Lesson Description
You learn two ways to select by “chromatic information”: (1) masks per channel, based on the luminosity of a channel (R, G, B...); and (2) color-based masks, based on color similarity in the composited image. You will see clear examples (the red of an airplane and the green of an aurora), along with a typical narrowband astrophotography example where selecting by channel usually works better than selecting by color.
What you will learn
- Differentiate when a mask is appropriate by color (well-defined color) vs. by channel (separate signal per channel).
- Select and adjust only a red/green area without affecting the rest, using color masks.
- In astrophoto, hide the star layer before creating the mask so as not to “contaminate” the selection.
- Enhance the blue of OIII with Hue/Saturation using a blue channel mask and then return the stars intact.
Pro Tip:
In the “starless + stars” flow, generates the mask only from the starless layer (hiding stars before) and apply the adjustment; at the end, show stars again so that they are outside the adjustment.
Practical Checklist
- I choose a color mask (red/green) for a clearly defined color and check that it selects only that area.
- In narrowband, I hide stars and try a channel mask to isolate the signal.
- I apply Hue/Saturation with blue channel mask for OIII and restore the star layer at the end.
Technical Glossary
Starless: Starless layer used to apply adjustments to the object/background without touching stars.
RGB channel: a black-and-white image representing the intensity of a color in each pixel (red, green, blue).
Narrow band: a workflow in which each channel can represent different signals (e.g., Hα, OIII, SII) assigned to the final color.