding.

If I have an image in prophoto and 16 bits and I want to put it on a webpage is it important that I convert it to sRGB before converting it to 8 bit to save as a jpeg?

Allan
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Schewe

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Re: The order of changing bit depth and color profile
« Reply #1 on: May 30, 2015, 03:03:57 am »

If I have an image in prophoto and 16 bits and I want to put it on a webpage is it important that I convert it to sRGB before converting it to 8 bit to save as a jpeg?

Correct...when possible always do color space transforms in 16-bit before mode changing to 8-bit.
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Bart_van_der_Wolf

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Re: The order of changing bit depth and color profile
« Reply #2 on: May 30, 2015, 03:51:02 am »

I have read that if one is working in a large color space such as prophoto one should also be working in a high bit depth to avoid possible banding.

If I have an image in prophoto and 16 bits and I want to put it on a webpage is it important that I convert it to sRGB before converting it to 8 bit to save as a jpeg?

Hi Allan,

How important it is, will depend on the image in question. But in general it's best to do the major number crunching (= profile conversion), with as much precision (e.g. 16-bit/channel) as feasible, before totally dropping part of the precision (going from 16-b/ch to 8-b/ch), and additional (lossy) compression into JPEG.

Doing it that way will accumulate the least amount of round-off and/or truncation errors.

The ultimate consequence would be that it may even help (a bit) to first convert 8-b/ch source images to 16-b/ch before doing things like tone curve adjustments and profile conversions, even though we already start with one arm tied behind our back. This becomes even more the case if we will subject the original image data to several conversions in sequence, before arriving at an 8-b/ch end result.

Cheers,
Bart
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walter.sk

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Re: The order of changing bit depth and color profile
« Reply #3 on: June 11, 2015, 02:21:51 pm »

Hope this doesn't hijack the thread, but I had the above question as well as changing the pixel dimensions and then converting to jpeg, for projection on a projector/computer combination that are profiled, but I have no way to softproof for.  My original images are Tiffs, 16-bit color and ProPhoto RGB color space, and post-processed from Canon 5DIII CRW's.  They get reduced to 1024 pixels on the longer side, 8-bit and sRGB.  The sequence I had been using is:

1) Convert the color space to sRGB,
2) Downsample to 1024 x 688 or whatever the other dimension comes to),
3) Convert from 16-bit to 8-bit color,
4) Save as Jpeg.

Is there a better sequence to use, or does it not really matter that much?
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Bart_van_der_Wolf

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Re: The order of changing bit depth and color profile
« Reply #4 on: June 11, 2015, 03:44:54 pm »

Hope this doesn't hijack the thread, but I had the above question as well as changing the pixel dimensions and then converting to jpeg, for projection on a projector/computer combination that are profiled, but I have no way to softproof for.  My original images are Tiffs, 16-bit color and ProPhoto RGB color space, and post-processed from Canon 5DIII CRW's.  They get reduced to 1024 pixels on the longer side, 8-bit and sRGB.  The sequence I had been using is:

1) Convert the color space to sRGB,
2) Downsample to 1024 x 688 or whatever the other dimension comes to),
3) Convert from 16-bit to 8-bit color,
4) Save as Jpeg.

Is there a better sequence to use, or does it not really matter that much?

Hi Walter,

I'd use  2, 1, 3, 4 as order of events.

Down-sampling could (depending on algorithm used) result in lower contrast and blended (=new) colors that may map differently in the smaller gamut colorspace. That's why it may be beneficial to down-sample first, but it also depends on the resampling algorithm and subsequent sharpening how big a difference it creates.

Cheers,
Bart
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walter.sk

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Re: The order of changing bit depth and color profile
« Reply #5 on: June 11, 2015, 04:36:29 pm »

Thanks, Bart.  I use Photoshop CC to downsample, on the "auto" setting.  I guess it uses Bicubic Sharper, which seems to look fine when projected.
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