Using my camera as a color meter, forgot how...

[themichael]

It has been quite some time since visiting the forums and trying out new builds. My Canon AE-1P doesn't run Magic Lantern. 

About 5 years ago I had figured out how to use LV and a white balance setting to get a Kelvin and G/M readout near the spot meter square. I know this was an off schedule use of the firmware; but now I have need of it and forgotten how I did it. I even have the old version on a card but couldn't figure it out. My wife would kill me if I drop $5K on a colorimeter!

Help please and thanks.

[ilia3101]

Cameras are not very good colorimeters (or whatever u measure colour with), but mostly close enough for non science purposes.

Kelvin and G/M readout near the spot meter square.

This isn't a very meaningful reading of colour. Do you just want that, or would you like something more meaningful? Like XYZ/RGB or xyY values? I can help with that.

[Danne]

The ml Auto adjust Kelvin + G/M is actually very good indeed. To reach it. Stand on White balance and press q button. Or tap screen(eosm) and there it is.

[themichael]

Ilia3101 - It is a non-scientific reason and a good way to measure color for my purpose.   XYZ/RGB or xyY is getting too deep. I need to stack filters to get close to a tungsten/halogen color under various lighting conditions. 

Danne - Tried that but I may have not done it right, nothing happened and the WB didn't change. Going to try that again.

On a podcast they were talking about positive paper prints like Polaroids without the negative part. It involved R4 color paper used in a reversal process at room temp.  I think they used a contrast 00 and a contrast 1/2 filters (BW printing filters) and got good results under later day daylight.

Thanks for the fast response.  I may even shoot more digital now; great stuff in the newer ML.

[a1ex]

Actually I've been thinking a few times to add a Kelvin readout directly in the spotmeter, as an option, but never got around to it. A raw-based "RGB" readout could be useful as well; the current one is YUV-based. Once that is done, adding XYZ / xyY to the spotmeter would be probably straightforward.

Auto adjust Kelvin + G/M uses Canon's color science, and fine-tunes their WB parameters until getting a "gray" image in LiveView (in the YUV buffer, as rendered by Canon). It's an iterative process, so... it's slow.

[garry23]

@a1ex

Jumping in on the mention of the spotmeter.

If it was possible, being able to change the size of the spotmeter would allow the user to make the spotmeter focal length aware.

Also, if you could move the variable size spotmeter in Lua, you would have an ability to start to think about scripts that looked at scene aware decision making, i.e. exposure.

Just a thought 

[a1ex]

Hijacking some unrelated topic, with a (duplicate) feature request, is not appreciated.

Just a thought 

[garry23]

@a1ex

Ouch 

I was going to make a link to my previous suggestion, but thought I shouldn't.

Suitably and correctly chastised.

Cheers

Garry

[Kharak]

Actually I've been thinking a few times to add a Kelvin readout directly in the spotmeter, as an option, but never got around to it. A raw-based "RGB" readout could be useful as well; the current one is YUV-based. Once that is done, adding XYZ / xyY to the spotmeter would be probably straightforward.

Auto adjust Kelvin + G/M uses Canon's color science, and fine-tunes their WB parameters until getting a "gray" image in LiveView (in the YUV buffer, as rendered by Canon). It's an iterative process, so... it's slow.

I remember you mentioning tjis years ago! RAW WB spotmeter, that is some next level s***. How come no major camera manufacturer has done this?

[themichael]

Forgive the noobness of the question:

Where in the code can I find the white balancing and spotmeter sections? Or some "terms" to search with. Looked but couldn't find.  I'm an idiot. Searching in the right folder helps.

I wanted to look at the Auto adjust Kelvin + G/M maths for another project to get K and G/M from YUV or RGB but it sounds like that is using Canon routines.

My C was learned at uni on a PDP-11 without the + or ++ and definitely without the #  Going is slow.

[a1ex]

It's a binary search, attempting to minimize the absolute value of B - R (for Kelvin), or (R+B)/2 - G (for green/magenta) on Canon's 8-bit image buffer. It doesn't compute Kelvin value directly; it's only checking whether the resulting image is "gray" or not, and in which direction it might require adjustments (whether one of the above differences is positive or negative).

For computing Kelvin values from a raw image (camera-specific "RGB" values), I've extracted the routines from ufraw some time ago (kelvin.c).

[themichael]

Thank you a1ex

[ilia3101]

A raw-based "RGB" readout could be useful as well; the current one is YUV-based. Once that is done, adding XYZ / xyY to the spotmeter would be probably straightforward.

This would be amazing!

[DeafEyeJedi]

The ml Auto adjusts Kelvin + G/M is very good indeed. To reach it. Stand on White balance and press the q button. Or tap screen (eosm) and there it is.

Indeed I use this quite often. Whenever I find myself in a difficult situation for white balance I simply find anything closer to white or at least 18% grey.

Push/tab the button (5D3/EOSM) while pointing and from there it actually does wonders for you in a matter of seconds. Not too long at all.