RGB imaging - Can you synthesize the blue channel?

[swag72]

Gathering the data for my first proper serious colour attempt, I should have gathered the blue data after the red so that I could at least synthesize a green channel. But no .............

.......... So I have my luminance, red and green. Is there any way I can synthesize a blue channel?

[steppenwolf]

Yup, blend the red and the green together 40% red, 60% green - this is not an exact science but it should give a full coloured 'effect'. Also try 30% red and 70% green. Reduce brightness of the new channel.

[GlassWalker]

Haven't tried it myself, but to an extent, couldn't you subtract both R and G from L, what's left should be mostly blue... with appropriate channel scaling of course.

[steppenwolf]

Hmmm - like that idea, must give it a try. I love this forum !!!

[swag72]

Thanks for that info Steve - I've done it with limited success, in that the colours are just awful. Will it be better with a proper blue channel?

How do you subtract green and red from the luminance? Is't it a grayscale image? PI wouldn't let me do do it.

[GlassWalker]

I assume the Pixelmath function in PI might do it, but I never figured out that one myself. Strictly speaking, R+B+G doesn't exactly equal L for typical filter sets depending on their responses, but unless you get real B, it's probably the next best thing.

[steppenwolf]

I've done it with limited success, in that the colours are just awful.

I tried it with one of my own images at 70% green on red and the result was pretty good - are you getting too much magenta? If so, reduce the brightness of the composite red/green

[swag72]

I managed to subtract the blue from the luminance and that worked much better than the green / red mix.

Some great ideas, thanks all!!

[martin_h]

Here is another way....

http://billsnyderastrophotography.com/?page_id=2289

[ollypenrice]

Synthesized blue? I don't believe I'm reading this!!!

Synthesized blue?

SYNTHESIZED blue?

Synthesized BLUE?

Synthesized blue?

Go and get some real blue, you scandalous person, you!!!

Olly

[swag72]

Thanks Martin - That makes an interesting read!

@Olly - I just wanted to play before I had all the data - I did go and collect real blue, honest Guv!