avoid histogram combing effect?

peter wrote:

After editing levels (e.g. lower the max input from 255 to 240 or something, change the gamma), some histogram become "combed". Is there any ways to avoid this?

I understand this is caused by integer math. I wonder if changing the image from 8 bit / channel to 16 bit / channel would help. On the other hand, making this change already creates combs since out of 65536 brightness value, only 256 are possible from the 8 bit image. Unless, the conversion from 8 bit to 16 bit per channel involves some heuristics to generate more brightness values.

What keeps you from trying this yourself?

—
Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl

peter wrote:

After editing levels (e.g. lower the max input from 255 to 240 or something,
change the gamma), some histogram become "combed". Is there any ways to avoid this?

The bad news is "No", there is no way to change an channel with 255 different values to one with 244 values without altering the appearance of the histogram. The good news is that there is no need to avoid it for color photographs, since the combing will not be visible in the final image, even after fairly extreme changes.

It is possible to sabotage an image, by repeated extreme curve moves and the like, but starting from an original image from the camera, extreme edits to not cause problems in 8 bit color images in one of the popular working color spaces such as Adobe RGB. I have an open invitation out for any image that shows differently.

I understand this is caused by integer math. I wonder if changing the image
from 8 bit / channel to 16 bit / channel would help. On the other hand, making this change already creates combs since out of 65536 brightness value, only 256 are possible from the 8 bit image. Unless, the conversion from 8 bit to 16 bit per channel involves some heuristics to generate more
brightness values.

It does. By default, Photoshop adds about 1/2 bit of noise aka dither when converting from 16 to 8 bits. This is often mistaken for cleaner 16 bit math.

My advice is not to worry about histogram combing, or even the histogram. This is a tool that has it’s attractions, but has done more harm than good in the Photography world.
—
Mike Russell
www.curvemeister.com/forum/

"Mike Russell" wrote in message

The bad news is "No", there is no way to change an channel with 255 different values to one with 244 values without altering the appearance of the histogram. The good news is that there is no need to avoid it for color photographs, since the combing will not be visible in the final image, even after fairly extreme changes.

It is possible to sabotage an image, by repeated extreme curve moves and the like, but starting from an original image from the camera, extreme edits to not cause problems in 8 bit color images in one of the popular working color spaces such as Adobe RGB. I have an open invitation out for any image that shows differently.

Here’s a counterexample with a mild curve layer:
http://www.warrensarle.com/misc/Temp.psd

"peter" wrote in message

After editing levels (e.g. lower the max input from 255 to 240 or something, change the gamma), some histogram become "combed". Is there any ways to avoid this?

I understand this is caused by integer math. I wonder if changing the image from 8 bit / channel to 16 bit / channel would help. On the other hand, making this change already creates combs since out of 65536 brightness value, only 256 are possible from the 8 bit image. Unless, the conversion from 8 bit to 16 bit per channel involves some heuristics to generate more brightness values.

I don’t understand the question
I never get combng unless I hit AUTO Levels
soemtimes ugly or not the best adjustment

when I move the sliders manually I do not get combing?
I also try and only use the midtone slider
I think it prevents clipping?

KatWoman wrote:

"peter" wrote in message

After editing levels (e.g. lower the max input from 255 to 240 or something, change the gamma), some histogram become "combed". Is there any ways to avoid this?

I understand this is caused by integer math. I wonder if changing the image from 8 bit / channel to 16 bit / channel would help. On the other hand, making this change already creates combs since out of 65536 brightness value, only 256 are possible from the 8 bit image. Unless, the conversion from 8 bit to 16 bit per channel involves some heuristics to generate more brightness values.

I don’t understand the question
I never get combng unless I hit AUTO Levels
soemtimes ugly or not the best adjustment

Not every adjustment causes combing in the histogram. It depends on the image and it depends on what you do. An image with a ‘full’ histogram will not easily show this effect with a small adjustment, but a ‘flat’ image that lacks deep shadows and bright highlights will show combing after you corrected it. That applies to manual correction as well as automatic correction. It has nothing to do with using AUTO.

It’s important to note however that we are in the business of improving photograps, not in the business of improving histograms. If the photograph clearly improved as a result of your correction, who cares about what the histogram looks like?

—
Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl

"KatWoman" wrote in message
SNIP

I never get combng unless I hit AUTO Levels

The standard histogram display is only 194 pixels wide. It won’t be possible to show each and every of the 256 histogram bins. When the image is low contrast, people can pull in either the black point and/or the white point, e.g with the Levels sliders. Some tools, like Auto Contrast will by default clip a certain percentage at both ends of the histogram. Those changes would only show combing of the histogram if the number of populated bins falls below 194.

The Extended histogram option will enlarge the histogram to display 256 bins. They too will show combing if there are fewer than 256 bins populated.

when I move the sliders manually I do not get combing?
I also try and only use the midtone slider
I think it prevents clipping?

The middle slider in the Levels control, only redistributes the the bin values. With extreme adjustments there may occur some combing at the opposite side of the histogram.

Combing by itself says little about image quality. An original image with only, say, 100 levels of brightness, will just show those levels as it should, and they will be combed unless the levels are all contiguous. Besides, combing can be reduced by blurring the image, but then I wouldn’t always call that an improvement … 😉

—
Bart

"Johan W. Elzenga" wrote in message

KatWoman wrote:

"peter" wrote in message

After editing levels (e.g. lower the max input from 255 to 240 or something, change the gamma), some histogram become "combed". Is there any
ways to avoid this?

I understand this is caused by integer math. I wonder if changing the image from 8 bit / channel to 16 bit / channel would help. On the other hand, making this change already creates combs since out of 65536 brightness value, only 256 are possible from the 8 bit image. Unless, the
conversion from 8 bit to 16 bit per channel involves some heuristics to generate more brightness values.

I don’t understand the question
I never get combng unless I hit AUTO Levels
soemtimes ugly or not the best adjustment

Not every adjustment causes combing in the histogram. It depends on the image and it depends on what you do. An image with a ‘full’ histogram will not easily show this effect with a small adjustment, but a ‘flat’ image that lacks deep shadows and bright highlights will show combing after you corrected it. That applies to manual correction as well as automatic correction. It has nothing to do with using AUTO.
It’s important to note however that we are in the business of improving photograps, not in the business of improving histograms. If the photograph clearly improved as a result of your correction, who cares about what the histogram looks like?

a good point Johan

I guess I am fortunate to work with an excellent photographer who never gives me images that need so much adjusting.
He uses the histogram in the camera to make sure the highlights are not getting burnt, but this can be mis-leading in many shots that are (for instance) all white subjects like set and clothes are white back lighting and exposing for the front of the subject will give you a "bad" histogram and look nice in the image.