The “two” andromeda galaxies?

[Laurin Dave]

26 minutes ago, wimvb said:

Arcsinh stretch preserves colour, but I'm not sure it preserves colour ratios.

That's what it says it does on the tin...  Wim..

Dave

 

[wimvb]

51 minutes ago, Laurin Dave said:

That's what it says it does on the tin...  Wim..

Dave

 

Nice!

[ollypenrice]

If my images are as close to nature as an impressionist painting I guess I'll be happy enough. The impressionists were, after all, figurative painters though their intention was not so much to capture 'what was there' as what they perceived to be there. Still, I reckon this gives a remarkably precise impression of Sunday breakfast at my place...

🤩lly

[vlaiv]

1 hour ago, wimvb said:

But the final image is no more a true rendition of reality than an impressionist's painting of a pond and trees.

I don't want this to turn into debate on this topic, however I do think that there are certain aspects of photography that make it a bit more true rendition of reality than artist's impression of reality. Even we, as a community, sometimes debate - how much processing is "allowed" in order not to cross that documentary line of photography.

[Laurin Dave]

11 minutes ago, ollypenrice said:

If my images are as close to nature as an impressionist painting I guess I'll be happy enough. The impressionists were, after all, figurative painters though their intention was not so much to capture 'what was there' as what they perceived to be there. Still, I reckon this gives a remarkably precise impression of Sunday breakfast at my place...

🤩lly

I can vouch for that.. looks like you've just popped off to the kitchen though!

[Ships and Stars]

Just out of curiosity, I took the second image and inverted the colours in GIMP to show how far you can manipulate images with a few clicks. Interestingly, though of course it is unrealistic in the visual sense, the individual stars and some of the dust clouds seem easier to point out this way. 

[Laurin Dave]

1 hour ago, wimvb said:

Adam Block has a tutorial on lrgb combination, which is not restricted to Pixinsight,  even though he uses it to make his point. It shows his workflow to preserve colour.

https://adamblockstudios.com/articles/Fundamentals_LRGB

 

Just watched that video, very informative..  so thanks Wim

[vlaiv]

For those that don't use PI, here is "little tutorial" on how to do color combination in Gimp. I created a gradient of red and green colors, always maintaining ratio of 10:1 (red goes from 0% to 10% and green from 0% to 1% and then it's stretched).

Here is result of regular stretch:

Same thing stretched without loosing color saturation:

Second look less bright - but that is limitation of display rather than technique. If you maintain color you can't make it as bright as you like - you can't make red have value larger than 1, so green is limited to value 0.1 (if we keep 10:1 color ratio from my previous example).

Here is procedure:

Take RGB unstretched image and decompose into R, G and B channels (select decomposition in layers). Set each layer mode to 100% lighten (which is in fact max). Flatten this image - this is max of our three colors.

Again decompose RGB unstretched image, this time produce three images - each of R, G and B instead of layers. Copy/Paste max that we created in previous step on top of each of these three. Set its layer mode to division 100% - flatten each. These are ratio images.

Take luminance and do histogram stretch to your liking.

Make three copies of this luminance (will be final red, green and blue). Now copy each of ratio images and paste into respective copy of luminance. Set layer mode to multiply 100% - flatten each.

RGB combine resulting images.

[wimvb]

1 hour ago, Ships and Stars said:

Just out of curiosity, I took the second image and inverted the colours in GIMP to show how far you can manipulate images with a few clicks. Interestingly, though of course it is unrealistic in the visual sense, the individual stars and some of the dust clouds seem easier to point out this way. 

In scientific articles it is common to show any structure that is very close to the background in an inverted b/w image. My guess is that this is done because either we se faint variations better that way, or because the printing process simply isn't accurate enough to show small differences in dark tones. I sometimes invert a copy of my Luminance master to see what is hidden in the background. I stretch this copy by moving the black point and white point untill I get good contrast in the areas of interest.