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diff --git a/sd/source/ui/slidesorter/cache/SlsBitmapCompressor.hxx b/sd/source/ui/slidesorter/cache/SlsBitmapCompressor.hxx
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+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/*
+ * This file is part of the LibreOffice project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * This file incorporates work covered by the following license notice:
+ *
+ *   Licensed to the Apache Software Foundation (ASF) under one or more
+ *   contributor license agreements. See the NOTICE file distributed
+ *   with this work for additional information regarding copyright
+ *   ownership. The ASF licenses this file to you under the Apache
+ *   License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0 .
+ */
+
+#pragma once
+
+#include <sal/types.h>
+#include <memory>
+
+class BitmapEx;
+
+namespace sd::slidesorter::cache
+{
+class BitmapReplacement;
+
+/** This interface class provides the minimal method set for classes that
+    implement the compression and decompression of preview bitmaps.
+*/
+class BitmapCompressor
+{
+public:
+    /** Compress the given bitmap into a replacement format that is specific
+        to the compressor class.
+    */
+    virtual std::shared_ptr<BitmapReplacement> Compress(const BitmapEx& rBitmap) const = 0;
+
+    /** Decompress the given replacement data into a preview bitmap.
+        Depending on the compression technique the returned bitmap may
+        differ from the original bitmap given to the Compress() method.  It
+        may even of the wrong size or empty or the NULL pointer.  It is the
+        task of the caller to create a new preview bitmap if the returned
+        one is not as desired.
+    */
+    virtual BitmapEx Decompress(const BitmapReplacement& rBitmapData) const = 0;
+
+    /** Return whether the compression and decompression is lossless.  This
+        value is used by the caller of Decompress() to decide whether to use
+        the returned bitmap as is or if a new preview has to be created.
+    */
+    virtual bool IsLossless() const = 0;
+
+protected:
+    ~BitmapCompressor() {}
+};
+
+/** Interface for preview bitmap replacements.  Each bitmap
+    compressor/decompressor has to provide an implementation that is
+    suitable to store the compressed bitmaps.
+*/
+class BitmapReplacement
+{
+public:
+    virtual sal_Int32 GetMemorySize() const { return 0; }
+
+protected:
+    ~BitmapReplacement() {}
+};
+
+/** This is one trivial bitmap compressor.  It stores bitmaps unmodified
+    instead of compressing them.
+    This compressor is lossless.
+*/
+class NoBitmapCompression : public BitmapCompressor
+{
+    class DummyReplacement;
+
+public:
+    virtual ~NoBitmapCompression() {}
+    virtual std::shared_ptr<BitmapReplacement> Compress(const BitmapEx& rpBitmap) const override;
+    virtual BitmapEx Decompress(const BitmapReplacement& rBitmapData) const override;
+    virtual bool IsLossless() const override;
+};
+
+/** This is another trivial bitmap compressor.  Instead of compressing a
+    bitmap, it throws the bitmap away.  Its Decompress() method returns a
+    NULL pointer.  The caller has to create a new preview bitmap instead.
+    This compressor clearly is not lossless.
+*/
+class CompressionByDeletion : public BitmapCompressor
+{
+public:
+    virtual ~CompressionByDeletion() {}
+    virtual std::shared_ptr<BitmapReplacement> Compress(const BitmapEx& rBitmap) const override;
+    virtual BitmapEx Decompress(const BitmapReplacement& rBitmapData) const override;
+    virtual bool IsLossless() const override;
+};
+
+/** Compress a preview bitmap by reducing its resolution.  While the aspect
+    ratio is maintained the horizontal resolution is scaled down to 100
+    pixels.
+    This compressor is not lossless.
+*/
+class ResolutionReduction : public BitmapCompressor
+{
+    class ResolutionReducedReplacement;
+    static const sal_Int32 mnWidth = 100;
+
+public:
+    virtual ~ResolutionReduction() {}
+    virtual std::shared_ptr<BitmapReplacement> Compress(const BitmapEx& rpBitmap) const override;
+    /** Scale the replacement bitmap up to the original size.
+    */
+    virtual BitmapEx Decompress(const BitmapReplacement& rBitmapData) const override;
+    virtual bool IsLossless() const override;
+};
+
+/** Compress preview bitmaps using the PNG format.
+    This compressor is lossless.
+*/
+class PngCompression : public BitmapCompressor
+{
+    class PngReplacement;
+
+public:
+    virtual ~PngCompression() {}
+    virtual std::shared_ptr<BitmapReplacement> Compress(const BitmapEx& rBitmap) const override;
+    virtual BitmapEx Decompress(const BitmapReplacement& rBitmapData) const override;
+    virtual bool IsLossless() const override;
+};
+
+} // end of namespace ::sd::slidesorter::cache
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab: */