Add scanline decoding to SkCodec.

include/codec/SkScanlineDecoder.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkCodec.h"
+#include "SkImageGenerator.h"
+#include "SkImageInfo.h"
+
+class SkStream;
+
+class SkScanlineDecoder : public SkNoncopyable {
+public:
+    // Note for implementations: An SkScanlineDecoder should be deleted
+    // *before* its associated SkCodec, in case the order matters.
+    virtual ~SkScanlineDecoder() {}
+
+    /**
+     *  Write the next scanline into dst.
+     *
+     *  @param dst Must be non-null, and large enough to hold a scanline based
+     *      on the info used to create this object.
+     *  @return kSuccess or error code explaining failure.
+     */
+    SkImageGenerator::Result getNextScanline(void* dst) {
+        return this->onGetNextScanline(dst);
+    }
+
+    /**
+     *  Write the next countLines scanlines into dst.
+     *
+     *  @param dst Must be non-null, and large enough to hold countLines
+     *      scanlines based on rowBytes.
+     *  @param countLines Number of lines to write.
+     *  @param rowBytes Number of bytes per row.
+     */
+    SkImageGenerator::Result getScanlines(void* dst, int countLines, size_t rowBytes) {
+        // Should we keep track of how many scanlines have been read, at
+        // least in debug mode? Then we can make an even smarter check
+        // regarding countLines.
+        if (rowBytes < fDstInfo.minRowBytes() || countLines < 0
+                || countLines > fDstInfo.height()) {
+            return SkCodec::kInvalidParameters;
+        }
+        return this->onGetScanlines(dst, countLines, rowBytes);
+    }
+
+    /**
+     *  Skip count scanlines.
+     *
+     *  The default version just calls onGetScanline and discards the dst.
+     */
+    SkImageGenerator::Result skipScanlines(int count) {
+        return this->onSkipScanlines(count);
+    }
+
+    /**
+     *  Call this function after reading/skipping all scanlines.
+     */
+    SkImageGenerator::Result finish() {
+        return this->onFinish();
+    }
+
+    /**
+     *  Some images may initially report that they have alpha due to the format
+     *  of the encoded data, but then never use any colors which have alpha
+     *  less than 100%. This function can be called *after* decoding to
+     *  determine if such an image truly had alpha. Calling it before decoding
+     *  is undefined.
+     */
+    bool reallyHasAlpha() const {
+        return this->onReallyHasAlpha();
+    }
+
+protected:
+    SkScanlineDecoder(const SkImageInfo& original, const SkImageInfo& requested)
+        : fSrcInfo(original)
+        , fDstInfo(requested) {}
+
+    virtual bool onReallyHasAlpha() const { return false; }
+
+private:
+    // FIXME: Is fSrcInfo needed for subsetting? Not needed so far...
+    const SkImageInfo   fSrcInfo;
+    const SkImageInfo   fDstInfo;
+
+    virtual SkImageGenerator::Result onGetNextScanline(void* dst) = 0;
+
+    // Naive default version just calls onGetNextScanline on temp memory.
+    virtual SkImageGenerator::Result onSkipScanlines(int count) {
+        SkAutoMalloc storage(fDstInfo.minRowBytes());
+        for (int i = 0; i < count; ++i) {
+            SkImageGenerator::Result res = this->onGetNextScanline(storage.get());
+            if (res != SkImageGenerator::kSuccess) {
+                return res;
+            }
+        }
+        return SkCodec::kSuccess;
+    }
+
+    // Naive default version just calls onGetNextScanline
+    virtual SkImageGenerator::Result onGetScanlines(void* dst, int countLines, size_t rowBytes) {
+        void* dstRow = dst;
+        for (int i = 0; i < countLines; ++i) {
+            const SkImageGenerator::Result result = this->onGetNextScanline(dstRow);
+            if (result != SkCodec::kSuccess) {
+                return result;
+            }
+            dstRow = SkTAddOffset<void*>(dstRow, rowBytes);
+        }
+        return SkCodec::kSuccess;
+    }
+
+    virtual SkImageGenerator::Result onFinish() {
+        // Return success by default, since this may not be necessary for all codecs.
+        return SkImageGenerator::kSuccess;
+    }
+};