109 lines
5.4 KiB
Markdown
109 lines
5.4 KiB
Markdown
# Theme constraints
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The following general constraints are followed as palette options are mapped onto
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concrete themes:
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+ Harshness levels have monotone differences of a single shade.
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+ "Hard" themes anchor their background to the most extreme shade appropriate for the
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scheme (i.e., lightest shade for "light," darkest shade for "dark"), ensuring the
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palette's "monotone width" is fully spanned by the theme options.
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+ App-specific monotone settings have differences of a single shade compared to the
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system monotone settings.
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+ Shade differences between corresponding background/foreground settings should be
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constant (e.g., between `bg0` and `fg3`, `bg1` and `fg2`, etc)
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The primary goal of these constraints is to ensure each theme in a collection defined
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around a single palette is 1) sufficiently _distinct_, 2) attains sufficient _breadth_
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under the palette, and 3) upholds _relative invariance_ under key properties (e.g.,
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lightness differences).
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## Example
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The following is a natural solution to these constraints, demonstrated on a general
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example setting: a possible useful analogy is a sliding window that, on its own spans a
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given theme's `bg0-fg0` settings, while globally sliding across all available values
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in the palette. If associating integers `0-10` to indices in a list of monotone shades, and
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`bg-fg` is the syntax used to indicate that theme's shade range, we might have the
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following for dark mode themes across harshness levels:
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```
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Dark (system) 0-7 ; 1-8 ; 2-9
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Dark (app) 1-8 ; 2-9 ; 3-10
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```
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There are sliding windows at both the system-app level *and* the harshness-level, in a
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sense. Constraints are followed:
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+ Harshness levels, separated by semicolon, differ by a single shade from hard to soft.
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+ The hard theme anchors its background to the darkest available shade.
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+ Monotones between system and app differ by a single shade.
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+ Differences between bg/fg (value of 7) remains constant across all themes.
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Mapping this onto the common values used in my theme definition files:
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```
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System, dark
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| Hard | Regular | Soft
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| bg0 <- l15 | bg0 <- l20 | bg0 <- l25
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| fg0 <- l80 | fg0 <- l85 | fg0 <- l90
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App, dark
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| Hard | Regular | Soft
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| bg0 <- l20 | bg0 <- l25 | bg0 <- l30
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| fg0 <- l85 | fg0 <- l90 | fg0 <- l95
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System, light
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| Hard | Regular | Soft
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| bg0 <- l95 | bg0 <- l90 | bg0 <- l85
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| fg0 <- l30 | fg0 <- l25 | fg0 <- l20
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App, light
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| Hard | Regular | Soft
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| bg0 <- l90 | bg0 <- l85 | bg0 <- l80
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| fg0 <- l25 | fg0 <- l20 | fg0 <- l15
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```
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# Accent contrast
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Each group of biome monotones have nearly identical (WCAG 2) contrast ratios against
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white/black for all lightness levels (ratios identical between biomes). These are
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selected in a heavily constrained OKLCH context, and given the perceptual uniformity
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attached to lightness, we can expect very similar contrast ratios for each accent under
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a given biome lightness (e.g., the `l65` red tone will have the same ratio under the
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grassland, tundra, and savanna monotones).
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In terms of selecting accents for themes (by harshness and scheme), what matters is
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at what lightness level all accent colors meet/exceed a particular contrast threshold.
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Again, the ratios themselves are effectively constant across biome monotones, and thus
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dependent entirely on the monotone lightness being used. This of course is determined
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primarily by whether the theme is a light or dark one, and what level of harshness is
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being used. The following are the relevant values for making a decision. We want to
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ensure all accents can reach >4.5 WCAG 2 contrast ratio (the standard requirement for
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small text on the web) against all biome monotones for each theme:
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+ For BG l20 (harsh, dark) -> l65 is min lightness where all accents have CR >=4.5
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+ For BG l25 (regular, dark) -> l65 is min lightness where all accents have CR >=4.5
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+ For BG l30 (soft, dark) -> l70 is min lightness where all accents have CR >=4.5
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+ For BG l90 (harsh, dark) -> l45 is max lightness where all accents have CR >=4.5
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+ For BG l85 (regular, dark) -> l45 is min lightness where all accents have CR >=4.5
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+ For BG l80 (soft, dark) -> l40 is min lightness where all accents have CR >=4.5
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For the monotone boundaries (l15 and l95, neither of which are possible backgrounds for
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terminal or nvim in the current theme definitions), the relevant lightness levels are
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l60 and l50, respectively.
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While not necessary, it feels intuitive for us to shift the accent colors up/down by the
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relative change in monotones across harshness levels. This has led to the choice of l60
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accents for the harsh-dark theme, l65 for regular-dark, and l70 for soft-dark. This
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technically breaks the 4.5 ratio requirement, though, for the harsh theme, so you
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ultimately need to pick one: either soften the contrast constraint, or allow different
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harshness levels to use the same accent lightness. I think either is acceptable, but for
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now I've gone with the former, loosening the contrast to a ratio of >4.0 with respect to
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the background. This allows for the slightly tighter group of accent lightnesses:
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l45-l50-l55 for light, l60-l65-l70 for dark. Note that the "center shade" of the l15-l95
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shade group is l55, meaning these groups are very central (the light triplet could move
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down by one shade step, but we want these accents to be as bright as we can get away
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with; otherwise, they are extremely dull in the light modes, and we thus don't mind bias
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toward a brighter lightness).
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