Colorizing (only) with a Radial Gradient/ Alpha Channel

Impish · **Joined:** Sep 09, 2019 **Posts:** 5

GIMP Version: 2.8.14
Operating System: Windows
OS Version: Windows 10
GIMP Experience: New User

Hi… I’m an amateur astronomical photographer. I’m also new to GIMP and this forum….. After years of imaging nighttime celestial objects, I started imaging the sun through a solar telescope. Best practice calls for imaging the sun with a monochrome camera, which is what I do (see my result in frame “A” of the attached/ referenced image). Many who share this hobby with me color the sun in post-processing. My attempt at doing this, using GIMP’s Colorize feature has produced what can be seen in frame “B”. While that’s OK, I want to get results that look like, what another amateur has produced and is pictured in frame “C”. I’m guessing some steps involving Alpha Channels, Radial Gradients and the color palette pictured in frame “D” is needed to make this happen, but I’ve yet to figure out the sequence of steps and settings to make this happen….. Any advise would be most appreciated!

dinasset · **Posted:** Mon Sep 09, 2019 2:07 pm **(#2)**

Did you try the simple Brightness/Contrast feature of Gimp?
And Saturation, Exposure?
This is a very rough attempt of modifying B with those.

Attachment:

GIMP-Desired-Sun_rough-adjust-of-B.jpg [ 1.21 MiB | Viewed 4746 times ]

Impish · **Joined:** Sep 09, 2019 **Posts:** 5

Thanks, that is an improvement!

I have played with brightness/ contrast and saturation but not exposure. I'll give it a go. That said, I still want to introduce some gold tone to the center of the disk, something that I'm fairly sure requires some combination of gradients and alpha channels.

Wallace · **Posted:** Mon Sep 09, 2019 2:55 pm **(#4)**

This is what I came up with.
Using your Black and White image A.

I basically played with Color > Levels until the haze was nearly gone from the black.
After this,
I did a Color To Alpha on a copy,
using Black.
I then selected the outer portion,
using the Magic Wand/Fuzzy Select Tool. :fuzzsel

Being sure to select as much of the outer portion of the canvas as possible.
Grow this selection by 1 pixel.

Invert the selection,
created a new layer below,
fill it with black and Marge these layers together.

This should give you a base to work with.
To add color or whatever.

Attachment:

GIMP-Desired-Sun_Wallace(2).png [ 1.03 MiB | Viewed 4730 times ]

Impish · **Joined:** Sep 09, 2019 **Posts:** 5

Thanks Wallace! I appreciate the effort.

You've created an attractive image, but in the process you've removed details from the solar edge and its flares. Cosmetics are nice, but not at the expense of losing details.

Cheers!

Krikor · **Joined:** Dec 19, 2018 **Posts:** 158

Particularly I think Wallace's result is excellent.
One question is whether image C also originated from image A or another image.

Impish · **Joined:** Sep 09, 2019 **Posts:** 5

That's the best "B" result yet, Krikor. Thanks!

Image "C" was created by another solar imaging hobbyist. He's obviously nailed the technique of radially blending two colors.

Impish · **Joined:** Sep 09, 2019 **Posts:** 5

After some assistance from other resources, I think I've more or less achieved what I wanted.

The secret ingredient is applying a radial gradient with appropriate FG and BG colors AND setting the Gradient Mode to something other than Normal. The attached image has the Gradient Mode set to HSL Color but other modes also work well.

Thanks all for your help!

Wallace · **Posted:** Mon Sep 09, 2019 9:53 pm **(#9)**

Impish wrote:

After some assistance from other resources, I think I've more or less achieved what I wanted.

The secret ingredient is applying a radial gradient with appropriate FG and BG colors AND setting the Gradient Mode to something other than Normal. The attached image has the Gradient Mode set to HSL Color but other modes also work well.

Thanks all for your help!

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