PixInsight Forum (historical)

PixInsight => General => Off-topic => Topic started by: Ioannis Ioannou on 2011 June 08 01:40:11

Title: Combining mono L with L from RGB ?
Post by: Ioannis Ioannou on 2011 June 08 01:40:11

I was wondering, is there any benefit to extract the (pseudo) L from RGB , integrate it with the original L and then rejoin the new L with RGB ?
Or the LRGB tool will do this anyway during the combination L + RGB ? Assuming that L and RGB are of similar quality etc.

Thank you in advance

Title: Re: Combining mono L with L from RGB ?
Post by: Juan Conejero on 2011 June 08 12:17:24

Hi John,

The LRGBCombination tool has four channel weight parameters that allow you to combine source LRGB components with the corresponding components of a target image. By default, the four weights are all set to one, which means that the source images specified on the LRGBCombination tool (Channels / Source Images section) will replace those of the target image.

However, mixing an L frame (lightness) with the implicit lightness of an RGB image is generally a bad idea. Doing so is incoherent with the normal purpose of the LRGB combination process. We perform LRGB combinations because the L frame has better SNR than the RGB frames. So, why combine a good L with a poor L? :)

Title: Re: Combining mono L with L from RGB ?
Post by: Ioannis Ioannou on 2011 June 08 20:19:24

Just a play of the mind (and thank you for your reply, I understand that your time is precious right now)

Lets consider this case: 2 hours L , 1 hour each of RGB. Same time for each subframe.
If I extract the luminance from the final RGB and add it to the original L, this will not give me 2 + ((3 x 1h) / 3)  = 3 hours L ?
Or I missed something?