#apertus IRC Channel Logs

off for now ... bbl

@Bertl_oO Been on a long adventure. Have some interesting colorimetry stuffs to report.

share your adventure stories with us soon!

se6astian: Not really "back" per se. Just have been on a long deep dive into desaturation explorations.

not much happened on that front in recent times

That's depressing.

off to bed now

relatively fine ... the water cooling on my work system failed on saturday (as usual) and I have to wait till tomorrow to fix it

Bertl: It has to do with some interesting compression along the saturation domain in some higher end cameras.

Bertl: Here's a consumer grade DSLR demonstration that shows the issue largely: http://pasteall.org/pic/show.php?id=111281

Bertl: That is a simple two stop differential of exposure.

And all the colors areway off

Sucks to be them, I shoot B&W! https://www.youtube.com/watch?v=ewjZmd1AcZo

Surprisingly, the curves on B&W are still extremely complex in some cases.

Surely that wouldn't be there in a raw image.

B&W is not very anchored

Ah.

Spirit532: The camera has a limited range from minimum to maximum of sensitivity (aka output / device / display referred)

Spirit532: with me?

It's still n bits per channel, be it RGB or just brightness

Spirit532: To an infinitely large value (there might be a maximum in your scene, including the sun, but if we imagine, we can visualize that there are forever larger and more intense potential realities, so our upper end is infinite)

Spirit532: So when attempts to encode the scene into a device referred context, transforms have to take place that anchor those values.

Spirit532: In the case of a colour camera, we have issues at the higher end of the values. That is, if we consider each photosite is gathering up an arbitrary bunch of wavelengths that end up being the "green" filter, the "red" filter, and the "blue" filter, we define our colours based on ratios of those accumulator wells.

for simplicity's sake

Spirit532: But at high intensities, our wells fill up and can't record any further information. That data becomes "non data" because we are no longer recording the scene.

Spirit532: But the lower well totals continue to accumulate. Hence the wrong ratios. Hence the wrong colours.

Spirit532: The more correct approach would be for the hardware to implement a carefully designed 3D LUT transform that would, at a certain point, begin to compress the values such that the colorimetry of the scene is preserved in terms of the correct "hue" of the scene's object.

Spirit532: That way, while the actual chromaticity cannot be preserved (the xy position of the colour of the object at a lower exposure), we can preserve the axis that the colour lives on if you will.

you can't apply different gains to pixels

oh.

That makes sense.

And where is that implemented?

At the hardware level in the labyrinthine chain somewhere.

Pardon?

It's somewhat impossible to implement at a codec level as the horses have already left the barn.

So I'm going to have access to absolute raw data and all sensor controls

Assuming that some work has been done to stabilize the raw data and get a good solid photosite filter to XYZ transform.

Probably. Think of it as if it's the Axiom. Except it's not, and it's USB3.

The problem you've explained to me(which I didn't know the name for) is one of my main concerns. That, and having to grade everything.

The biggest scare for me at the moment is that it *only* shoots raw. Compression has to be done on the (thankfully beefy) PC.

Once proper profiling is done, to transform the sensor data to "aesthetic" bits, it is a very simple transform.

Bertl: In terms of ratios, the chromaticity coordinates of a given colour are a byproduct of the native "primaries" (albeit artificial) of the photosites expressed as a ratio.

put in*

Bertl: It's sort of "well duh" obvious, but something that has been elusive for a few years to me.

well, maybe I don't see it ... let's do an example with sensel values

let's assume raw linear values in 8 bit

Sure.

Right.

128, 255, 64

255 can't go higher

Right. So in this case you'd set a well total value at say 90%.

what is a well total value?

(assuming you've unbotched all of the bits and we are dealing with proper information and not non-data)

Fully exposed at 8 bit let's assume is correctly mapped to full saturation.

(Again, for the sake of discussion to keep things clear)

I have no idea what you are talking about to be honest

Let's call it in terms of chromaticity coordinates.

at this point, nothing will help you

the data is not linear unless you've fully built the QE & bayer curves into the hardware

just kidding

Now as that exposure goes up, you'd take your max() and realize that you are nearing saturation.

Instead of recording the other two channels.)

right?

doesn't make sense to me