skia2/include/codec/SkCodec.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.
 */

#ifndef SkCodec_DEFINED
#define SkCodec_DEFINED

#include "../private/SkTemplates.h"
#include "SkColor.h"
#include "SkEncodedFormat.h"
#include "SkImageInfo.h"
#include "SkSize.h"
#include "SkStream.h"
#include "SkTypes.h"

class SkData;

/**
 *  Abstraction layer directly on top of an image codec.
 */
class SkCodec : SkNoncopyable {
public:
    /**
     *  If this stream represents an encoded image that we know how to decode,
     *  return an SkCodec that can decode it. Otherwise return NULL.
     *
     *  If NULL is returned, the stream is deleted immediately. Otherwise, the
     *  SkCodec takes ownership of it, and will delete it when done with it.
     */
    static SkCodec* NewFromStream(SkStream*);

    /**
     *  If this data represents an encoded image that we know how to decode,
     *  return an SkCodec that can decode it. Otherwise return NULL.
     *
     *  Will take a ref if it returns a codec, else will not affect the data.
     */
    static SkCodec* NewFromData(SkData*);

    virtual ~SkCodec();

    /**
     *  Return the ImageInfo associated with this codec.
     */
    const SkImageInfo& getInfo() const { return fSrcInfo; }

    /**
     *  Return a size that approximately supports the desired scale factor.
     *  The codec may not be able to scale efficiently to the exact scale
     *  factor requested, so return a size that approximates that scale.
     *  The returned value is the codec's suggestion for the closest valid
     *  scale that it can natively support
     */
    SkISize getScaledDimensions(float desiredScale) const {
        // Negative and zero scales are errors.
        SkASSERT(desiredScale > 0.0f);
        if (desiredScale <= 0.0f) {
            return SkISize::Make(0, 0);
        }

        // Upscaling is not supported. Return the original size if the client
        // requests an upscale.
        if (desiredScale >= 1.0f) {
            return this->getInfo().dimensions();
        }
        return this->onGetScaledDimensions(desiredScale);
    }

    /**
     *  Return (via desiredSubset) a subset which can decoded from this codec,
     *  or false if this codec cannot decode subsets or anything similar to
     *  desiredSubset.
     *
     *  @param desiredSubset In/out parameter. As input, a desired subset of
     *      the original bounds (as specified by getInfo). If true is returned,
     *      desiredSubset may have been modified to a subset which is
     *      supported. Although a particular change may have been made to
     *      desiredSubset to create something supported, it is possible other
     *      changes could result in a valid subset.
     *      If false is returned, desiredSubset's value is undefined.
     *  @return true if this codec supports decoding desiredSubset (as
     *      returned, potentially modified)
     */
    bool getValidSubset(SkIRect* desiredSubset) const {
        return this->onGetValidSubset(desiredSubset);
    }

    /**
     *  Format of the encoded data.
     */
    SkEncodedFormat getEncodedFormat() const { return this->onGetEncodedFormat(); }

    /**
     *  Used to describe the result of a call to getPixels().
     *
     *  Result is the union of possible results from subclasses.
     */
    enum Result {
        /**
         *  General return value for success.
         */
        kSuccess,
        /**
         *  The input is incomplete. A partial image was generated.
         */
        kIncompleteInput,
        /**
         *  The generator cannot convert to match the request, ignoring
         *  dimensions.
         */
        kInvalidConversion,
        /**
         *  The generator cannot scale to requested size.
         */
        kInvalidScale,
        /**
         *  Parameters (besides info) are invalid. e.g. NULL pixels, rowBytes
         *  too small, etc.
         */
        kInvalidParameters,
        /**
         *  The input did not contain a valid image.
         */
        kInvalidInput,
        /**
         *  Fulfilling this request requires rewinding the input, which is not
         *  supported for this input.
         */
        kCouldNotRewind,
        /**
         *  startScanlineDecode() was not called before calling getScanlines.
         */
        kScanlineDecodingNotStarted,
        /**
         *  This method is not implemented by this codec.
         *  FIXME: Perhaps this should be kUnsupported?
         */
        kUnimplemented,
    };

    /**
     *  Whether or not the memory passed to getPixels is zero initialized.
     */
    enum ZeroInitialized {
        /**
         *  The memory passed to getPixels is zero initialized. The SkCodec
         *  may take advantage of this by skipping writing zeroes.
         */
        kYes_ZeroInitialized,
        /**
         *  The memory passed to getPixels has not been initialized to zero,
         *  so the SkCodec must write all zeroes to memory.
         *
         *  This is the default. It will be used if no Options struct is used.
         */
        kNo_ZeroInitialized,
    };

    /**
     *  Additional options to pass to getPixels.
     */
    struct Options {
        Options()
            : fZeroInitialized(kNo_ZeroInitialized)
            , fSubset(NULL)
        {}

        ZeroInitialized fZeroInitialized;
        /**
         *  If not NULL, represents a subset of the original image to decode.
         *
         *  Must be within the bounds returned by getInfo().
         *
         *  If the EncodedFormat is kWEBP_SkEncodedFormat (the only one which
         *  currently supports subsets), the top and left values must be even.
         */
        SkIRect*        fSubset;
    };

    /**
     *  Decode into the given pixels, a block of memory of size at
     *  least (info.fHeight - 1) * rowBytes + (info.fWidth *
     *  bytesPerPixel)
     *
     *  Repeated calls to this function should give the same results,
     *  allowing the PixelRef to be immutable.
     *
     *  @param info A description of the format (config, size)
     *         expected by the caller.  This can simply be identical
     *         to the info returned by getInfo().
     *
     *         This contract also allows the caller to specify
     *         different output-configs, which the implementation can
     *         decide to support or not.
     *
     *         A size that does not match getInfo() implies a request
     *         to scale. If the generator cannot perform this scale,
     *         it will return kInvalidScale.
     *
     *  If info is kIndex8_SkColorType, then the caller must provide storage for up to 256
     *  SkPMColor values in ctable. On success the generator must copy N colors into that storage,
     *  (where N is the logical number of table entries) and set ctableCount to N.
     *
     *  If info is not kIndex8_SkColorType, then the last two parameters may be NULL. If ctableCount
     *  is not null, it will be set to 0.
     *
     *  If a scanline decode is in progress, scanline mode will end, requiring the client to call
     *  startScanlineDecode() in order to return to decoding scanlines.
     *
     *  @return Result kSuccess, or another value explaining the type of failure.
     */
    Result getPixels(const SkImageInfo& info, void* pixels, size_t rowBytes, const Options*,
                     SkPMColor ctable[], int* ctableCount);

    /**
     *  Simplified version of getPixels() that asserts that info is NOT kIndex8_SkColorType and
     *  uses the default Options.
     */
    Result getPixels(const SkImageInfo& info, void* pixels, size_t rowBytes);

    /**
     *  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.
     *  FIXME: see skbug.com/3582.
     */
    bool reallyHasAlpha() const {
        return this->onReallyHasAlpha();
    }

    /**
     * The remaining functions revolve around decoding scanlines.
     */

    /**
     *  Prepare for a scanline decode with the specified options.
     *
     *  After this call, this class will be ready to decode the first scanline.
     *
     *  This must be called in order to call getScanlines or skipScanlines.
     *
     *  This may require rewinding the stream.
     *
     *  Not all SkCodecs support this.
     *
     *  @param dstInfo Info of the destination. If the dimensions do not match
     *      those of getInfo, this implies a scale.
     *  @param options Contains decoding options, including if memory is zero
     *      initialized.
     *  @param ctable A pointer to a color table.  When dstInfo.colorType() is
     *      kIndex8, this should be non-NULL and have enough storage for 256
     *      colors.  The color table will be populated after decoding the palette.
     *  @param ctableCount A pointer to the size of the color table.  When
     *      dstInfo.colorType() is kIndex8, this should be non-NULL.  It will
     *      be modified to the true size of the color table (<= 256) after
     *      decoding the palette.
     *  @return Enum representing success or reason for failure.
     */
    Result startScanlineDecode(const SkImageInfo& dstInfo, const SkCodec::Options* options,
                 SkPMColor ctable[], int* ctableCount);

    /**
     *  Simplified version of startScanlineDecode() that asserts that info is NOT
     *  kIndex8_SkColorType and uses the default Options.
     */
    Result startScanlineDecode(const SkImageInfo& dstInfo);

    /**
     *  Write the next countLines scanlines into dst.
     *
     *  Not valid to call before calling startScanlineDecode().
     *
     *  @param dst Must be non-null, and large enough to hold countLines
     *      scanlines of size rowBytes.
     *  @param countLines Number of lines to write.
     *  @param rowBytes Number of bytes per row. Must be large enough to hold
     *      a scanline based on the SkImageInfo used to create this object.
     */
    Result getScanlines(void* dst, int countLines, size_t rowBytes);

    /**
     *  Skip count scanlines.
     *
     *  Not valid to call before calling startScanlineDecode().
     *
     *  The default version just calls onGetScanlines and discards the dst.
     *  NOTE: If skipped lines are the only lines with alpha, this default
     *  will make reallyHasAlpha return true, when it could have returned
     *  false.
     */
    Result skipScanlines(int countLines);

    /**
     *  The order in which rows are output from the scanline decoder is not the
     *  same for all variations of all image types.  This explains the possible
     *  output row orderings.
     */
    enum SkScanlineOrder {
        /*
         * By far the most common, this indicates that the image can be decoded
         * reliably using the scanline decoder, and that rows will be output in
         * the logical order.
         */
        kTopDown_SkScanlineOrder,

        /*
         * This indicates that the scanline decoder reliably outputs rows, but
         * they will be returned in reverse order.  If the scanline format is
         * kBottomUp, the nextScanline() API can be used to determine the actual
         * y-coordinate of the next output row, but the client is not forced
         * to take advantage of this, given that it's not too tough to keep
         * track independently.
         *
         * For full image decodes, it is safe to get all of the scanlines at
         * once, since the decoder will handle inverting the rows as it
         * decodes.
         *
         * For subset decodes and sampling, it is simplest to get and skip
         * scanlines one at a time, using the nextScanline() API.  It is
         * possible to ask for larger chunks at a time, but this should be used
         * with caution.  As with full image decodes, the decoder will handle
         * inverting the requested rows, but rows will still be delivered
         * starting from the bottom of the image.
         *
         * Upside down bmps are an example.
         */
        kBottomUp_SkScanlineOrder,

        /*
         * This indicates that the scanline decoder reliably outputs rows, but
         * they will not be in logical order.  If the scanline format is
         * kOutOfOrder, the nextScanline() API should be used to determine the
         * actual y-coordinate of the next output row.
         *
         * For this scanline ordering, it is advisable to get and skip
         * scanlines one at a time.
         *
         * Interlaced gifs are an example.
         */
        kOutOfOrder_SkScanlineOrder,

        /*
         * Indicates that the entire image must be decoded in order to output
         * any amount of scanlines.  In this case, it is a REALLY BAD IDEA to
         * request scanlines 1-by-1 or in small chunks.  The client should
         * determine which scanlines are needed and ask for all of them in
         * a single call to getScanlines().
         *
         * Interlaced pngs are an example.
         */
        kNone_SkScanlineOrder,
    };

    /**
     *  An enum representing the order in which scanlines will be returned by
     *  the scanline decoder.
     */
    SkScanlineOrder getScanlineOrder() const { return this->onGetScanlineOrder(); }

    /**
     *  Returns the y-coordinate of the next row to be returned by the scanline
     *  decoder.  This will be overridden in the case of
     *  kOutOfOrder_SkScanlineOrder and should be unnecessary in the case of
     *  kNone_SkScanlineOrder.
     *
     *  Results are undefined when not in scanline decoding mode.
     */
    int nextScanline() const {
        return this->onNextScanline();
    }

protected:
    SkCodec(const SkImageInfo&, SkStream*);

    virtual SkISize onGetScaledDimensions(float /* desiredScale */) const {
        // By default, scaling is not supported.
        return this->getInfo().dimensions();
    }

    virtual SkEncodedFormat onGetEncodedFormat() const = 0;

    virtual Result onGetPixels(const SkImageInfo& info,
                               void* pixels, size_t rowBytes, const Options&,
                               SkPMColor ctable[], int* ctableCount) = 0;

    virtual bool onGetValidSubset(SkIRect* /* desiredSubset */) const {
        // By default, subsets are not supported.
        return false;
    }

    virtual bool onReallyHasAlpha() const { return false; }

    /**
     *  If the stream was previously read, attempt to rewind.
     *
     *  If the stream needed to be rewound, call onRewind.
     *  @returns true if the codec is at the right position and can be used.
     *      false if there was a failure to rewind.
     *
     *  This is called by getPixels() and start(). Subclasses may call if they
     *  need to rewind at another time.
     */
    bool SK_WARN_UNUSED_RESULT rewindIfNeeded();

    /**
     *  Called by rewindIfNeeded, if the stream needed to be rewound.
     *
     *  Subclasses should do any set up needed after a rewind.
     */
    virtual bool onRewind() {
        return true;
    }

    /**
     * Get method for the input stream
     */
    SkStream* stream() {
        return fStream.get();
    }

    /**
     *  The remaining functions revolve around decoding scanlines.
     */

    /**
     *  Most images types will be kTopDown and will not need to override this function.
     */
    virtual SkScanlineOrder onGetScanlineOrder() const { return kTopDown_SkScanlineOrder; }

    /**
     *  Most images will be kTopDown and will not need to override this function.
     */
    virtual int onNextScanline() const { return fCurrScanline; }

    /**
     *  Update the next scanline. Used by interlaced png.
     */
    void updateNextScanline(int newY) { fCurrScanline = newY; }

    const SkImageInfo& dstInfo() const { return fDstInfo; }

    const SkCodec::Options& options() const { return fOptions; }

private:
    const SkImageInfo       fSrcInfo;
    SkAutoTDelete<SkStream> fStream;
    bool                    fNeedsRewind;
    // These fields are only meaningful during scanline decodes.
    SkImageInfo             fDstInfo;
    SkCodec::Options        fOptions;
    int                     fCurrScanline;

    // Methods for scanline decoding.
    virtual SkCodec::Result onStartScanlineDecode(const SkImageInfo& dstInfo,
            const SkCodec::Options& options, SkPMColor ctable[], int* ctableCount) {
        return kUnimplemented;
    }

    // Naive default version just calls onGetScanlines on temp memory.
    virtual SkCodec::Result onSkipScanlines(int countLines) {
        SkAutoMalloc storage(fDstInfo.minRowBytes());
        // Note that we pass 0 to rowBytes so we continue to use the same memory.
        // Also note that while getScanlines checks that rowBytes is big enough,
        // onGetScanlines bypasses that check.
        // Calling the virtual method also means we do not double count
        // countLines.
        return this->onGetScanlines(storage.get(), countLines, 0);
    }

    virtual SkCodec::Result onGetScanlines(void* dst, int countLines,
                                                    size_t rowBytes) {
        return kUnimplemented;
    }

};
#endif // SkCodec_DEFINED