_color_scale.py

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# This file is part of pipe_tasks.
#
# Developed for the LSST Data Management System.
# This product includes software developed by the LSST Project
# (https://www.lsst.org).
# See the COPYRIGHT file at the top-level directory of this distribution
# for details of code ownership.
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
from __future__ import annotations
 
__all__ = ("ColorScaler",)
 
import numpy as np
 
from lsst.pipe.tasks.prettyPictureMaker.types import FloatImagePlane
from lsst.pex.config.configurableActions import ConfigurableAction
from lsst.pex.config import Field, ListField
 
from .._equalizers import contrast_equalizer
 
 
class ColorScaler(ConfigurableAction):
    saturation = Field[float](
        doc=(
            "The overall saturation factor with the scaled luminance between zero and one. "
            "A value of one is not recommended as it makes bright pixels very saturated"
        ),
        default=0.6,
    )
    maxChroma = Field[float](
        doc=(
            "The maximum chromaticity in the OKLCh color space, large "
            "values will cause bright pixels to fall outside the RGB gamut."
        ),
        default=0.4,
    )
    brightFalloff = Field[float](
        doc="The rate that saturation scaling should slow with brightness", default=800
    )
    equalizer_levels = ListField[float](
        doc=(
            "A list of factors to modify the color constrast in a scale dependent way. "
            "One coefficient for each spatial scale, starting from the largest. "
            "Values large than 1 increase contrast, while less than 1 decreases"
            "Only scales upto and including the largest to be modified need specified"
            "IE [1.1,0.9] modifieds the first two [1.1,1,0.9] modifies the first three."
        ),
        optional=True,
    )
 
    def __call__(
        self, old_lum: FloatImagePlane, new_lum: FloatImagePlane, a: FloatImagePlane, b: FloatImagePlane
    ) -> tuple[FloatImagePlane, FloatImagePlane]:
        """
        Adjusts the color saturation while keeping the hue constant.
 
        This function adjusts the chromaticity (a, b) of colors to maintain a
        consistent saturation level, based on their original luminance. It uses
        the CIELAB color space representation and the `luminance` is the new target
        luminance for all colors.
 
        Parameters
        ----------
        old_lum : `FloatImagePlane`
            Luminance values of the original colors.
        new_lum : `FloatImagePlane`
            Target luminance values for the transformed colors.
        a : `FloatImagePlane`
            Chromaticity parameter 'a' corresponding to green-red axis in CIELAB.
        b : `FloatImagePlane`
            Chromaticity parameter 'b' corresponding to blue-yellow axis in CIELAB.
 
        Returns
        -------
        new_a : `FloatImagePlane`
            New a values representing the adjusted chromaticity.
        new_b : `FloatImagePlane`
            New b values representing the adjusted chromaticity.
        """
        # Calculate the square of the chroma, which is the distance from origin in
        # the a-b plane.
        chroma1_2 = a**2 + b**2
        chroma1 = np.sqrt(chroma1_2)
 
        # Calculate the hue angle, taking the absolute value to ensure non-negative
        # angle representation.
        chromaMask = chroma1 == 0
        chroma1[chromaMask] = 1
        # Calculate sine and cosine of hue angle from chromaticity coordinates.
        # In CIELAB a-b plane: sin(hue) = b/chroma, cos(hue) = a/chroma
        # These are used below to preserve hue while adjusting chroma.
        sinHue = b / chroma1
        cosHue = a / chroma1
        sinHue[chromaMask] = 0
        cosHue[chromaMask] = 0
 
        # Compute a divisor for saturation calculation, adding 1 to avoid division
        # by zero.
        div = chroma1_2 + old_lum**2
        div[div <= 0] = 1
 
        # Calculate the square of the new chroma based on desired saturation
        sat_original_2 = chroma1_2 / div
        # chroma2_2 = self.saturation * sat_original_2 * new_lum**2 / (1 - sat_original_2)
        correction_factor = np.arcsinh(abs(new_lum - 1) * self.brightFalloff) / np.arcsinh(self.brightFalloff)
        chroma2_2 = (
            self.saturation
            * correction_factor**2
            * sat_original_2
            * new_lum**2
            / (1 - sat_original_2 * correction_factor**2)
        )
 
        # Compute new 'a' values using the square root of adjusted chroma and
        # considering hue direction.
        chroma2 = np.sqrt(chroma2_2)
        if self.equalizer_levels is not None:
            chroma2 = contrast_equalizer(chroma2, self.equalizer_levels)
        # Cap the chroma to avoid excessive values that are visually unrealistic
        chroma2[chroma2 > self.maxChroma] = self.maxChroma
 
        new_a = chroma2 * cosHue
 
        # Compute new 'b' values using the root of the adjusted chroma and hue
        # direction.
        new_b = chroma2 * sinHue
 
        return new_a, new_b