src.com.android.internal.graphics.cam.Frame Maven / Gradle / Ivy
/*
* Copyright (C) 2021 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.android.internal.graphics.cam;
import android.annotation.NonNull;
import android.util.MathUtils;
/**
* The frame, or viewing conditions, where a color was seen. Used, along with a color, to create a
* color appearance model representing the color.
*
* To convert a traditional color to a color appearance model, it requires knowing what
* conditions the color was observed in. Our perception of color depends on, for example, the tone
* of the light illuminating the color, how bright that light was, etc.
*
*
This class is modelled separately from the color appearance model itself because there are a
* number of calculations during the color => CAM conversion process that depend only on the viewing
* conditions. Caching those calculations in a Frame instance saves a significant amount of time.
*/
public final class Frame {
// Standard viewing conditions assumed in RGB specification - Stokes, Anderson, Chandrasekar,
// Motta - A Standard Default Color Space for the Internet: sRGB, 1996.
//
// White point = D65
// Luminance of adapting field: 200 / Pi / 5, units are cd/m^2.
// sRGB ambient illuminance = 64 lux (per sRGB spec). However, the spec notes this is
// artificially low and based on monitors in 1990s. Use 200, the sRGB spec says this is the
// real average, and a survey of lux values on Wikipedia confirms this is a comfortable
// default: somewhere between a very dark overcast day and office lighting.
// Per CAM16 introduction paper (Li et al, 2017) Ew = pi * lw, and La = lw * Yb/Yw
// Ew = ambient environment luminance, in lux.
// Yb/Yw is taken to be midgray, ~20% relative luminance (XYZ Y 18.4, CIELAB L* 50).
// Therefore La = (Ew / pi) * .184
// La = 200 / pi * .184
// Image surround to 10 degrees = ~20% relative luminance = CIELAB L* 50
//
// Not from sRGB standard:
// Surround = average, 2.0.
// Discounting illuminant = false, doesn't occur for self-luminous displays
public static final Frame DEFAULT =
Frame.make(
CamUtils.WHITE_POINT_D65,
(float) (200.0f / Math.PI * CamUtils.yFromLstar(50.0f) / 100.f), 50.0f, 2.0f,
false);
private final float mAw;
private final float mNbb;
private final float mNcb;
private final float mC;
private final float mNc;
private final float mN;
private final float[] mRgbD;
private final float mFl;
private final float mFlRoot;
private final float mZ;
float getAw() {
return mAw;
}
float getN() {
return mN;
}
float getNbb() {
return mNbb;
}
float getNcb() {
return mNcb;
}
float getC() {
return mC;
}
float getNc() {
return mNc;
}
@NonNull
float[] getRgbD() {
return mRgbD;
}
float getFl() {
return mFl;
}
float getFlRoot() {
return mFlRoot;
}
float getZ() {
return mZ;
}
private Frame(float n, float aw, float nbb, float ncb, float c, float nc, float[] rgbD,
float fl, float fLRoot, float z) {
mN = n;
mAw = aw;
mNbb = nbb;
mNcb = ncb;
mC = c;
mNc = nc;
mRgbD = rgbD;
mFl = fl;
mFlRoot = fLRoot;
mZ = z;
}
/** Create a custom frame. */
@NonNull
public static Frame make(@NonNull float[] whitepoint, float adaptingLuminance,
float backgroundLstar, float surround, boolean discountingIlluminant) {
// Transform white point XYZ to 'cone'/'rgb' responses
float[][] matrix = CamUtils.XYZ_TO_CAM16RGB;
float[] xyz = whitepoint;
float rW = (xyz[0] * matrix[0][0]) + (xyz[1] * matrix[0][1]) + (xyz[2] * matrix[0][2]);
float gW = (xyz[0] * matrix[1][0]) + (xyz[1] * matrix[1][1]) + (xyz[2] * matrix[1][2]);
float bW = (xyz[0] * matrix[2][0]) + (xyz[1] * matrix[2][1]) + (xyz[2] * matrix[2][2]);
// Scale input surround, domain (0, 2), to CAM16 surround, domain (0.8, 1.0)
float f = 0.8f + (surround / 10.0f);
// "Exponential non-linearity"
float c = (f >= 0.9) ? MathUtils.lerp(0.59f, 0.69f, ((f - 0.9f) * 10.0f)) : MathUtils.lerp(
0.525f, 0.59f, ((f - 0.8f) * 10.0f));
// Calculate degree of adaptation to illuminant
float d = discountingIlluminant ? 1.0f : f * (1.0f - ((1.0f / 3.6f) * (float) Math.exp(
(-adaptingLuminance - 42.0f) / 92.0f)));
// Per Li et al, if D is greater than 1 or less than 0, set it to 1 or 0.
d = (d > 1.0) ? 1.0f : (d < 0.0) ? 0.0f : d;
// Chromatic induction factor
float nc = f;
// Cone responses to the whitepoint, adjusted for illuminant discounting.
//
// Why use 100.0 instead of the white point's relative luminance?
//
// Some papers and implementations, for both CAM02 and CAM16, use the Y
// value of the reference white instead of 100. Fairchild's Color Appearance
// Models (3rd edition) notes that this is in error: it was included in the
// CIE 2004a report on CIECAM02, but, later parts of the conversion process
// account for scaling of appearance relative to the white point relative
// luminance. This part should simply use 100 as luminance.
float[] rgbD = new float[]{d * (100.0f / rW) + 1.0f - d, d * (100.0f / gW) + 1.0f - d,
d * (100.0f / bW) + 1.0f - d, };
// Luminance-level adaptation factor
float k = 1.0f / (5.0f * adaptingLuminance + 1.0f);
float k4 = k * k * k * k;
float k4F = 1.0f - k4;
float fl = (k4 * adaptingLuminance) + (0.1f * k4F * k4F * (float) Math.cbrt(
5.0 * adaptingLuminance));
// Intermediate factor, ratio of background relative luminance to white relative luminance
float n = CamUtils.yFromLstar(backgroundLstar) / whitepoint[1];
// Base exponential nonlinearity
// note Schlomer 2018 has a typo and uses 1.58, the correct factor is 1.48
float z = 1.48f + (float) Math.sqrt(n);
// Luminance-level induction factors
float nbb = 0.725f / (float) Math.pow(n, 0.2);
float ncb = nbb;
// Discounted cone responses to the white point, adjusted for post-chromatic
// adaptation perceptual nonlinearities.
float[] rgbAFactors = new float[]{(float) Math.pow(fl * rgbD[0] * rW / 100.0, 0.42),
(float) Math.pow(fl * rgbD[1] * gW / 100.0, 0.42), (float) Math.pow(
fl * rgbD[2] * bW / 100.0, 0.42)};
float[] rgbA = new float[]{(400.0f * rgbAFactors[0]) / (rgbAFactors[0] + 27.13f),
(400.0f * rgbAFactors[1]) / (rgbAFactors[1] + 27.13f),
(400.0f * rgbAFactors[2]) / (rgbAFactors[2] + 27.13f), };
float aw = ((2.0f * rgbA[0]) + rgbA[1] + (0.05f * rgbA[2])) * nbb;
return new Frame(n, aw, nbb, ncb, c, nc, rgbD, fl, (float) Math.pow(fl, 0.25), z);
}
}