The balance report

Before any look, measure each shot against the film as a whole, and list the shots that sit apart.

What you measure. Take three frames from the part of each graded live shot that plays (at about 20%, 50% and 80% of it). On each, read:

• the white point: the colour temperature of the near-white areas, or of a neutral object you choose, in kelvin. Read from a frame, it is always an estimate, never a meter reading;

• the black level: the darkest 1% of the picture's brightness, on a 0–100 scale;

• saturation: the average colour strength, measured as the average chroma in Lab (C*, the colour space that matches how the eye sees colour difference);

• skin hue and its spread, where a face is in frame: the Lab hue angle of the skin, its median and how far it spreads.

Average the three frames per shot. Then find the film's median for each figure and list every shot that sits outside it by more than the starting thresholds [best available, not yet validated]:

These are starting values. Once you have balanced a film by eye, set them again from your own figures.

How to read it. Scopes show which way a shot leans. The flag figures need numbers, so read them from exported frames, every shot on the same scale, then compare the shots by eye against one whose figures sit near the film's median (steps below). A shot with no near-white area gives no white point. List it as "not measurable" and check it by eye, or pick a neutral object in it.

What the report is not. It reads frames and changes nothing. Skin hue misleads under coloured light, so a skin flag is a flag, never a verdict.

Do it yourself

Export the three frames (ffmpeg)

1. Work out each time in the take's own file (its in point plus 20%, 50% or 80% of the part that plays).

2. Export each frame as PNG:

ffmpeg -ss 2.4 -i S03_t2.mov -frames:v 1 -pix_fmt rgb24 S03_a.png

The -pix_fmt rgb24 keeps every frame at 8 bits, so every shot is read on the same scale, in Photoshop and in ffmpeg alike. Without it, a 10-bit take exports as a 16-bit PNG, and its figures land on a scale far from the 8-bit takes'.

Read the white point and skin hue (Photoshop, a spreadsheet)

1. Open the frame in Photoshop and place Color Samplers, each set to Sample Size 5 by 5 Average: one on a near-white area or a neutral object, and three to five across the skin.

2. Set the Info panel to show Lab and RGB.

3. The white point's a and b show the cast (b* above zero is warm, below zero is cool). For the figure in kelvin, enter the sampler's RGB values in any RGB-to-colour-temperature (CCT) calculator. The result is an estimate, as every colour temperature read from a frame is, and one sampler approximates the average of all the near-white areas, so place it on the largest clean one.

4. Skin hue is the Lab hue angle, h = atan2(b, a), in degrees. In a spreadsheet, with a* first (Excel and Google Sheets take the arguments in that order):

=DEGREES(ATAN2(a, b))

1. Take the median of the skin samplers. This is an approximation: the full measure reads every skin pixel in a box. Your samplers' range stands in for the spread.

Measure saturation (ffmpeg)

1. ffmpeg's signalstats prints a frame's average saturation on its SATAVG line:

ffmpeg -i S03_a.png -vf "signalstats,metadata=print:file=-" -f null -

1. This is an approximation too: it is measured in video colour, not Lab C*, so its numbers are on another scale. The flag is relative (20% of the film's median), so it works when every shot is read the same way; check any shot close to the line by eye.

Read the black level (Premiere, DaVinci Resolve)

1. Read the floor of the Waveform, ignoring a few stray specks. This is an approximation as well: the floor you read by eye stands in for the darkest 1%.

2. Premiere's Lumetri Scopes read 0–100 directly.

3. On DaVinci Resolve's 10-bit scale, 3 points is about 31 steps of 0–1023.

Compare shot against shot, by eye (DaVinci Resolve)

1. In Resolve's Color page (the free version is enough), grab a still of a shot whose figures sit near the film's median, put it in the Gallery and wipe against it.

2. The RGB Parade shows a cast: on a neutral, the three traces sit level, and red above blue is warm.

3. The Vectorscope's skin-tone indicator shows skin drifting off the line. Its degrees are not Lab degrees, so take the 5° skin flag from the Lab figure.

Sources

• Julien Alary and Denny Cooper (LBB, "An Eye for Colour") (balance first, and match colour temperature, contrast and saturation scene to scene).

• The thresholds are this Guide's own starting values.