Hi, I think this post is going outside the original question.
The original question was about a grey card and white balance (I I understood corrrectly). And what to do if the grey card doesnt appear "neutral" in darktable. 1 - Grey card and white balance. The fact that the card is 18%, 22% or anything else is unrelevant if we speak of white balance (the 18% is useful for exposure control... well, I will no enter the debate about calibration and the various mid-grey, median-grey or so) So, yes, we can set the white balance using the "spot" white balance on a grey card as soon as the grey is neutral. Neutral means "the same value for red, green, blue". 2 - Card is not neutral If the grey is not neutral, say a colour cast with one of the channel with a different value, your white balance will give a colour cast in the opposite direction. You can correct that in dt with the module "colour balance" for instance by selecting the neutral area of interest with the colour picker and trying to have the same value for all the 3 channels. But, in my opinion, if your grey card is reasonably neutral, you well not see (with your eyes) any difference. A simple sheet of (white) paper can also do the job. About the reflectivity of the neutral grey : it is an other matter. Such grey card can be used to adjust the exposure. But as there are strong controversies about what should be the neutrality of a neutral grey, this subject is prone to leave to various trolls. Regards Jean-Luc 2017-02-22 8:36 GMT+01:00 Lorenzo Bolzani <l.bolz...@gmail.com>: > Printing a gray card is a bad idea: you need a very good an perfectly > calibrated printer for this and good non-glossy paper. Otherwise you do not > get perfect gray and you get a random shade of color in all your shots > where you use it. Just buy one. > > For white balance, as Ivanov said, any gray is good, actually you > typically use a white card. So *just take a white sheet from the printer > a use that.* I did this in several occasions and works really fine. The > minor problem is that paper is not "perfectly neutral white" but can vary > slightly so if you are looking for ultimate perfection buy a WB card > (X-Rite, Opteka, etc.). > > The gray 18% thing is about setting the exposure, not the white balance. > Of course you can use the gray card for white balance too but it's not its > main purpose. Typically gray 18 targets are white on the other side for WB. > > Often white balance cards came in set of three pieces: white, gray and > black. Almost always the gray here is not gray 18 so it is not good for > exposure reference only for WB. > > > Bye > > Lorenzo > > > 2017-02-22 5:57 GMT+01:00 I. Ivanov <iv3...@gmail.com>: > >> For the purpose of white balance - I don't think it really matters if it >> is 18% or less or more. It is just pure gray. As long as you use a card and >> then use it to measure white balance you should be fine. >> >> On 2017-02-21 08:21 PM, Michael wrote: >> >> Well, I guess I can try to print a gray card! >> Anyone know what the code is for 18% gray? If 18% is right in the middle >> of the shades of gray I found one page (http://www.computerhope.com/c >> gi-bin/htmlcolor.pl?c=808080) that says the code is: >> 808080 >> and according to the same page it is made up of equal part RGB >> *W3C Color Name:* Grey >> *RGB:* 128, 128, 128 >> *HSL:* 0.00, 0.00, 0.50 >> so what is the authoritative answer? >> >> On Tue, Feb 21, 2017 at 7:53 PM, Marcus Sundman <sund...@iki.fi> wrote: >> >>> On 21/02/17 23:16, Michael wrote: >>> >>> is there something where we can take a picture of a gray card and then >>>> we click on it and then dt will adjust all of the colors so that the gray >>>> card is >>>> 33-33-33%? >>>> >>> Usually you use "18% gray" meaning a card that reflects 18% of the light >>> in the visible spectrum and equal amounts of red, green and blue. >>> >>> It might actually be nice if the whitebalance module would support >>> exposure adjustment as well, to make a selected area a specific brightness, >>> or what do others think? >>> >>> by the way: what is the color of the remaining 1% >>>> >>> That "33-33-33%" is your invention, so nobody but you can know what >>> "remaining 1%" you are talking about. If you were talking about the >>> reflected % of the individual color channels then there is no "remaining >>> 1%", but the "remaining" (absorbed) amounts of light are 67%, 67% and 67%, >>> respectively. >>> >>> >>> - Marcus >>> >>> >> >> >> -- >> :-)~MIKE~(-: >> >> ____________________________________________________________________________ >> darktable user mailing list to unsubscribe send a mail to >> darktable-user+unsubscr...@lists.darktable.org >> >> >> >> ____________________________________________________________________________ >> darktable user mailing list to unsubscribe send a mail to >> darktable-user+unsubscr...@lists.darktable.org >> > > > ____________________________________________________________________________ > darktable user mailing list to unsubscribe send a mail to > darktable-user+unsubscr...@lists.darktable.org > ____________________________________________________________________________ darktable user mailing list to unsubscribe send a mail to darktable-user+unsubscr...@lists.darktable.org
