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+/****************************************************************************
+**
+** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the plugins of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** No Commercial Usage
+** This file contains pre-release code and may not be distributed.
+** You may use this file in accordance with the terms and conditions
+** contained in the Technology Preview License Agreement accompanying
+** this package.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain additional
+** rights.  These rights are described in the Nokia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** If you have questions regarding the use of this file, please contact
+** Nokia at qt-info@nokia.com.
+**
+**
+**
+**
+**
+**
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+// This is an implementation of the 32bit => 16bit Floyd-Steinberg dithering.
+// The alghorithm used here is not the fastest possible but it's prolly fast enough:
+// uses look-up tables, integer-only arthmetics and works in one pass on two lines
+// at a time. It's a high-quality dithering using 1/8 diffusion precission.
+// Two functions here to look at:
+//
+//   * convertRGBA32_to_RGB565
+//   * convertRGBA32_to_RGBA4444
+//
+// Each channel (RGBA) is diffused independently and alpha is dithered too.
+
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+
+// Gets a component (red = 1, green = 2...) from a RGBA data structure.
+// data is unsigned char. stride is the number of bytes per line.
+#define GET_RGBA_COMPONENT(data, x, y, stride, c) (data[(y * stride) + (x << 2) + c])
+
+// Writes a new pixel with r, g, b to data in 565 16bit format. Data is a short.
+#define PUT_565(data, x, y, width, r, g, b) (data[(y * width) + x] = (r << 11) | (g << 5) | b)
+
+// Writes a new pixel with r, g, b, a to data in 4444 RGBA 16bit format. Data is a short.
+#define PUT_4444(data, x, y, width, r, g, b, a) (data[(y * width) + x] = (r << 12) | (g << 8) | (b << 4) | a)
+
+// Writes(ads) a new value to the diffusion accumulator. accumulator is a short.
+// x, y is a position in the accumulation buffer. y can be 0 or 1 -- we operate on two lines at time.
+#define ACCUMULATE(accumulator, x, y, width, v) if (x < width && x > 0) accumulator[(y * width) + x] += v
+
+// Clamps a value to be in 0..255 range.
+#define CLAMP_256(v) if (v > 255) v = 255; if (v < 0) v = 0;
+
+// Converts incoming RGB32 (QImage::Format_RGB32) to RGB565. Returns the newly allocated data.
+unsigned short* convertRGB32_to_RGB565(const unsigned char *in, int width, int height, int stride)
+{
+    // Will store output
+    unsigned short *out = (unsigned short *) malloc(width * height * 2);
+
+    // Lookup tables for the 8bit => 6bit and 8bit => 5bit conversion
+    unsigned char lookup_8bit_to_5bit[256];
+    short lookup_8bit_to_5bit_diff[256];
+    unsigned char lookup_8bit_to_6bit[256];
+    short lookup_8bit_to_6bit_diff[256];
+
+    // Macros for the conversion using the lookup table.
+    #define CONVERT_8BIT_TO_5BIT(v) (lookup_8bit_to_5bit[v])
+    #define DIFF_8BIT_TO_5BIT(v) (lookup_8bit_to_5bit_diff[v])
+
+    #define CONVERT_8BIT_TO_6BIT(v) (lookup_8bit_to_6bit[v])
+    #define DIFF_8BIT_TO_6BIT(v) (lookup_8bit_to_6bit_diff[v])
+
+    int i;
+    int x, y, c; // Pixel we're processing. c is component number (0, 1, 2 for r, b, b)
+    short component[3]; // Stores the new components (r, g, b) for pixel produced during conversion
+    short diff; // The difference between the converted value and the original one. To be accumulated.
+    short accumulator[3][width * 2]; // Three acumulators for r, g, b. Each accumulator is two lines.
+
+    // Produce the conversion lookup tables.
+    for (i = 0; i < 256; i++) {
+        lookup_8bit_to_5bit[i] = round(i / 8.0);
+        if (lookup_8bit_to_5bit[i] > 31)
+            lookup_8bit_to_5bit[i] -= 1;
+
+        // Before bitshifts: (i * 8) - (... * 8 * 8)
+        lookup_8bit_to_5bit_diff[i] = (i << 3) - (lookup_8bit_to_5bit[i] << 6);
+
+        lookup_8bit_to_6bit[i] = round(i / 4.0);
+        if (lookup_8bit_to_6bit[i] > 63)
+            lookup_8bit_to_6bit[i] -= 1;
+
+        // Before bitshifts: (i * 8) - (... * 4 * 8)
+        lookup_8bit_to_6bit_diff[i] = (i << 3) - (lookup_8bit_to_6bit[i] << 5);
+    }
+
+    // Clear the accumulators
+    memset(accumulator[0], 0, width * 4);
+    memset(accumulator[1], 0, width * 4);
+    memset(accumulator[2], 0, width * 4);
+
+    // For each line...
+    for (y = 0; y < height; y++) {
+
+        // For each accumulator, move the second line (index 1) to replace the first line (index 0).
+        // Clear the second line (index 1)
+        memcpy(accumulator[0], accumulator[0] + width, width * 2);
+        memset(accumulator[0] + width, 0, width * 2);
+
+        memcpy(accumulator[1], accumulator[1] + width, width * 2);
+        memset(accumulator[1] + width, 0, width * 2);
+
+        memcpy(accumulator[2], accumulator[2] + width, width * 2);
+        memset(accumulator[2] + width, 0, width * 2);
+
+        // For each column....
+        for (x = 0; x < width; x++) {
+
+            // For each component (r, g, b)...
+            for (c = 0; c < 3; c++) {
+
+                // Get the 8bit value from the original image
+                component[c] = GET_RGBA_COMPONENT(in, x, y, stride, c);
+
+                // Add the diffusion for this pixel we stored in the accumulator.
+                // >> 7 because the values in accumulator are stored * 128
+                component[c] += accumulator[c][x] >> 7;
+
+                // Make sure we're not over the boundaries.
+                CLAMP_256(component[c]);
+
+                // For green component we use 6 bits. Otherwise 5 bits.
+                // Store the difference from converting 8bit => 6 bit and the orig pixel.
+                // Convert 8bit => 6(5) bit.
+                if (c == 1) {
+                    diff = DIFF_8BIT_TO_6BIT(component[c]);
+                    component[c] = CONVERT_8BIT_TO_6BIT(component[c]);
+                } else {
+                    diff = DIFF_8BIT_TO_5BIT(component[c]);
+                    component[c] = CONVERT_8BIT_TO_5BIT(component[c]);
+                }
+
+                // Distribute the difference according to the matrix in the
+                // accumulation bufffer.
+                ACCUMULATE(accumulator[c], x + 1, 0, width, diff * 7);
+                ACCUMULATE(accumulator[c], x - 1, 1, width, diff * 3);
+                ACCUMULATE(accumulator[c], x, 1, width, diff * 5);
+                ACCUMULATE(accumulator[c], x + 1, 1, width, diff * 1);
+            }
+
+            // Write the newly produced pixel
+            PUT_565(out, x, y, width, component[2], component[1], component[0]);
+        }
+    }
+
+    return out;
+}
+
+// Converts incoming RGBA32 (QImage::Format_ARGB32_Premultiplied) to RGB565. Returns the newly allocated data.
+// This function is similar (yet different) to the _565 variant but it makes sense to duplicate it here for simplicity.
+unsigned short* convertARGB32_to_RGBA4444(const unsigned char *in, int width, int height, int stride)
+{
+    // Will store output
+    unsigned short *out = (unsigned short *) malloc(width * height * 2);
+
+    // Lookup tables for the 8bit => 4bit conversion
+    unsigned char lookup_8bit_to_4bit[256];
+    short lookup_8bit_to_4bit_diff[256];
+
+    // Macros for the conversion using the lookup table.
+    #define CONVERT_8BIT_TO_4BIT(v) (lookup_8bit_to_4bit[v])
+    #define DIFF_8BIT_TO_4BIT(v) (lookup_8bit_to_4bit_diff[v])
+
+    int i;
+    int x, y, c; // Pixel we're processing. c is component number (0, 1, 2, 3 for r, b, b, a)
+    short component[4]; // Stores the new components (r, g, b, a) for pixel produced during conversion
+    short diff; // The difference between the converted value and the original one. To be accumulated.
+    short accumulator[4][width * 2]; // Four acumulators for r, g, b, a. Each accumulator is two lines.
+
+    // Produce the conversion lookup tables.
+    for (i = 0; i < 256; i++) {
+        lookup_8bit_to_4bit[i] = round(i / 16.0);
+        if (lookup_8bit_to_4bit[i] > 15)
+            lookup_8bit_to_4bit[i] -= 1;
+
+        // Before bitshifts: (i * 8) - (... * 16 * 8)
+        lookup_8bit_to_4bit_diff[i] = (i << 3) - (lookup_8bit_to_4bit[i] << 7);
+    }
+
+    // Clear the accumulators
+    memset(accumulator[0], 0, width * 4);
+    memset(accumulator[1], 0, width * 4);
+    memset(accumulator[2], 0, width * 4);
+    memset(accumulator[3], 0, width * 4);
+
+    // For each line...
+    for (y = 0; y < height; y++) {
+
+        // For each component (r, g, b, a)...
+        memcpy(accumulator[0], accumulator[0] + width, width * 2);
+        memset(accumulator[0] + width, 0, width * 2);
+
+        memcpy(accumulator[1], accumulator[1] + width, width * 2);
+        memset(accumulator[1] + width, 0, width * 2);
+
+        memcpy(accumulator[2], accumulator[2] + width, width * 2);
+        memset(accumulator[2] + width, 0, width * 2);
+
+        memcpy(accumulator[3], accumulator[3] + width, width * 2);
+        memset(accumulator[3] + width, 0, width * 2);
+
+        // For each column....
+        for (x = 0; x < width; x++) {
+
+            // For each component (r, g, b, a)...
+            for (c = 0; c < 4; c++) {
+
+                // Get the 8bit value from the original image
+                component[c] = GET_RGBA_COMPONENT(in, x, y, stride, c);
+
+                // Add the diffusion for this pixel we stored in the accumulator.
+                // >> 7 because the values in accumulator are stored * 128
+                component[c] += accumulator[c][x] >> 7;
+
+                // Make sure we're not over the boundaries.
+                CLAMP_256(component[c]);
+
+                // Store the difference from converting 8bit => 4bit and the orig pixel.
+                // Convert 8bit => 4bit.
+                diff = DIFF_8BIT_TO_4BIT(component[c]);
+                component[c] = CONVERT_8BIT_TO_4BIT(component[c]);
+
+                // Distribute the difference according to the matrix in the
+                // accumulation bufffer.
+                ACCUMULATE(accumulator[c], x + 1, 0, width, diff * 7);
+                ACCUMULATE(accumulator[c], x - 1, 1, width, diff * 3);
+                ACCUMULATE(accumulator[c], x, 1, width, diff * 5);
+                ACCUMULATE(accumulator[c], x + 1, 1, width, diff * 1);
+            }
+
+            // Write the newly produced pixel
+            PUT_4444(out, x, y, width, component[0], component[1], component[2], component[3]);
+        }
+    }
+
+    return out;
+}