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// SPDX-License-Identifier: GPL-2.0-or-later
/** \file
* Provide methods to calculate grayscale values (e.g. convert rgba value to grayscale rgba value)
*/
/*
* Author:
* Johan Engelen <goejendaagh@zonnet.nl>
*
* Copyright (C) 2011 Author
*
* Released under GNU GPL v2+, read the file 'COPYING' for more information.
*/
#include "display/grayscale.h"
#include "color.h"
namespace Grayscale {
/* Original values from Johan:
const float red_factor = 0.3;
const float green_factor = 0.59;
const float blue_factor = 0.11;
*/
// Values below are from the SVG specification
const float red_factor = 0.2125;
const float green_factor = 0.7154;
const float blue_factor = 0.0721;
guint32 process(guint32 rgba) {
return process(SP_RGBA32_R_U(rgba), SP_RGBA32_G_U(rgba), SP_RGBA32_B_U(rgba), SP_RGBA32_A_U(rgba));
}
guint32 process(unsigned char r, unsigned char g, unsigned char b, unsigned char a) {
/** To reduce banding in gradients, this calculation is tweaked a bit
* by outputting blue+1 or red+1 or both. The luminance is calculated
* times 4. Then last two bits are used to determine if red and/or blue
* can be increased by one. Then these two bits are discarded.
* So the output color it still looks gray, but has more than 256 steps.
* The assumption is that the eye is most sensitive to green, then red, then blue.
* (hope this trick works :-) Johan)
*/
guint32 luminance = ( red_factor * (r << 3)
+ green_factor * (g << 3)
+ blue_factor * (b << 3) );
unsigned blue_plus_one = (luminance & 0x01) ? 1 : 0;
unsigned red_plus_one = (luminance & 0x02) ? 1 : 0;
unsigned green_plus_one = (luminance & 0x04) ? 1 : 0;
luminance = luminance >> 3;
if (luminance >= 0xff) {
return SP_RGBA32_U_COMPOSE(0xff, 0xff, 0xff, a);
} else {
return SP_RGBA32_U_COMPOSE(luminance + red_plus_one, luminance + green_plus_one, luminance + blue_plus_one, a);
}
}
unsigned char luminance(unsigned char r, unsigned char g, unsigned char b) {
guint32 luminance = ( red_factor * r
+ green_factor * g
+ blue_factor * b );
if (luminance > 0xff) {
luminance = 0xff;
}
return luminance & 0xff;
}
};
/*
Local Variables:
mode:c++
c-file-style:"stroustrup"
c-file-offsets:((innamespace . 0)(inline-open . 0)(case-label . +))
indent-tabs-mode:nil
fill-column:99
End:
*/
// vim: filetype=cpp:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:fileencoding=utf-8:textwidth=99 :
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