Add Scanlines (SCAN) Axis

[felipesanches]

Fonts to be onboarded that have this axis:

• Add Homecomputer VF fonts google/fonts#3425

[RosaWagner]

Discussed in google/fonts#3699

[RosaWagner]

Copy pasting Jens issue here:

My Homecomputer fonts use custom variation axes for visual artefacts related to display on a cathode ray tube: (horizontal) bleed (tag: BLED) and (vertical) scan lines (tag: SCAN).

I propose to add those axes to the Google Fonts axis registry:

Scanlines (SCAN)

On CRT screens

Scanlines of pixels separated by a varying amount of black space are characteristic of CRT screens.

The Scanlines axis allows to controls the height of the lines, and as a result of this, the amount of vertical space between the lines. The space may also disappear completely, causing the simulated "pixels" to blur into each other.

SCAN=default (0)

SCAN=min

SCAN=max

Other uses

The axis could also be used to segment glyphs in a way that is not reminiscent of CRT screens, similar to the "Hatch" filter in Glyphs.

Axis proposal

tag: "SCAN"
display_name: "Scanlines"
min_value: -1000
max_value: 1000
default_value: 0
precision: 0
fallback {
  name: "Normal"
  value: 0
}
description:
  "Segment glyphs horizontally without any changes overall width, inter-letter spacing,"
  " or kerning, so there are no changes to line breaks or page layout."
  " Negative values causes the glyphs to become visually segmented,"
  " while positive values may cause the segments to overlap and thus"
  " cause a boldening effect."

[RosaWagner]

The addition of this axis is still to discuss because it could be a weight axis.

cc @davelab6

[vv-monsalve]

After the last revision, it was decided to include this axis in the axis registry as a relative axis, with a percent range instead of an absolute one.
This decision was made because the current axis range values used do not match any specific font measurement. Additionally, this change will make the axis more versatile and applicable to other scenarios where it can be used to adjust the number of lines (as in the example shown above) instead of the space between them. This change will allow for various implementations to be used in the future.

The description needs to be refined.

Definitive metadata fields:

# Scanlines, based on https://github.com/jenskutilek/homecomputer-fonts
tag: "SCAN"
display_name: "Scanlines"
min_value: -100
max_value: 100
default_value: 0
precision: 0
fallback {
  name: "Default"
  value: 0
}
fallback_only: false
description:
  "Segment glyphs horizontally without any changes overall width, inter-letter spacing,"
  " or kerning, so there are no line breaks or page layout changes. Negative values"
  " cause the glyphs to become visually segmented, while positive values may cause
  " the segments even overlap and create a bold effect."

[jenskutilek]

How about this?

description:
  "Break up shapes into horizontal segments without any changes in overall width, inter-letter spacing,"
  " or kerning, so there are no line breaks or page layout changes. Negative values"
  " cause the glyphs to become visually segmented, while positive values may cause
  " the segments to overlap and create a bold effect."

I don't know how specific the description has to be. Should it be mentioned that the segments could be rectangular, but may also take on other forms, like in the rounded lines image, or even be irregular in shape as long as they create the impression of a vertical segmentation?

[vv-monsalve]

I don't know how specific the description has to be.

This level of definition sounds good. The description has to be clear and universal enough at the same time. So it doesn't need to go that much into details, like specific shapes.