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Mustafa Umut Sarac
05-01-2010, 07:49 AM
And did you notice that autochrome histograms are smoother ?

Q.G.
05-01-2010, 08:51 AM
QC , Lets continiue to reverse engineering. May be We can post process an image as you told and see the result. This is called histogram matching or something similar to it .
Is there anyone who knows how to create such a histogram on a image or do I need to ask to hybrid photo ? This is not digital photography but get help from a calculator.

You could produce such an image easily (in theory) in any image processing software. Just fill an image with a gradual transition from black to white.
You have to make sure that the image is as many pixels wide as there are tone steps in the colour mode (or a whole number multiple of that), so that each tone does indeed find a place.

In theory, because you need to be able to control the gradient fill tool so that it does indeed use the same step size for every part of the tone scale (gamma 1?).

In such an image, every tone is present as often as any other tone: flat histogram.

But recreating such an image would indeed be something for hybridphoto.


Oh, and no: i haven't noticed any extraordinary smoothness in the histograms of the Autochrome images i have picked of the web and tried.

Mustafa Umut Sarac
05-01-2010, 09:34 AM
Can you select a image and convert it to flat histogram as you told ?
By this way people do not have to imagine but see .
If you want , you can carry it to hybridphoto but it is not a crowded environment but harder to find help !
By this way , may be you will want to discuss to create a flat histogram by analog meanings !

Q.G.
05-01-2010, 10:01 AM
That would be a lot of work. And i mean a lot of work. So, no.

You can create a flat histogram using film by taking a picture of a continous wedge, and trace that with a densitometer.
Whether it will be really flat depends on exposure and processing though. To help, no toe or shoulder is allowed. The curve itself must not be a curve, but be straight. And gamma must be 1.

lxdude
05-01-2010, 10:06 AM
What is a "histogram"? I am sure someone at Hybrid Photo dot com knows!

They give you allergies this time of year. That's why people take anti-histograms. ;)

Kirk Keyes
05-01-2010, 10:08 AM
Doesn't a "flat histogram" indicate that there is not a lot of color saturation in the image?

Q.G.
05-01-2010, 10:25 AM
Doesn't a "flat histogram" indicate that there is not a lot of color saturation in the image?

I don't think so.

Athiril
05-01-2010, 10:49 AM
flat histograms to me respresent a lot of colour separation and high saturation (usually), not something I think of when thinking of Autochrome.

For example, here is the histogram of some Kodak 50D that has very very little colour information, due to the telecine operator setting the white point to the negative density of when the camera was running at only a few frames per second and thus several stops overexposed instead of the maximum density of when it was running at 25 fps - he also compressed it out to 1997 HDCAM without the correct levels set, which was then recompressed out to ProRes,

Long story short, what we end up with is a piece of crap with 5.27 bits of information on average per colour channel.
http://i39.tinypic.com/27xlead.jpg


Now if we compare that to something with reasonable colour separation and saturation
http://i40.tinypic.com/9i7j3q.jpg

nick mulder
05-01-2010, 03:32 PM
This reminds me of the time I put a photoshop standard gradient (the default option) from black to white vertically or horizontally across a large enough resolution file then looked at the histogram...

certainly not linear ! although I suspect the issue was with the gradient tool, rather than the histogram - although I do wonder ...

Q.G.
05-01-2010, 06:54 PM
This reminds me of the time I put a photoshop standard gradient (the default option) from black to white vertically or horizontally across a large enough resolution file then looked at the histogram...

certainly not linear ! although I suspect the issue was with the gradient tool, rather than the histogram - although I do wonder ...

That has to do with what i wrote before, i think.
A histogram is nothing but a list of all possible tones, with information about how often each of those is found in an image.

So a gradient from black to white should produce a completely flat line histogram.

But only if the gradient tool is set not to favour one, or boths ends of the gradient. (An issue that can be be confounded by the image profile that a image processing software applies to that image - such a profile usually also contains a preferred gamma setting, i.e. applying the profile will change the tone distribution even if the gradient tool is completely 'true'.)

And if the gradient length matches the number of possible different tones (or a whole number multiple), so that it does not need to be made to fit. (A 256 tone scale from black to white, for instance, is hard to fit in a 200 pixel wide image without having to drop quite a few tones.)

Photo Engineer
05-01-2010, 09:20 PM
The idea of a histogram of a color photo is pretty much of digital origin.

It is very difficult to derive full color information from one of these! You must remember that the designers of many of the software packages that work with images are rather clueless as far as color science and engineering goes. This is why they often mess up color negative scanning AAMOF. They cannot visualize the original from the negative.

PE