aboutsummaryrefslogtreecommitdiffhomepage
path: root/include/codec/SkCodec.h
blob: 629274d21ccacc734259c981b670d60cf1d37c5e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
/*
 * 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"
#include "SkYUVSizeInfo.h"

class SkColorSpace;
class SkData;
class SkPngChunkReader;
class SkSampler;

/**
 *  Abstraction layer directly on top of an image codec.
 */
class SkCodec : SkNoncopyable {
public:
    /**
     *  Minimum number of bytes that must be buffered in SkStream input.
     *
     *  An SkStream passed to NewFromStream must be able to use this many
     *  bytes to determine the image type. Then the same SkStream must be
     *  passed to the correct decoder to read from the beginning.
     *
     *  This can be accomplished by implementing peek() to support peeking
     *  this many bytes, or by implementing rewind() to be able to rewind()
     *  after reading this many bytes.
     */
    static size_t MinBufferedBytesNeeded();

    /**
     *  If this stream represents an encoded image that we know how to decode,
     *  return an SkCodec that can decode it. Otherwise return NULL.
     *
     *  As stated above, this call must be able to peek or read
     *  MinBufferedBytesNeeded to determine the correct format, and then start
     *  reading from the beginning. First it will attempt to peek, and it
     *  assumes that if less than MinBufferedBytesNeeded bytes (but more than
     *  zero) are returned, this is because the stream is shorter than this,
     *  so falling back to reading would not provide more data. If peek()
     *  returns zero bytes, this call will instead attempt to read(). This
     *  will require that the stream can be rewind()ed.
     *
     *  If SkPngChunkReader is not NULL, take a ref and pass it to libpng if
     *  the image is a png.
     *
     *  If the SkPngChunkReader is not NULL then:
     *      If the image is not a PNG, the SkPngChunkReader will be ignored.
     *      If the image is a PNG, the SkPngChunkReader will be reffed.
     *      If the PNG has unknown chunks, the SkPngChunkReader will be used
     *      to handle these chunks.  SkPngChunkReader will be called to read
     *      any unknown chunk at any point during the creation of the codec
     *      or the decode.  Note that if SkPngChunkReader fails to read a
     *      chunk, this could result in a failure to create the codec or a
     *      failure to decode the image.
     *      If the PNG does not contain unknown chunks, the SkPngChunkReader
     *      will not be used or modified.
     *
     *  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*, SkPngChunkReader* = NULL);

    /**
     *  If this data represents an encoded image that we know how to decode,
     *  return an SkCodec that can decode it. Otherwise return NULL.
     *
     *  If the SkPngChunkReader is not NULL then:
     *      If the image is not a PNG, the SkPngChunkReader will be ignored.
     *      If the image is a PNG, the SkPngChunkReader will be reffed.
     *      If the PNG has unknown chunks, the SkPngChunkReader will be used
     *      to handle these chunks.  SkPngChunkReader will be called to read
     *      any unknown chunk at any point during the creation of the codec
     *      or the decode.  Note that if SkPngChunkReader fails to read a
     *      chunk, this could result in a failure to create the codec or a
     *      failure to decode the image.
     *      If the PNG does not contain unknown chunks, the SkPngChunkReader
     *      will not be used or modified.
     *
     *  Will take a ref if it returns a codec, else will not affect the data.
     */
    static SkCodec* NewFromData(SkData*, SkPngChunkReader* = NULL);

    virtual ~SkCodec();

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

    /**
     *  Returns the color space associated with the codec.
     *  Does not affect ownership.
     *  Might be NULL.
     */
    SkColorSpace* getColorSpace() const { return fColorSpace.get(); }

    enum Origin {
        kTopLeft_Origin     = 1, // Default
        kTopRight_Origin    = 2, // Reflected across y-axis
        kBottomRight_Origin = 3, // Rotated 180
        kBottomLeft_Origin  = 4, // Reflected across x-axis
        kLeftTop_Origin     = 5, // Reflected across x-axis, Rotated 90 CCW
        kRightTop_Origin    = 6, // Rotated 90 CW
        kRightBottom_Origin = 7, // Reflected across x-axis, Rotated 90 CW
        kLeftBottom_Origin  = 8, // Rotated 90 CCW
        kDefault_Origin     = kTopLeft_Origin,
        kLast_Origin        = kLeftBottom_Origin,
    };

    /**
     *  Returns the image orientation stored in the EXIF data.
     *  If there is no EXIF data, or if we cannot read the EXIF data, returns kTopLeft.
     */
    Origin getOrigin() const { return fOrigin; }

    /**
     *  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,
        /**
         *  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.
         *
         *  In getPixels, we will attempt to decode the exact rectangular
         *  subset specified by fSubset.
         *
         *  In a scanline decode, it does not make sense to specify a subset
         *  top or subset height, since the client already controls which rows
         *  to get and which rows to skip.  During scanline decodes, we will
         *  require that the subset top be zero and the subset height be equal
         *  to the full height.  We will, however, use the values of
         *  subset left and subset width to decode partial scanlines on calls
         *  to getScanlines().
         */
        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);

    /**
     *  If decoding to YUV is supported, this returns true.  Otherwise, this
     *  returns false and does not modify any of the parameters.
     *
     *  @param sizeInfo   Output parameter indicating the sizes and required
     *                    allocation widths of the Y, U, and V planes.
     *  @param colorSpace Output parameter.  If non-NULL this is set to kJPEG,
     *                    otherwise this is ignored.
     */
    bool queryYUV8(SkYUVSizeInfo* sizeInfo, SkYUVColorSpace* colorSpace) const {
        if (nullptr == sizeInfo) {
            return false;
        }

        return this->onQueryYUV8(sizeInfo, colorSpace);
    }

    /**
     *  Returns kSuccess, or another value explaining the type of failure.
     *  This always attempts to perform a full decode.  If the client only
     *  wants size, it should call queryYUV8().
     *
     *  @param sizeInfo   Needs to exactly match the values returned by the
     *                    query, except the WidthBytes may be larger than the
     *                    recommendation (but not smaller).
     *  @param planes     Memory for each of the Y, U, and V planes.
     */
    Result getYUV8Planes(const SkYUVSizeInfo& sizeInfo, void* planes[3]) {
        if (nullptr == planes || nullptr == planes[0] || nullptr == planes[1] ||
                nullptr == planes[2]) {
            return kInvalidInput;
        }

        if (!this->rewindIfNeeded()) {
            return kCouldNotRewind;
        }

        return this->onGetYUV8Planes(sizeInfo, planes);
    }

    /**
     * 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.
     *  @return the number of lines successfully decoded.  If this value is
     *      less than countLines, this will fill the remaining lines with a
     *      default value.
     */
    int 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.
     *
     *  @return true if the scanlines were successfully skipped
     *          false on failure, possible reasons for failure include:
     *              An incomplete input image stream.
     *              Calling this function before calling startScanlineDecode().
     *              If countLines is less than zero or so large that it moves
     *                  the current scanline past the end of the image.
     */
    bool 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.
     *
     *  This is undefined before startScanlineDecode() is called.
     */
    SkScanlineOrder getScanlineOrder() const { return this->onGetScanlineOrder(); }

    /**
     *  Returns the y-coordinate of the next row to be returned by the scanline
     *  decoder.
     *
     *  This will equal fCurrScanline, except in the case of strangely
     *  encoded image types (bottom-up bmps, interlaced gifs).
     *
     *  Results are undefined when not in scanline decoding mode.
     */
    int nextScanline() const { return this->outputScanline(fCurrScanline); }

    /**
     *  Returns the output y-coordinate of the row that corresponds to an input
     *  y-coordinate.  The input y-coordinate represents where the scanline
     *  is located in the encoded data.
     *
     *  This will equal inputScanline, except in the case of strangely
     *  encoded image types (bottom-up bmps, interlaced gifs).
     */
    int outputScanline(int inputScanline) const;

protected:
    /**
     *  Takes ownership of SkStream*
     */
    SkCodec(const SkImageInfo&,
            SkStream*,
            sk_sp<SkColorSpace> = nullptr,
            Origin = kTopLeft_Origin);

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

    // FIXME: What to do about subsets??
    /**
     *  Subclasses should override if they support dimensions other than the
     *  srcInfo's.
     */
    virtual bool onDimensionsSupported(const SkISize&) {
        return false;
    }

    virtual SkEncodedFormat onGetEncodedFormat() const = 0;

    /**
     * @param rowsDecoded When the encoded image stream is incomplete, this function
     *                    will return kIncompleteInput and rowsDecoded will be set to
     *                    the number of scanlines that were successfully decoded.
     *                    This will allow getPixels() to fill the uninitialized memory.
     */
    virtual Result onGetPixels(const SkImageInfo& info,
                               void* pixels, size_t rowBytes, const Options&,
                               SkPMColor ctable[], int* ctableCount,
                               int* rowsDecoded) = 0;

    virtual bool onQueryYUV8(SkYUVSizeInfo*, SkYUVColorSpace*) const {
        return false;
    }

    virtual Result onGetYUV8Planes(const SkYUVSizeInfo&, void*[3] /*planes*/) {
        return kUnimplemented;
    }

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

    /**
     * On an incomplete input, getPixels() and getScanlines() will fill any uninitialized
     * scanlines.  This allows the subclass to indicate what value to fill with.
     *
     * @param colorType Destination color type.
     * @return          The value with which to fill uninitialized pixels.
     *
     * Note that we can interpret the return value as an SkPMColor, a 16-bit 565 color,
     * an 8-bit gray color, or an 8-bit index into a color table, depending on the color
     * type.
     */
    uint32_t getFillValue(SkColorType colorType) const {
        return this->onGetFillValue(colorType);
    }

    /**
     * Some subclasses will override this function, but this is a useful default for the color
     * types that we support.  Note that for color types that do not use the full 32-bits,
     * we will simply take the low bits of the fill value.
     *
     * kN32_SkColorType: Transparent or Black, depending on the src alpha type
     * kRGB_565_SkColorType: Black
     * kGray_8_SkColorType: Black
     * kIndex_8_SkColorType: First color in color table
     */
    virtual uint32_t onGetFillValue(SkColorType /*colorType*/) const {
        return kOpaque_SkAlphaType == fSrcInfo.alphaType() ? SK_ColorBLACK : SK_ColorTRANSPARENT;
    }

    /**
     * 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; }

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

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

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

    /**
     *  Returns the number of scanlines that have been decoded so far.
     *  This is unaffected by the SkScanlineOrder.
     *
     *  Returns -1 if we have not started a scanline decode.
     */
    int currScanline() const { return fCurrScanline; }

    virtual int onOutputScanline(int inputScanline) const;

private:
    const SkImageInfo           fSrcInfo;
    SkAutoTDelete<SkStream>     fStream;
    bool                        fNeedsRewind;
    sk_sp<SkColorSpace>         fColorSpace;
    const Origin                fOrigin;

    // These fields are only meaningful during scanline decodes.
    SkImageInfo                 fDstInfo;
    SkCodec::Options            fOptions;
    int                         fCurrScanline;

    /**
     *  Return whether these dimensions are supported as a scale.
     *
     *  The codec may choose to cache the information about scale and subset.
     *  Either way, the same information will be passed to onGetPixels/onStart
     *  on success.
     *
     *  This must return true for a size returned from getScaledDimensions.
     */
    bool dimensionsSupported(const SkISize& dim) {
        return dim == fSrcInfo.dimensions() || this->onDimensionsSupported(dim);
    }

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

    virtual bool onSkipScanlines(int /*countLines*/) { return false; }

    virtual int onGetScanlines(void* /*dst*/, int /*countLines*/, size_t /*rowBytes*/) { return 0; }

    /**
     * On an incomplete decode, getPixels() and getScanlines() will call this function
     * to fill any uinitialized memory.
     *
     * @param dstInfo        Contains the destination color type
     *                       Contains the destination alpha type
     *                       Contains the destination width
     *                       The height stored in this info is unused
     * @param dst            Pointer to the start of destination pixel memory
     * @param rowBytes       Stride length in destination pixel memory
     * @param zeroInit       Indicates if memory is zero initialized
     * @param linesRequested Number of lines that the client requested
     * @param linesDecoded   Number of lines that were successfully decoded
     */
    void fillIncompleteImage(const SkImageInfo& dstInfo, void* dst, size_t rowBytes,
            ZeroInitialized zeroInit, int linesRequested, int linesDecoded);

    /**
     *  Return an object which will allow forcing scanline decodes to sample in X.
     *
     *  May create a sampler, if one is not currently being used. Otherwise, does
     *  not affect ownership.
     *
     *  Only valid during scanline decoding.
     */
    virtual SkSampler* getSampler(bool /*createIfNecessary*/) { return nullptr; }

    friend class SkSampledCodec;
    friend class SkIcoCodec;
};
#endif // SkCodec_DEFINED