Autochrome Histogram - Suprise for me

[Kirk Keyes]

Doesn't a "flat histogram" indicate that there is not a lot of color saturation in the image?

 

[Q.G.]

Doesn't a "flat histogram" indicate that there is not a lot of color saturation in the image?

I don't think so.

 

[Athiril]

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.

Now if we compare that to something with reasonable colour separation and saturation

 

[nick mulder]

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.]

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]

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