1 files changed, 83 insertions, 0 deletions
diff --git a/src/display/grayscale.cpp b/src/display/grayscale.cpp
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+++ b/src/display/grayscale.cpp
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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 :