Gray > Brightness

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Marpel
Posts: 288
Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Gray > Brightness

Postby Marpel » February 15th, 2017, 6:09 pm

Trying to understand the Gray > Brightness transform a bit better. Was trying to change tones in an image and this transform did' t work quite as expected. I'm sure it is user error/misunderstanding.

To figure things out, I took an image whose dynamic range did not extend the full width of the histogram, and in Levels and Color, the right and left ends were at 39.2 and 83.9 respectively.

I applied a 14.3% increase in Gray > Brightness (this amount was arbitrary).

The first time, I clicked on "Black and White" (to maintain both black and white ends, presumably) and re-measured the resultant image in Levels and Color. Readings were 43.9 and 87.1.

I then did the same process, but clicked on "Black". Measurements were now 48.6 and 100.

In the Help literature for Gray > Brightness, it advises that clicking on "Black" it applies a curve but "keeps black black" while for "Black and White" it further states "keeps black black and white white".

Question 1 -Does the change in the numbers in Levels and Color actually mean that Gray > Brightness only keeps the blacks and whites in the same general area but does not maintain the exact darkest point (as in 39.2 to 43.9 etc) or brightest point (as in 83.9 to 87.1)?

Question 2 - Why does the difference between the darkest and brightest points differ between the original and resultant images (i.e 39.2 to 83.9 is 44.7 while 43.9 to 87.1 is 43.2)? In other words it seems to shrink the dynamic range of the image (obviously not by much)?

Question 3 - If Gray > Brightness is actually working the way it is designed, how then do I increase the brightness of all the tones in an image (except the two end points of course) while maintaining the darkest and brightest points?

Hopefully my explanation and questions are understandable.

Thanks,

Marv

jsachs
Posts: 780
Joined: January 22nd, 2009, 11:03 pm

Re: Gray > Brightness

Postby jsachs » February 15th, 2017, 6:42 pm

Depending on the Preserve setting, one of four sets of curves are applied to the image as illustrated below:

Brightness.jpg
Brightness.jpg (62.41 KiB) Viewed 469 times


None of these preserve the black or white points of the image unless they are at pure black or pure white.

To do what you want, I recommend using Brightness Curve with a curve something like the following:

Brightness Curve.jpg
Brightness Curve.jpg (40.53 KiB) Viewed 469 times


These curves will make the image lighter or darker without moving the black and white points of the image.

Alternatively, you could expand the image to full range, use Brightness preserving both black and white and then compress the dynamic range back to its original.
Jonathan Sachs
Digital Light & Color

Marpel
Posts: 288
Joined: September 13th, 2009, 3:19 pm
What is the make/model of your primary camera?: Nikon D810
Location: Port Coquitlam, British Columbia

Re: Gray > Brightness

Postby Marpel » February 16th, 2017, 10:11 pm

Thanks Jonathan,

I think your comment about not preserving the black and/or white points unless they are full black or full white (and of course, the image I was working on did not have full dynamic range) helps clear things up. I will try out your two suggested methods to see which works best.

Marv


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