Documentation
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Overview ¶
Package palette provides colours and colour sequences for charts.
The default qualitative sequence is Okabe-Ito, which is designed to remain distinguishable under the common forms of colour vision deficiency. Charts should also encode series redundantly (dash patterns, marker shapes) — colour alone is never sufficient — but starting from a safe palette costs nothing.
Index ¶
- Variables
- func Lerp(a, b ir.Color, t float64) ir.Color
- func Luminance(c ir.Color) float64
- func QualitativeName(q Qualitative) (string, bool)
- func QualitativeNames() []string
- func RampName(r Ramp) (string, bool)
- func RampNames() []string
- func RegisterQualitative(name string, q Qualitative)
- func RegisterRamp(name string, r Ramp)
- type Qualitative
- type Ramp
Constants ¶
This section is empty.
Variables ¶
var ( Blue = ir.RGB(0x00, 0x72, 0xB2) Orange = ir.RGB(0xE6, 0x9F, 0x00) SkyBlue = ir.RGB(0x56, 0xB4, 0xE9) Green = ir.RGB(0x00, 0x9E, 0x73) Yellow = ir.RGB(0xF0, 0xE4, 0x42) Vermilion = ir.RGB(0xD5, 0x5E, 0x00) Purple = ir.RGB(0xCC, 0x79, 0xA7) Black = ir.RGB(0x00, 0x00, 0x00) White = ir.RGB(0xFF, 0xFF, 0xFF) Red = ir.RGB(0xD5, 0x5E, 0x00) Gray = ir.RGB(0x88, 0x88, 0x88) )
Named colours used by the examples and the default theme.
var BlueOrange = Ramp{ ir.RGB(0x05, 0x30, 0x61), Blue, ir.RGB(0x92, 0xC5, 0xDE), ir.RGB(0xF2, 0xF2, 0xF2), ir.RGB(0xF4, 0xA5, 0x82), Vermilion, ir.RGB(0x7F, 0x27, 0x04), }
BlueOrange is the default diverging ramp: blue at one end, vermilion at the other, a near-neutral light grey in the middle.
Blue against orange is the one hue contrast that survives every common form of colour vision deficiency, which is why both ends are taken from Okabe-Ito rather than from the red/green pairing diverging ramps usually reach for.
var Blues = Ramp{ ir.RGB(0xF7, 0xFB, 0xFF), ir.RGB(0xDE, 0xEB, 0xF7), ir.RGB(0xC6, 0xDB, 0xEF), ir.RGB(0x9E, 0xCA, 0xE1), ir.RGB(0x6B, 0xAE, 0xD6), ir.RGB(0x42, 0x92, 0xC6), ir.RGB(0x21, 0x71, 0xB5), ir.RGB(0x08, 0x51, 0x9C), ir.RGB(0x08, 0x30, 0x6B), }
Blues is a single-hue sequential ramp, light to dark. Use it when the chart already carries a hue meaning and a second one would compete.
ColorBrewer 2.0, Brewer & Harrower.
var Cividis = Ramp{ ir.RGB(0x00, 0x20, 0x4D), ir.RGB(0x00, 0x30, 0x6F), ir.RGB(0x39, 0x48, 0x6B), ir.RGB(0x57, 0x5D, 0x6D), ir.RGB(0x70, 0x71, 0x73), ir.RGB(0x8A, 0x87, 0x79), ir.RGB(0xA6, 0x9D, 0x75), ir.RGB(0xC4, 0xB5, 0x6C), ir.RGB(0xE4, 0xCF, 0x5B), ir.RGB(0xFF, 0xEA, 0x46), }
Cividis is a sequential ramp optimised so that readers with and without deuteranomaly see the same ordering of colours, not merely a distinguishable one.
Nuñez, Anderton & Renslow, "Optimizing colormaps with consideration for color vision deficiency" (PLOS ONE, 2018).
var Default = OkabeIto
Default is the palette figure uses when a theme does not override it.
var DefaultRamp = Viridis
DefaultRamp is the sequential ramp a colour scale uses when none is given.
var Greys = Ramp{ ir.RGB(0xF7, 0xF7, 0xF7), ir.RGB(0xD9, 0xD9, 0xD9), ir.RGB(0xBD, 0xBD, 0xBD), ir.RGB(0x96, 0x96, 0x96), ir.RGB(0x73, 0x73, 0x73), ir.RGB(0x52, 0x52, 0x52), ir.RGB(0x25, 0x25, 0x25), }
Greys is a neutral sequential ramp, light to dark.
var Magma = Ramp{ ir.RGB(0x00, 0x00, 0x04), ir.RGB(0x18, 0x0F, 0x3D), ir.RGB(0x44, 0x0F, 0x76), ir.RGB(0x72, 0x1F, 0x81), ir.RGB(0x9E, 0x2F, 0x7F), ir.RGB(0xCD, 0x40, 0x71), ir.RGB(0xF1, 0x60, 0x5D), ir.RGB(0xFD, 0x96, 0x68), ir.RGB(0xFE, 0xC9, 0x8D), ir.RGB(0xFC, 0xFD, 0xBF), }
Magma is a sequential ramp from black through purple and red to near-white. Like Viridis it is perceptually uniform and monotone in lightness.
var OkabeIto = Qualitative{ Blue, Vermilion, Green, Yellow, SkyBlue, Orange, Purple, ir.RGB(0x00, 0x00, 0x00), }
OkabeIto is the default colourblind-safe qualitative palette.
Okabe & Ito, "Color Universal Design" (2008).
var PurpleGreen = Ramp{ ir.RGB(0x40, 0x00, 0x4B), ir.RGB(0x9A, 0x70, 0xAB), ir.RGB(0xE7, 0xD4, 0xE8), ir.RGB(0xF7, 0xF7, 0xF7), ir.RGB(0xD9, 0xF0, 0xD3), ir.RGB(0x5A, 0xAE, 0x61), ir.RGB(0x00, 0x44, 0x1B), }
PurpleGreen is a second diverging ramp, for when a chart already spends blue or orange on something else. Purple against green is the next safest pairing after blue against orange.
var Viridis = Ramp{ ir.RGB(0x44, 0x01, 0x54), ir.RGB(0x48, 0x28, 0x78), ir.RGB(0x3E, 0x4A, 0x89), ir.RGB(0x31, 0x68, 0x8E), ir.RGB(0x26, 0x82, 0x8E), ir.RGB(0x1F, 0x9E, 0x89), ir.RGB(0x35, 0xB7, 0x79), ir.RGB(0x6D, 0xCD, 0x59), ir.RGB(0xB4, 0xDE, 0x2C), ir.RGB(0xFD, 0xE7, 0x25), }
Viridis is the default sequential ramp: perceptually uniform, monotone in lightness — so it survives being printed in greyscale — and legible under every common form of colour vision deficiency.
Smith & van der Walt, matplotlib (2015).
Functions ¶
func Lerp ¶
Lerp blends a into b by t in [0, 1], interpolating the colour channels in linear light and the alpha channel directly.
Alpha is not gamma-encoded, so it interpolates as it is. The colours are non-premultiplied, which means blending two colours of different alpha interpolates the *stated* colour rather than the composited one — for a colour ramp, where the anchors are normally opaque, that is what a caller expects.
func Luminance ¶
Luminance is the relative luminance of c, from 0 for black to 1 for white.
It is WCAG's definition, which is the one a contrast decision is made against: the channels are decoded to linear light and weighted for the eye's sensitivity to each, so that a saturated yellow reads as light and a saturated blue as dark, which is what a reader sees and what averaging the encoded bytes would get wrong. It is the same decode Lerp blends through, for the same reason.
Alpha is ignored: a translucent fill is composited over something this package cannot see, so its luminance is the luminance of the colour it states. A caller who knows what is behind it can blend the two with Lerp first.
func QualitativeName ¶
func QualitativeName(q Qualitative) (string, bool)
QualitativeName reports the name a qualitative palette is registered under. Like RampName it compares colours rather than identity, so a copy is still recognised. ok is false for a palette nobody registered.
func QualitativeNames ¶
func QualitativeNames() []string
QualitativeNames lists the registered qualitative palettes, sorted.
func RampName ¶
RampName reports the name a ramp is registered under.
It compares colours rather than identity, so a ramp that was copied — which is what Ramp.Reverse and any slice expression produce — is still recognised. ok is false for a ramp nobody registered.
func RampNames ¶
func RampNames() []string
RampNames lists the registered ramps, sorted, so that a caller iterating them gets the same order every time.
func RegisterQualitative ¶
func RegisterQualitative(name string, q Qualitative)
RegisterQualitative adds a qualitative palette under a name, replacing any palette already there. It is how a third-party palette becomes reachable from a spec or a config file.
func RegisterRamp ¶
RegisterRamp adds a ramp under a name, replacing any ramp already there. It is how a third-party ramp becomes reachable from a spec or a config file.
Types ¶
type Qualitative ¶
Qualitative is a sequence of categorical colours.
func QualitativeByName ¶
func QualitativeByName(name string) (Qualitative, bool)
QualitativeByName looks up a registered qualitative palette.
type Ramp ¶
Ramp is a continuous colour ramp: anchor colours spaced evenly over [0, 1] and interpolated between.
Interpolation happens in linear light, not in sRGB byte space. Averaging two gamma-encoded bytes darkens the midpoint of a ramp by roughly 20% — a band that reads as a seam across an otherwise smooth gradient. gg composites in linear space for the same reason, so a ramp evaluated here and a gradient rasterised there agree.
func (Ramp) At ¶
At returns the colour at position t, clamped to [0, 1]. An empty Ramp returns ir.Transparent; a one-colour Ramp is constant.
Source Files