LRGB via H.S.L. MaximDL Plugins

This pair of plugins for MaximDL allows the composition of an LRGB image in the H.S.L. (Hue Saturation Luminance) color space.

MaximDL contains a built-in method for creating LRGB luminance layered compositions, using the L.a.b. color space. This converts the RGB image into the L.a.b. color space and then replaces the computed Luminance channel with the one supplied by the user, before converting back into the RGB color space.
Whilst this works reasonably well, it results in a color shift from the original due to the way the L.a.b. color space works.

In the L.a.b. color space, the three channels are not completely separate. The "a" channel is tied loosely with the L channel, but the "b" channel is not significantly connected. The result is that if you modify the L channel only, you will still change the "a” and “b" channels proportionately. The "a" channel controls the reds and oranges, and the "b" channel controls the greens and blues. Since the "b" channel is not significantly altered, it does not change hue as the brightness of the L channel goes up. However, as you increase the brightness of the L channel, the "a" channel shifts its hue. The result is that reds become more orange resulting in the infamous "salmon pink" coloration, while blues stay pretty much the same.

In addition to the hue shift which affects only the "a" channel, as the Luminosity in the "L" channel increases, both the "a" and "b" channels will loose their saturation. Since this is a non linear function, it is quite complex to predict a correction factor.

An alternative method is to translate the RGB image into the H.S.L. color space and then replace the computed Luminance channel with the one supplied by the user, before converting back into the RGB color space. This has the advantage of not changing the image hue at all, thus preserving the original image colors accurately.


Here is an example M33 image I captured 2x2 binned with RGB filters, alongside the same RGB image LRGB combined with an unfiltered image captured binned 1x1.
The LRGB image was created in MaximDL using the built-in LRGB combine in the L.a.b color space.


It can be seen that the LRGB image has benefited from the better resolution of the unbinned image, but the color has been affected as described above resulting in paler reds.

Next, we see the same RGB image next to an LRGB image created in MaximDL in the H.S.L. color space with my plugins:


One can see that the H.S.L. LRGB image has the same benefit of the greater resolution, but without any shift in the image color leaving the correct red colors.



First, construct a fully corrected RGB image from your original RGB data, which will be the target color of the final LRGB image. This image should have a maximum ADU level of 65535.

With this image selected, launch the “RGB to HSL” plugin, which will split the RGB image into three new images labeled “Hue”, “Sat” and “Lum”.
The idea is to now replace the “Lum” image with the more detailed unfiltered/unbinned luminance image you have already captured and processed.
At this point it is also possible to tweak the “Sat” image to boost the saturation to counteract the desaturation mentioned above. This can be done with a mild curves application.

To recreate an RGB image with a replacement Luminance channel, one must simply launch the sibling plugin “HSL to RGB” with the replacement Luminance image selected. There must also be the “Hue” and “Sat” images open in the same copy of MaximDL.
This will create a new document labeled “HSLtoRGB” containing the resulting LRGB image, which should have exactly the same colors as the original RGB image.


Version history:
v1.0 - Initial release.
v1.1 – Improve normalization – now copes with input levels > 65535.


If you download this to give it a go, please post feedback on the MaximDL Yahoo group, or email me directly at: winfij_AT_gmail_DOT_com
One final note - if you have any errors, please note the exact error messages and let me know!



Up to Home Page