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apca.lua
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apca.lua
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--------------------------------------------------------------------------------
--[[ @preserve
----/ *** APCA VERSION for W3 and WCAG 3 ***
----/
----/ SAPC - S-Luv Advanced Perceptual Contrast - Beta Algorithm 0.98e_d12e
----/ *** With the NEW SmoothScale extension ***
----/ *** Optimized for the Font Select Extension ***
----/
----/ Functions to parse color values and determine SAPC/APCA contrast
----/ Copyright © 2019-2021 by Andrew Somers. All Rights Reserved.
----/ LICENSE: GNU AGPL v3 https:--www.gnu.org/licenses/
----/ CONTACT: For SAPC/APCA Please use the ISSUES tab at:
----/ https:--github.com/Myndex/SAPC-APCA/
-- ]]
--------------------------------------------------------------------------------
----/
----/ SAPC Method and APCA Algorithm
----/ •••• Version 0.98e_d12e with SmoothScale™ by Andrew Somers ••••
----/
----/ GITHUB: https:--github.com/Myndex/SAPC-APCA
----/ DEVELOPER SITE: https:--www.myndex.com/WEB/Perception
----/
----/ Thanks To:
----/ • This project references the research and work of Dr.Legge, Dr.Arditi,
----/ Dr.Lovie-Kitchin, M.Fairchild, R.Hunt, M.Stone, Dr.Poynton, L.Arend, &
----/ many others — see refs at https:--www.myndex.com/WEB/WCAG_CE17polarity
----/ • Stoyan Stefanov for his input parsing idea, Twitter @stoyanstefanov
----/ • Bruce Bailey of USAccessBoard for his encouragement, ideas, & feedback
----/ • Chris Loiselle of Oracle for getting us back on track in a pandemic
----/
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
----/
----/ ***** SAPC BLOCK *****
----/
----/ For Evaluations, this is referred to as: SAPC-8, D-series constants
----/ S-LUV Advanced Perceptual Contrast
----/ Copyright © 2019-2021 by Andrew Somers. All Rights Reserved.
----/
----/
----/ INCLUDED Extensions or Model Features:
----/ • SAPC-8 Core Contrast
----/ • SmoothScale™ scaling technique
----/ • SoftToe black level soft clamp
----/
----/ NOT INCLUDED — This Version Does NOT Have These Extensions:
----/ • Color Vision Module
----/ • Spatial Frequency Module
----/ • Light Adaptation Module
----/ • Dynamics Module
----/ • Alpha Module
----/ • Personalization Module
----/ • Multiway Module
----/ • DynaFont™ font display
----/ • ResearchMode middle contrast explorer
----/ • ResearchMode static target
----/ • CIE function suite
----/ • SAPColor listings and sorting suite
----/ • RGBcolor() colorString parsing
----/
----/
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
------------------------------------------------------------------------------/
----/ BEGIN SAPC/APCA CONTRAST BLOCK \--------------------------------------
---- \------------------------------------
----------------------------------------------------------------------------
----/ SAPC Function with SmoothScale \------------------------------------
---- \----------------------------------
--/
----/ *** Polarity is Important: do not mix up background and text *** ----/
----/ Input value must be integer in RGB order (RRGGBB for 0xFFFFFF) ----/
----/ DO NOT use a Y from any other method ----/
local APCA = {}
function APCA.contrast (background, text)
Rbg = (background & 0xFF0000) >> 16
Gbg = (background & 0x00FF00) >> 8
Bbg = (background & 0x0000FF)
Rtxt = (text & 0xFF0000) >> 16
Gtxt = (text & 0x00FF00) >> 8
Btxt = (text & 0x0000FF)
----/ MAGICAL NUMBERS ------------------------------/
----/ sRGB Conversion to Relative Luminance (Y) ----/
mainTRC = 2.4 -- Transfer Curve (aka "Gamma") for sRGB linearization
-- Simple power curve vs piecewise described in docs
-- Essentially, 2.4 best models actual display
-- characteristics in combination with the total method
mainTRCencode = 0.41666666666666666667 -- = 1.0/mainTRC
Rco = 0.2126729 -- sRGB Red Coefficient (from matrix)
Gco = 0.7151522 -- sRGB Green Coefficient (from matrix)
Bco = 0.0721750 -- sRGB Blue Coefficient (from matrix)
----/ For Finding Raw SAPC Contrast from Relative Luminance (Y) ----/
normBG = 0.55 -- Constants for SAPC Power Curve Exponents
normTXT = 0.58 -- One pair for normal text, and one for reverse
revTXT = 0.57 -- These are the "beating heart" of SAPC
revBG = 0.62
----/ For Clamping and Scaling Values ----/
blkThrs = 0.03 -- Level that triggers the soft black clamp
blkClmp = 1.45 -- Exponent for the soft black clamp curve
deltaYmin = 0.0005 -- Lint trap
scaleBoW = 1.25 -- Scaling for dark text on light
scaleWoB = 1.25 -- Scaling for light text on dark
loConThresh = 0.078 -- Threshold for new simple offset scale
loConFactor = 12.82051282051282 -- = 1/0.078,
loConOffset = 0.06 -- The simple offset
loClip = 0.001 -- Output clip (lint trap #2)
-- We are only concerned with Y at this point
-- Ybg and Ytxt: divide sRGB to 0.0-1.0 range, linearize,
-- and then apply the standard coefficients and sum to Y.
-- Note that the Y we create here is unique and designed
-- exclusively for SAPC. Do not use Y from other methods.
Ybg = math.pow(Rbg/255.0, mainTRC) * Rco +
math.pow(Gbg/255.0, mainTRC) * Gco +
math.pow(Bbg/255.0, mainTRC) * Bco
Ytxt = math.pow(Rtxt/255.0, mainTRC) * Rco +
math.pow(Gtxt/255.0, mainTRC) * Gco +
math.pow(Btxt/255.0, mainTRC) * Bco
SAPC = 0.0 -- For holding raw SAPC values
outputContrast = 0.0 -- For weighted final values
----/ TUTORIAL ----/
-- Take Y and soft clamp black, return 0 for very close luminances
-- determine polarity, and calculate SAPC raw contrast
-- Then apply the output scaling
-- Note that reverse contrast (white text on black)
-- intentionally returns a negative number
-- Proper polarity is important!
---------- BLACK SOFT CLAMP & INPUT CLIP --------------------------------
-- Soft clamp Y when near black.
-- Now clamping all colors to prevent crossover errors
Ytxt = (Ytxt > blkThrs) and Ytxt or (Ytxt + math.pow(blkThrs - Ytxt, blkClmp))
Ybg = (Ybg > blkThrs) and Ybg or (Ybg + math.pow(blkThrs - Ybg, blkClmp))
----/ Return 0 Early for extremely low ∆Y (lint trap #1) ----/
if (math.abs(Ybg - Ytxt) < deltaYmin) then
outputcontrast = 0
elseif ( Ybg > Ytxt ) then -- For normal polarity, black text on white
----/ Calculate the SAPC contrast value and scale
SAPC = ( math.pow(Ybg, normBG) - math.pow(Ytxt, normTXT) ) * scaleBoW
----/ NEW! SAPC SmoothScale™
-- Low Contrast Smooth Scale Rollout to prevent polarity reversal
-- and also a low clip for very low contrasts (lint trap #2)
-- much of this is for very low contrasts, less than 10
-- therefore for most reversing needs, only loConOffset is important
if (SAPC < loClip) then
outputContrast = 0.0
elseif ( SAPC < loConThresh ) then
outputContrast = SAPC - SAPC * loConFactor * loConOffset
else
outputContrast = SAPC - loConOffset
end
else -- For reverse polarity, light text on dark
-- WoB should always return negative value.
SAPC = ( math.pow(Ybg, revBG) - math.pow(Ytxt, revTXT) ) * scaleWoB
if (SAPC > -loClip) then
outputContrast = 0.0
elseif ( SAPC > -loConThresh ) then
outputContrast = SAPC - SAPC * loConFactor * loConOffset
else
outputContrast = SAPC + loConOffset
end
end
return outputContrast * 100
end -- Close APCAcontrast()
--
-- Test values to check
-- text, background, expected value from https://github.com/Myndex/SAPC-APCA Readme
--
-- local function assertsame( a, b )
-- print (math.abs( a - b ) < 0.000000000001)
-- end
-- assertsame(APCA.contrast(0xffffff, 0x888888), 66.89346308821438)
-- assertsame(APCA.contrast(0x000000, 0xaaaaaa), -60.438571788907524)
-- assertsame(APCA.contrast(0x112233, 0xddeeff), -98.44863435731266)
-- assertsame(APCA.contrast(0x223344, 0x112233), 1.276075977788573)
return APCA
----\ ------------------------------------------/\
----/\ END OF SAPC/APCA BLOCK --------------------------------------------/\
------------------------------------------------------------------------------\
------------------------------------------------------------------------------/\