Colour grading your band¶

Each band has a look (H-alpha orange-red, calcium violet and so on). The look is a starting point: here you make it yours. A grade adds hue, saturation, brightness (gamma), a black and a white point and a curve for each of red, green and blue, with a live preview, and it is saved as a named colour preset. The band look is kept; with every grade control at its neutral value the picture is exactly the look.

The capture. 15_32_09.avi, H-alpha, full disc, 250 of 1,000 frames kept, with the recommended wavelets.

The new H-alpha look¶

Several users found the old H-alpha pale and cream, not the red-orange of the line. The look is now closer to the common H-alpha palette: gains red 1.0, green 0.45, blue 0.10 (before the tone gamma; it was 1.0, 0.85, 0.60), exponents 0.70, 0.90, 1.40. A mid-grey now maps to RGB 169, 68, 11, where it was 169, 90, 33. The same look is used in helia lut and the colour tables, so a table saved for another program matches.

RGB value a mid-grey maps to in the old and the new H-alpha look, as printed on the page.

Step by step, in the app¶

1. Stack the capture and go to Colour. H-alpha is already chosen. Under Grade are eight controls, each with a slider and a number box you can type into; the preview follows every one.
2. Hue (-180 to 180 degrees) turns the colours round the colour wheel. -6 is a little more red, +8 a little more yellow-green.
3. Saturation (0 to 2): 0 is grey, 1 is the look as it is, 1.25 is stronger.
4. Brightness is a gamma on the picture before the look (0.3 to 3); above 1 brightens the mid-tones.
5. Black point and White point say which stack values become black and full brightness. Raise the black point a little (0.03) to deepen the sky.
6. Red, green and blue curves (0.3 to 3) lift or lower the mid-tones of one channel. Blue 0.9 warms the picture, blue 1.2 cools it.
7. Name it under My colour presets, press Save, and press its name to use it again. Reset grade returns to the look. Presets are small readable JSON files in $HELIA_COLOUR_PRESETS, else a colour-presets folder beside the edit presets (%APPDATA%\helia\colour-presets on Windows, ~/.config/helia/colour-presets elsewhere).

What you should see¶

The look as it is (left), the warm grade of step 7 (hue -6, saturation 1.25, gamma 1.1, black 0.03, red 1.05, blue 0.9; middle) and a cool one (hue 8, saturation 0.85, gamma 0.95, blue 1.2; right):

The same on the command line and in the API¶

helia stack 15_32_09.avi -o ha.tif --preset ha \
  --grade hue=-6,sat=1.25,gamma=1.1,black=0.03,r=1.05,b=0.9
helia stack 15_32_09.avi -o ha.tif --preset ha --grade "Warm Ha"   # a saved colour preset

The API takes the same text as grade in the job configuration. The grade is written into the file's history (grade hue=-6,...), and a colour table saved from the app carries it. A true-colour stack (--colour true) has no look to grade, so --grade is refused there.

The keys are hue (-180 to 180), sat (0 to 2), gamma (0.3 to 3), black (0 to 0.9), white (0.1 to 1, at least 0.05 above black) and r, g, b (0.3 to 3).

How it works¶

The grade is part of the colour table, so the CPU and the GPU agree (the parity tests cover it) and a .cube or .lut includes it. The mono value is first mapped through black, white and gamma, then through the band look, then the per-channel curves; hue rotates the colour about the grey axis and saturation scales its distance from its luma (Rec. 709 weights).

Troubleshooting¶

• A flat orange disc. Saturation or the red curve is too high and the red channel clipped. Lower them, or raise the white point.
• Black sky is not black. Raise the black point a little.
• The saved table looks different in another program. A table is the grade at the time you saved it; save again after changing it.
• "grade: ... out of range". The ranges are listed above.