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I'm not sure what the current formula used to convert images to grayscale in PureRef, but I want to propose an alternative formula to grayscale.

I think this method is called Luminosity.
You can do this in Photoshop or other programs by creating a new layer, fill it completely with black and set the layer blending mode to "Color". Now everything underneath that layer is converted to grayscale with the Luminosity method.

Me and quite a few other artists prefer to use this conversion, as we consider it a much more accurate method to obtaining values.

Can we have this formula as an option when doing grayscale conversions in PureRef? Your response will be very appreciated, PureRef dev!



Edited 1 time(s). Last edit at 2018-04-12 14:11 by panhartstuff.

I'm not a dev, I'm just a guy that looked into this. PureRef already uses an equation similar/compatible with the black color layer. If you search the forum it has already been talked about a couple of times, including myself. The conversion to grayscale is ITU-R BT.709, Luminosity as you say. As far as I can tell PureRef does that already, it's the only option and it's the correct option, there's no need for other options really - as far as color -> value conversion is concerned at least.

Which grayscale feature is it you're using? The one for individual images or the one that grayscales the entire scene at once? There's a possibility that they behave slightly different and if they do maybe one of them are more correct than the other.

@PureRef
I'm using both the individual and entire grayscale on PureRef, and they produce the same results, at least from the examples I'm using

@razvanc-r
I used a sample image and converted it to grayscale, both using the PureRef method and my Photoshop method, and it produced very different results

This is the original image:
https://i.imgur.com/ZEkqyVB.png

Grayscale converted using the Photoshop method I described above:
https://i.imgur.com/OWJcmA9.png

Grayscale converted using PureRef method:
https://i.imgur.com/JEbtie5.png

To make the differences more clear, here's an image comparison tool as an aid:
http://screenshotcomparison.com/comparison/113707
Mouse-over the image to see PureRef results and mouse-out to see Photoshop results

This is really unproductive when you're doing studies and trying to make the values on Photoshop accurate and consistent with your reference on PureRef

Will you develop a feature to accommodate these differences, PureRef dev?

I stand corrected, I did some tests in that color wheel, I don't know why I didn't try a color wheel myself in the first place, but yeah it seems that the grayscale algorithm in pureref is slightly different compared to the black layer on color blend mode. Now the question is, is this black layer with color blend the right algorithm? I'm not sure, both ITU-R BT.709 and ITU-R BT.601 are implemented in Krita and for what is worth the pureref looks very close to 601. The latest standard for HDR TV is ITU-R BT.2100. There's no real "correct" algorithm to convert to grayscale. Check "Converting color to grayscale" at en.m.wikipedia.org/wiki/Grayscale

@panhartstuff the easiest for you would be to just convert the image to grayscale in Photoshop and copy it to pureref. You have to understand this black layer on color blending mode isn't "the" correct way. There's no one correct way. But if the devs want to implement this algorithm, go for it.



Edited 1 time(s). Last edit at 2018-05-20 13:30 by razvanc-r.

As you say @razvanc-r there's many different standards and we haven't found any real "correct" way of doing it either. Currently PureRef uses this formula: (R * 11 + G * 16 + B * 5)/32 which might be slightly different than the standards you have mentioned.

We appreciate your research and discussion, as you notice it's a passionate subject, we'll look into if we want to add several different options for it in future versions of PureRef. :)