Add support for 8-bit PseudoColor visuals

This support involves allocating a 16-bit grayscale ramp as well
as a 5x5x5 RGB color cube. Afterwards, the 256 colors are queried
and an array is generated mapping from r3g3b3 colors to the closest
available color. Both the queried colors and the reverse mapping
are stored in a new visual-specific cairo_xlib_visual_info_t
structure which hangs off of the cairo_xlib_screen_info_t.

Both the color-cube allocation and the distance metric could be
improved by someone sufficiently motivated, (for example, allocating
and matching in a perceptually linear color space rather than just
in RGB space). Also, making this work well in the face of a changing
color map, or in the case of GrayScale, StaticGray, or DirectColor
visuals are left entirely as exercises for the reader. StaticColor
support should be fine as is, but is entirely untested.

1 files changed, 137 insertions, 0 deletions

diff --git a/src/cairo-xlib-visual.c b/src/cairo-xlib-visual.c
new file mode 100644
index 00000000..016ab00e
--- /dev/null
+++ b/src/cairo-xlib-visual.c
@@ -0,0 +1,137 @@
+/* Cairo - a vector graphics library with display and print output
+ *
+ * Copyright © 2008 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it either under the terms of the GNU Lesser General Public
+ * License version 2.1 as published by the Free Software Foundation
+ * (the "LGPL") or, at your option, under the terms of the Mozilla
+ * Public License Version 1.1 (the "MPL"). If you do not alter this
+ * notice, a recipient may use your version of this file under either
+ * the MPL or the LGPL.
+ *
+ * You should have received a copy of the LGPL along with this library
+ * in the file COPYING-LGPL-2.1; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ * You should have received a copy of the MPL along with this library
+ * in the file COPYING-MPL-1.1
+ *
+ * The contents of this file are subject to the Mozilla Public License
+ * Version 1.1 (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.mozilla.org/MPL/
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY
+ * OF ANY KIND, either express or implied. See the LGPL or the MPL for
+ * the specific language governing rights and limitations.
+ *
+ * The Original Code is the cairo graphics library.
+ *
+ * The Initial Developer of the Original Code is Red Hat, Inc.
+ *
+ * Contributor(s):
+ *	Carl D. Worth <cworth@cworth.org>
+ */
+
+#include "cairoint.h"
+
+#include "cairo-xlib-private.h"
+
+/* A perceptual distance metric between two colors. No sqrt needed
+ * since the square of the distance is still a valid metric. */
+
+/* XXX: This is currently using linear distance in RGB space which is
+ * decidedly not perceptually linear. If someone cared a lot about the
+ * quality, they might choose something else here. Then again, they
+ * might also choose not to use a PseudoColor visual... */
+static inline int
+_color_distance (unsigned short r1, unsigned short g1, unsigned short b1,
+		 unsigned short r2, unsigned short g2, unsigned short b2)
+{
+    r1 >>= 8; g1 >>= 8; b1 >>= 8;
+    r2 >>= 8; g2 >>= 8; b2 >>= 8;
+
+    return ((r2 - r1) * (r2 - r1) +
+	    (g2 - g1) * (g2 - g1) +
+	    (b2 - b1) * (b2 - b1));
+}
+
+cairo_xlib_visual_info_t *
+_cairo_xlib_visual_info_create (Display *dpy, int screen, VisualID visualid)
+{
+    cairo_xlib_visual_info_t *info;
+    Colormap colormap = DefaultColormap (dpy, screen);
+    XColor color;
+    int gray, red, green, blue;
+    int i, distance, min_distance, index;
+
+    const unsigned short index5_to_short[5] = {
+	0x0000, 0x4000, 0x8000, 0xc000, 0xffff
+    };
+    const unsigned short index8_to_short[8] = {
+	0x0000, 0x2492, 0x4924, 0x6db6,
+	0x9249, 0xb6db, 0xdb6d, 0xffff
+    };
+
+    info = malloc (sizeof (cairo_xlib_visual_info_t));
+    if (info == NULL)
+	return NULL;
+
+    info->visualid = visualid;
+
+    /* Allocate a 16-entry gray ramp and a 5x5x5 color cube. Give up
+     * as soon as failures start. */
+    for (gray = 0; gray < 16; gray++) {
+	color.red   = (gray << 12) | (gray << 8) | (gray << 4) | gray;
+	color.green = (gray << 12) | (gray << 8) | (gray << 4) | gray;
+	color.blue  = (gray << 12) | (gray << 8) | (gray << 4) | gray;
+	if (! XAllocColor (dpy, colormap, &color))
+	    goto DONE_ALLOCATE;
+    }
+
+    /* XXX: Could do this in a more clever order to have the best
+     * possible results from early failure. Could also choose a cube
+     * uniformly distributed in a better space than RGB. */
+    for (red = 0; red < 5; red++) {
+	for (green = 0; green < 5; green++) {
+	    for (blue = 0; blue < 5; blue++) {
+		color.red = index5_to_short[red];
+		color.green = index5_to_short[green];
+		color.blue = index5_to_short[blue];
+		color.pixel = 0;
+		color.flags = 0;
+		color.pad = 0;
+		if (! XAllocColor (dpy, colormap, &color))
+		    goto DONE_ALLOCATE;
+	    }
+	}
+    }
+  DONE_ALLOCATE:
+
+    for (i = 0; i < ARRAY_LENGTH (info->colors); i++)
+	info->colors[i].pixel = i;
+    XQueryColors (dpy, colormap, info->colors, ARRAY_LENGTH (info->colors));
+
+    /* Search for nearest colors within allocated colormap. */
+    for (red = 0; red < 8; red++) {
+	for (green = 0; green < 8; green++) {
+	    for (blue = 0; blue < 8; blue++) {
+		index = (red << 6) | (green << 3) | (blue);
+		for (i = 0; i < 256; i++) {
+		    distance = _color_distance (index8_to_short[red],
+						index8_to_short[green],
+						index8_to_short[blue],
+						info->colors[i].red,
+						info->colors[i].green,
+						info->colors[i].blue);
+		    if (i == 0 || distance < min_distance) {
+			info->rgb333_to_pseudocolor[index] = info->colors[i].pixel;
+			min_distance = distance;
+		    }
+		}
+	    }
+	}
+    }
+
+    return info;
+}