Revert of Delete SkImageDecoder

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index c3df5d10a874e5243b94d84c16b2e6f375aeb910..cd8152a36bbf808387bca3c7399f29424de2dad4 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -5,12 +5,14 @@
  * found in the LICENSE file.
  */
 
+#include "SkImageDecoder.h"
 #include "SkImageEncoder.h"
 #include "SkColor.h"
 #include "SkColorPriv.h"
 #include "SkDither.h"
 #include "SkMath.h"
 #include "SkRTConf.h"
+#include "SkScaledBitmapSampler.h"
 #include "SkStream.h"
 #include "SkTemplates.h"
 #include "SkUtils.h"
@@ -42,10 +44,88 @@
                 "Suppress most PNG warnings when calling image decode "
                 "functions.");
 
-///////////////////////////////////////////////////////////////////////////////
-
-#include "SkColorPriv.h"
-#include "SkUnPreMultiply.h"
+class SkPNGImageIndex {
+public:
+    // Takes ownership of stream.
+    SkPNGImageIndex(SkStreamRewindable* stream, png_structp png_ptr, png_infop info_ptr)
+        : fStream(stream)
+        , fPng_ptr(png_ptr)
+        , fInfo_ptr(info_ptr)
+        , fColorType(kUnknown_SkColorType) {
+        SkASSERT(stream != nullptr);
+    }
+    ~SkPNGImageIndex() {
+        if (fPng_ptr) {
+            png_destroy_read_struct(&fPng_ptr, &fInfo_ptr, png_infopp_NULL);
+        }
+    }
+
+    SkAutoTDelete<SkStreamRewindable>   fStream;
+    png_structp                         fPng_ptr;
+    png_infop                           fInfo_ptr;
+    SkColorType                         fColorType;
+};
+
+class SkPNGImageDecoder : public SkImageDecoder {
+public:
+    SkPNGImageDecoder() {
+        fImageIndex = nullptr;
+    }
+    Format getFormat() const override {
+        return kPNG_Format;
+    }
+
+    virtual ~SkPNGImageDecoder() { delete fImageIndex; }
+
+protected:
+    Result onDecode(SkStream* stream, SkBitmap* bm, Mode) override;
+
+private:
+    SkPNGImageIndex* fImageIndex;
+
+    bool onDecodeInit(SkStream* stream, png_structp *png_ptrp, png_infop *info_ptrp);
+    bool decodePalette(png_structp png_ptr, png_infop info_ptr, int bitDepth,
+                       bool * SK_RESTRICT hasAlphap, bool *reallyHasAlphap,
+                       SkColorTable **colorTablep);
+    bool getBitmapColorType(png_structp, png_infop, SkColorType*, bool* hasAlpha,
+                            SkPMColor* theTranspColor);
+
+    typedef SkImageDecoder INHERITED;
+};
+
+#ifndef png_jmpbuf
+#  define png_jmpbuf(png_ptr) ((png_ptr)->jmpbuf)
+#endif
+
+#define PNG_BYTES_TO_CHECK 4
+
+/* Automatically clean up after throwing an exception */
+struct PNGAutoClean {
+    PNGAutoClean(png_structp p, png_infop i): png_ptr(p), info_ptr(i) {}
+    ~PNGAutoClean() {
+        png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
+    }
+private:
+    png_structp png_ptr;
+    png_infop info_ptr;
+};
+
+static void sk_read_fn(png_structp png_ptr, png_bytep data, png_size_t length) {
+    SkStream* sk_stream = (SkStream*) png_get_io_ptr(png_ptr);
+    size_t bytes = sk_stream->read(data, length);
+    if (bytes != length) {
+        png_error(png_ptr, "Read Error!");
+    }
+}
+
+#ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
+static int sk_read_user_chunk(png_structp png_ptr, png_unknown_chunkp chunk) {
+    SkPngChunkReader* peeker = (SkPngChunkReader*)png_get_user_chunk_ptr(png_ptr);
+    // readChunk() returning true means continue decoding
+    return peeker->readChunk((const char*)chunk->name, chunk->data, chunk->size) ?
+            1 : -1;
+}
+#endif
 
 static void sk_error_fn(png_structp png_ptr, png_const_charp msg) {
     if (!c_suppressPNGImageDecoderWarnings) {
@@ -53,6 +133,577 @@
     }
     longjmp(png_jmpbuf(png_ptr), 1);
 }
+
+static void skip_src_rows(png_structp png_ptr, uint8_t storage[], int count) {
+    for (int i = 0; i < count; i++) {
+        uint8_t* tmp = storage;
+        png_read_rows(png_ptr, &tmp, png_bytepp_NULL, 1);
+    }
+}
+
+static bool pos_le(int value, int max) {
+    return value > 0 && value <= max;
+}
+
+static bool substituteTranspColor(SkBitmap* bm, SkPMColor match) {
+    SkASSERT(bm->colorType() == kN32_SkColorType);
+
+    bool reallyHasAlpha = false;
+
+    for (int y = bm->height() - 1; y >= 0; --y) {
+        SkPMColor* p = bm->getAddr32(0, y);
+        for (int x = bm->width() - 1; x >= 0; --x) {
+            if (match == *p) {
+                *p = 0;
+                reallyHasAlpha = true;
+            }
+            p += 1;
+        }
+    }
+    return reallyHasAlpha;
+}
+
+static bool canUpscalePaletteToConfig(SkColorType dstColorType, bool srcHasAlpha) {
+    switch (dstColorType) {
+        case kN32_SkColorType:
+        case kARGB_4444_SkColorType:
+            return true;
+        case kRGB_565_SkColorType:
+            // only return true if the src is opaque (since 565 is opaque)
+            return !srcHasAlpha;
+        default:
+            return false;
+    }
+}
+
+// call only if color_type is PALETTE. Returns true if the ctable has alpha
+static bool hasTransparencyInPalette(png_structp png_ptr, png_infop info_ptr) {
+    png_bytep trans;
+    int num_trans;
+
+    if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
+        png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans, nullptr);
+        return num_trans > 0;
+    }
+    return false;
+}
+
+void do_nothing_warning_fn(png_structp, png_const_charp) {
+    /* do nothing */
+}
+
+bool SkPNGImageDecoder::onDecodeInit(SkStream* sk_stream, png_structp *png_ptrp,
+                                     png_infop *info_ptrp) {
+    /* Create and initialize the png_struct with the desired error handler
+    * functions.  If you want to use the default stderr and longjump method,
+    * you can supply nullptr for the last three parameters.  We also supply the
+    * the compiler header file version, so that we know if the application
+    * was compiled with a compatible version of the library.  */
+
+    png_error_ptr user_warning_fn =
+        (c_suppressPNGImageDecoderWarnings) ? (&do_nothing_warning_fn) : nullptr;
+    /* nullptr means to leave as default library behavior. */
+    /* c_suppressPNGImageDecoderWarnings default depends on SK_DEBUG. */
+    /* To suppress warnings with a SK_DEBUG binary, set the
+     * environment variable "skia_images_png_suppressDecoderWarnings"
+     * to "true".  Inside a program that links to skia:
+     * SK_CONF_SET("images.png.suppressDecoderWarnings", true); */
+
+    png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
+        nullptr, sk_error_fn, user_warning_fn);
+    //   png_voidp user_error_ptr, user_error_fn, user_warning_fn);
+    if (png_ptr == nullptr) {
+        return false;
+    }
+
+    *png_ptrp = png_ptr;
+
+    /* Allocate/initialize the memory for image information. */
+    png_infop info_ptr = png_create_info_struct(png_ptr);
+    if (info_ptr == nullptr) {
+        png_destroy_read_struct(&png_ptr, png_infopp_NULL, png_infopp_NULL);
+        return false;
+    }
+    *info_ptrp = info_ptr;
+
+    /* Set error handling if you are using the setjmp/longjmp method (this is
+    * the normal method of doing things with libpng).  REQUIRED unless you
+    * set up your own error handlers in the png_create_read_struct() earlier.
+    */
+    if (setjmp(png_jmpbuf(png_ptr))) {
+        png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
+        return false;
+    }
+
+    /* If you are using replacement read functions, instead of calling
+    * png_init_io() here you would call:
+    */
+    png_set_read_fn(png_ptr, (void *)sk_stream, sk_read_fn);
+    /* where user_io_ptr is a structure you want available to the callbacks */
+    /* If we have already read some of the signature */
+//  png_set_sig_bytes(png_ptr, 0 /* sig_read */ );
+
+#ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
+    // hookup our peeker so we can see any user-chunks the caller may be interested in
+    png_set_keep_unknown_chunks(png_ptr, PNG_HANDLE_CHUNK_ALWAYS, (png_byte*)"", 0);
+    if (this->getPeeker()) {
+        png_set_read_user_chunk_fn(png_ptr, (png_voidp)this->getPeeker(), sk_read_user_chunk);
+    }
+#endif
+    /* The call to png_read_info() gives us all of the information from the
+    * PNG file before the first IDAT (image data chunk). */
+    png_read_info(png_ptr, info_ptr);
+    png_uint_32 origWidth, origHeight;
+    int bitDepth, colorType;
+    png_get_IHDR(png_ptr, info_ptr, &origWidth, &origHeight, &bitDepth,
+                 &colorType, int_p_NULL, int_p_NULL, int_p_NULL);
+
+    /* tell libpng to strip 16 bit/color files down to 8 bits/color */
+    if (bitDepth == 16) {
+        png_set_strip_16(png_ptr);
+    }
+#ifdef PNG_READ_PACK_SUPPORTED
+    /* Extract multiple pixels with bit depths of 1, 2, and 4 from a single
+     * byte into separate bytes (useful for paletted and grayscale images). */
+    if (bitDepth < 8) {
+        png_set_packing(png_ptr);
+    }
+#endif
+    /* Expand grayscale images to the full 8 bits from 1, 2, or 4 bits/pixel */
+    if (colorType == PNG_COLOR_TYPE_GRAY && bitDepth < 8) {
+        png_set_expand_gray_1_2_4_to_8(png_ptr);
+    }
+
+    return true;
+}
+
+SkImageDecoder::Result SkPNGImageDecoder::onDecode(SkStream* sk_stream, SkBitmap* decodedBitmap,
+                                                   Mode mode) {
+    png_structp png_ptr;
+    png_infop info_ptr;
+
+    if (!onDecodeInit(sk_stream, &png_ptr, &info_ptr)) {
+        return kFailure;
+    }
+
+    PNGAutoClean autoClean(png_ptr, info_ptr);
+
+    if (setjmp(png_jmpbuf(png_ptr))) {
+        return kFailure;
+    }
+
+    png_uint_32 origWidth, origHeight;
+    int bitDepth, pngColorType, interlaceType;
+    png_get_IHDR(png_ptr, info_ptr, &origWidth, &origHeight, &bitDepth,
+                 &pngColorType, &interlaceType, int_p_NULL, int_p_NULL);
+
+    SkColorType         colorType;
+    bool                hasAlpha = false;
+    SkPMColor           theTranspColor = 0; // 0 tells us not to try to match
+
+    if (!this->getBitmapColorType(png_ptr, info_ptr, &colorType, &hasAlpha, &theTranspColor)) {
+        return kFailure;
+    }
+
+    SkAlphaType alphaType = this->getRequireUnpremultipliedColors() ?
+                                kUnpremul_SkAlphaType : kPremul_SkAlphaType;
+    const int sampleSize = this->getSampleSize();
+    SkScaledBitmapSampler sampler(origWidth, origHeight, sampleSize);
+    decodedBitmap->setInfo(SkImageInfo::Make(sampler.scaledWidth(), sampler.scaledHeight(),
+                                             colorType, alphaType));
+
+    if (SkImageDecoder::kDecodeBounds_Mode == mode) {
+        return kSuccess;
+    }
+
+    // from here down we are concerned with colortables and pixels
+
+    // we track if we actually see a non-opaque pixels, since sometimes a PNG sets its colortype
+    // to |= PNG_COLOR_MASK_ALPHA, but all of its pixels are in fact opaque. We care, since we
+    // draw lots faster if we can flag the bitmap has being opaque
+    bool reallyHasAlpha = false;
+    SkColorTable* colorTable = nullptr;
+
+    if (pngColorType == PNG_COLOR_TYPE_PALETTE) {
+        decodePalette(png_ptr, info_ptr, bitDepth, &hasAlpha, &reallyHasAlpha, &colorTable);
+    }
+
+    SkAutoUnref aur(colorTable);
+
+    if (!this->allocPixelRef(decodedBitmap,
+                             kIndex_8_SkColorType == colorType ? colorTable : nullptr)) {
+        return kFailure;
+    }
+
+    SkAutoLockPixels alp(*decodedBitmap);
+
+    // Repeat setjmp, otherwise variables declared since the last call (e.g. alp
+    // and aur) won't get their destructors called in case of a failure.
+    if (setjmp(png_jmpbuf(png_ptr))) {
+        return kFailure;
+    }
+
+    /* Turn on interlace handling.  REQUIRED if you are not using
+    *  png_read_image().  To see how to handle interlacing passes,
+    *  see the png_read_row() method below:
+    */
+    const int number_passes = (interlaceType != PNG_INTERLACE_NONE) ?
+                              png_set_interlace_handling(png_ptr) : 1;
+
+    /* Optional call to gamma correct and add the background to the palette
+    *  and update info structure.  REQUIRED if you are expecting libpng to
+    *  update the palette for you (ie you selected such a transform above).
+    */
+    png_read_update_info(png_ptr, info_ptr);
+
+    if ((kAlpha_8_SkColorType == colorType || kIndex_8_SkColorType == colorType) &&
+            1 == sampleSize) {
+        if (kAlpha_8_SkColorType == colorType) {
+            // For an A8 bitmap, we assume there is an alpha for speed. It is
+            // possible the bitmap is opaque, but that is an unlikely use case
+            // since it would not be very interesting.
+            reallyHasAlpha = true;
+            // A8 is only allowed if the original was GRAY.
+            SkASSERT(PNG_COLOR_TYPE_GRAY == pngColorType);
+        }
+        for (int i = 0; i < number_passes; i++) {
+            for (png_uint_32 y = 0; y < origHeight; y++) {
+                uint8_t* bmRow = decodedBitmap->getAddr8(0, y);
+                png_read_rows(png_ptr, &bmRow, png_bytepp_NULL, 1);
+            }
+        }
+    } else {
+        SkScaledBitmapSampler::SrcConfig sc;
+        int srcBytesPerPixel = 4;
+
+        if (colorTable != nullptr) {
+            sc = SkScaledBitmapSampler::kIndex;
+            srcBytesPerPixel = 1;
+        } else if (kAlpha_8_SkColorType == colorType) {
+            // A8 is only allowed if the original was GRAY.
+            SkASSERT(PNG_COLOR_TYPE_GRAY == pngColorType);
+            sc = SkScaledBitmapSampler::kGray;
+            srcBytesPerPixel = 1;
+        } else if (hasAlpha) {
+            sc = SkScaledBitmapSampler::kRGBA;
+        } else {
+            sc = SkScaledBitmapSampler::kRGBX;
+        }
+
+        /*  We have to pass the colortable explicitly, since we may have one
+            even if our decodedBitmap doesn't, due to the request that we
+            upscale png's palette to a direct model
+         */
+        const SkPMColor* colors = colorTable ? colorTable->readColors() : nullptr;
+        if (!sampler.begin(decodedBitmap, sc, *this, colors)) {
+            return kFailure;
+        }
+        const int height = decodedBitmap->height();
+
+        if (number_passes > 1) {
+            SkAutoTMalloc<uint8_t> storage(origWidth * origHeight * srcBytesPerPixel);
+            uint8_t* base = storage.get();
+            size_t rowBytes = origWidth * srcBytesPerPixel;
+
+            for (int i = 0; i < number_passes; i++) {
+                uint8_t* row = base;
+                for (png_uint_32 y = 0; y < origHeight; y++) {
+                    uint8_t* bmRow = row;
+                    png_read_rows(png_ptr, &bmRow, png_bytepp_NULL, 1);
+                    row += rowBytes;
+                }
+            }
+            // now sample it
+            base += sampler.srcY0() * rowBytes;
+            for (int y = 0; y < height; y++) {
+                reallyHasAlpha |= sampler.next(base);
+                base += sampler.srcDY() * rowBytes;
+            }
+        } else {
+            SkAutoTMalloc<uint8_t> storage(origWidth * srcBytesPerPixel);
+            uint8_t* srcRow = storage.get();
+            skip_src_rows(png_ptr, srcRow, sampler.srcY0());
+
+            for (int y = 0; y < height; y++) {
+                uint8_t* tmp = srcRow;
+                png_read_rows(png_ptr, &tmp, png_bytepp_NULL, 1);
+                reallyHasAlpha |= sampler.next(srcRow);
+                if (y < height - 1) {
+                    skip_src_rows(png_ptr, srcRow, sampler.srcDY() - 1);
+                }
+            }
+
+            // skip the rest of the rows (if any)
+            png_uint_32 read = (height - 1) * sampler.srcDY() +
+                               sampler.srcY0() + 1;
+            SkASSERT(read <= origHeight);
+            skip_src_rows(png_ptr, srcRow, origHeight - read);
+        }
+    }
+
+    /* read rest of file, and get additional chunks in info_ptr - REQUIRED */
+    png_read_end(png_ptr, info_ptr);
+
+    if (0 != theTranspColor) {
+        reallyHasAlpha |= substituteTranspColor(decodedBitmap, theTranspColor);
+    }
+    if (reallyHasAlpha && this->getRequireUnpremultipliedColors()) {
+        switch (decodedBitmap->colorType()) {
+            case kIndex_8_SkColorType:
+                // Fall through.
+            case kARGB_4444_SkColorType:
+                // We have chosen not to support unpremul for these colortypes.
+                return kFailure;
+            default: {
+                // Fall through to finish the decode. This colortype either
+                // supports unpremul or it is irrelevant because it has no
+                // alpha (or only alpha).
+                // These brackets prevent a warning.
+            }
+        }
+    }
+
+    if (!reallyHasAlpha) {
+        decodedBitmap->setAlphaType(kOpaque_SkAlphaType);
+    }
+    return kSuccess;
+}
+
+
+
+bool SkPNGImageDecoder::getBitmapColorType(png_structp png_ptr, png_infop info_ptr,
+                                           SkColorType* colorTypep,
+                                           bool* hasAlphap,
+                                           SkPMColor* SK_RESTRICT theTranspColorp) {
+    png_uint_32 origWidth, origHeight;
+    int bitDepth, colorType;
+    png_get_IHDR(png_ptr, info_ptr, &origWidth, &origHeight, &bitDepth,
+                 &colorType, int_p_NULL, int_p_NULL, int_p_NULL);
+
+#ifdef PNG_sBIT_SUPPORTED
+    // check for sBIT chunk data, in case we should disable dithering because
+    // our data is not truely 8bits per component
+    png_color_8p sig_bit;
+    if (this->getDitherImage() && png_get_sBIT(png_ptr, info_ptr, &sig_bit)) {
+#if 0
+        SkDebugf("----- sBIT %d %d %d %d\n", sig_bit->red, sig_bit->green,
+                 sig_bit->blue, sig_bit->alpha);
+#endif
+        // 0 seems to indicate no information available
+        if (pos_le(sig_bit->red, SK_R16_BITS) &&
+            pos_le(sig_bit->green, SK_G16_BITS) &&
+            pos_le(sig_bit->blue, SK_B16_BITS)) {
+            this->setDitherImage(false);
+        }
+    }
+#endif
+
+    if (colorType == PNG_COLOR_TYPE_PALETTE) {
+        bool paletteHasAlpha = hasTransparencyInPalette(png_ptr, info_ptr);
+        *colorTypep = this->getPrefColorType(kIndex_SrcDepth, paletteHasAlpha);
+        // now see if we can upscale to their requested colortype
+        if (!canUpscalePaletteToConfig(*colorTypep, paletteHasAlpha)) {
+            *colorTypep = kIndex_8_SkColorType;
+        }
+    } else {
+        png_color_16p transpColor = nullptr;
+        int numTransp = 0;
+
+        png_get_tRNS(png_ptr, info_ptr, nullptr, &numTransp, &transpColor);
+
+        bool valid = png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS);
+
+        if (valid && numTransp == 1 && transpColor != nullptr) {
+            /*  Compute our transparent color, which we'll match against later.
+                We don't really handle 16bit components properly here, since we
+                do our compare *after* the values have been knocked down to 8bit
+                which means we will find more matches than we should. The real
+                fix seems to be to see the actual 16bit components, do the
+                compare, and then knock it down to 8bits ourselves.
+            */
+            if (colorType & PNG_COLOR_MASK_COLOR) {
+                if (16 == bitDepth) {
+                    *theTranspColorp = SkPackARGB32(0xFF, transpColor->red >> 8,
+                                                    transpColor->green >> 8,
+                                                    transpColor->blue >> 8);
+                } else {
+                    /* We apply the mask because in a very small
+                       number of corrupt PNGs, (transpColor->red > 255)
+                       and (bitDepth == 8), for certain versions of libpng. */
+                    *theTranspColorp = SkPackARGB32(0xFF,
+                                                    0xFF & (transpColor->red),
+                                                    0xFF & (transpColor->green),
+                                                    0xFF & (transpColor->blue));
+                }
+            } else {    // gray
+                if (16 == bitDepth) {
+                    *theTranspColorp = SkPackARGB32(0xFF, transpColor->gray >> 8,
+                                                    transpColor->gray >> 8,
+                                                    transpColor->gray >> 8);
+                } else {
+                    /* We apply the mask because in a very small
+                       number of corrupt PNGs, (transpColor->red >
+                       255) and (bitDepth == 8), for certain versions
+                       of libpng.  For safety we assume the same could
+                       happen with a grayscale PNG.  */
+                    *theTranspColorp = SkPackARGB32(0xFF,
+                                                    0xFF & (transpColor->gray),
+                                                    0xFF & (transpColor->gray),
+                                                    0xFF & (transpColor->gray));
+                }
+            }
+        }
+
+        if (valid ||
+            PNG_COLOR_TYPE_RGB_ALPHA == colorType ||
+            PNG_COLOR_TYPE_GRAY_ALPHA == colorType) {
+            *hasAlphap = true;
+        }
+
+        SrcDepth srcDepth = k32Bit_SrcDepth;
+        if (PNG_COLOR_TYPE_GRAY == colorType) {
+            srcDepth = k8BitGray_SrcDepth;
+            // Remove this assert, which fails on desk_pokemonwiki.skp
+            //SkASSERT(!*hasAlphap);
+        }
+
+        *colorTypep = this->getPrefColorType(srcDepth, *hasAlphap);
+        // now match the request against our capabilities
+        if (*hasAlphap) {
+            if (*colorTypep != kARGB_4444_SkColorType) {
+                *colorTypep = kN32_SkColorType;
+            }
+        } else {
+            if (kAlpha_8_SkColorType == *colorTypep) {
+                if (k8BitGray_SrcDepth != srcDepth) {
+                    // Converting a non grayscale image to A8 is not currently supported.
+                    *colorTypep = kN32_SkColorType;
+                }
+            } else if (*colorTypep != kRGB_565_SkColorType &&
+                       *colorTypep != kARGB_4444_SkColorType) {
+                *colorTypep = kN32_SkColorType;
+            }
+        }
+    }
+
+    // sanity check for size
+    {
+        int64_t size = sk_64_mul(origWidth, origHeight);
+        // now check that if we are 4-bytes per pixel, we also don't overflow
+        if (size < 0 || size > (0x7FFFFFFF >> 2)) {
+            return false;
+        }
+    }
+
+    // If the image has alpha and the decoder wants unpremultiplied
+    // colors, the only supported colortype is 8888.
+    if (this->getRequireUnpremultipliedColors() && *hasAlphap) {
+        *colorTypep = kN32_SkColorType;
+    }
+
+    if (fImageIndex != nullptr) {
+        if (kUnknown_SkColorType == fImageIndex->fColorType) {
+            // This is the first time for this subset decode. From now on,
+            // all decodes must be in the same colortype.
+            fImageIndex->fColorType = *colorTypep;
+        } else if (fImageIndex->fColorType != *colorTypep) {
+            // Requesting a different colortype for a subsequent decode is not
+            // supported. Report failure before we make changes to png_ptr.
+            return false;
+        }
+    }
+
+    bool convertGrayToRGB = PNG_COLOR_TYPE_GRAY == colorType && *colorTypep != kAlpha_8_SkColorType;
+
+    // Unless the user is requesting A8, convert a grayscale image into RGB.
+    // GRAY_ALPHA will always be converted to RGB
+    if (convertGrayToRGB || colorType == PNG_COLOR_TYPE_GRAY_ALPHA) {
+        png_set_gray_to_rgb(png_ptr);
+    }
+
+    // Add filler (or alpha) byte (after each RGB triplet) if necessary.
+    if (colorType == PNG_COLOR_TYPE_RGB || convertGrayToRGB) {
+        png_set_filler(png_ptr, 0xff, PNG_FILLER_AFTER);
+    }
+
+    return true;
+}
+
+typedef uint32_t (*PackColorProc)(U8CPU a, U8CPU r, U8CPU g, U8CPU b);
+
+bool SkPNGImageDecoder::decodePalette(png_structp png_ptr, png_infop info_ptr,
+                                      int bitDepth, bool *hasAlphap,
+                                      bool *reallyHasAlphap,
+                                      SkColorTable **colorTablep) {
+    int numPalette;
+    png_colorp palette;
+    png_bytep trans;
+    int numTrans;
+
+    png_get_PLTE(png_ptr, info_ptr, &palette, &numPalette);
+
+    SkPMColor colorStorage[256];    // worst-case storage
+    SkPMColor* colorPtr = colorStorage;
+
+    if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
+        png_get_tRNS(png_ptr, info_ptr, &trans, &numTrans, nullptr);
+        *hasAlphap = (numTrans > 0);
+    } else {
+        numTrans = 0;
+    }
+
+    // check for bad images that might make us crash
+    if (numTrans > numPalette) {
+        numTrans = numPalette;
+    }
+
+    int index = 0;
+    int transLessThanFF = 0;
+
+    // Choose which function to use to create the color table. If the final destination's
+    // colortype is unpremultiplied, the color table will store unpremultiplied colors.
+    PackColorProc proc;
+    if (this->getRequireUnpremultipliedColors()) {
+        proc = &SkPackARGB32NoCheck;
+    } else {
+        proc = &SkPreMultiplyARGB;
+    }
+    for (; index < numTrans; index++) {
+        transLessThanFF |= (int)*trans - 0xFF;
+        *colorPtr++ = proc(*trans++, palette->red, palette->green, palette->blue);
+        palette++;
+    }
+    bool reallyHasAlpha = (transLessThanFF < 0);
+
+    for (; index < numPalette; index++) {
+        *colorPtr++ = SkPackARGB32(0xFF, palette->red, palette->green, palette->blue);
+        palette++;
+    }
+
+    /*  BUGGY IMAGE WORKAROUND
+
+        Invalid images could contain pixel values that are greater than the number of palette
+        entries. Since we use pixel values as indices into the palette this could result in reading
+        beyond the end of the palette which could leak the contents of uninitialized memory. To
+        ensure this doesn't happen, we grow the colortable to the maximum size that can be
+        addressed by the bitdepth of the image and fill it with the last palette color or black if
+        the palette is empty (really broken image).
+    */
+    int colorCount = SkTMax(numPalette, 1 << SkTMin(bitDepth, 8));
+    SkPMColor lastColor = index > 0 ? colorPtr[-1] : SkPackARGB32(0xFF, 0, 0, 0);
+    for (; index < colorCount; index++) {
+        *colorPtr++ = lastColor;
+    }
+
+    *colorTablep = new SkColorTable(colorStorage, colorCount);
+    *reallyHasAlphap = reallyHasAlpha;
+    return true;
+}
+
+///////////////////////////////////////////////////////////////////////////////
+
+#include "SkColorPriv.h"
+#include "SkUnPreMultiply.h"
 
 static void sk_write_fn(png_structp png_ptr, png_bytep data, png_size_t len) {
     SkWStream* sk_stream = (SkWStream*)png_get_io_ptr(png_ptr);
@@ -334,11 +985,37 @@
 }
 
 ///////////////////////////////////////////////////////////////////////////////
+DEFINE_DECODER_CREATOR(PNGImageDecoder);
 DEFINE_ENCODER_CREATOR(PNGImageEncoder);
 ///////////////////////////////////////////////////////////////////////////////
 
+static bool is_png(SkStreamRewindable* stream) {
+    char buf[PNG_BYTES_TO_CHECK];
+    if (stream->read(buf, PNG_BYTES_TO_CHECK) == PNG_BYTES_TO_CHECK &&
+        !png_sig_cmp((png_bytep) buf, (png_size_t)0, PNG_BYTES_TO_CHECK)) {
+        return true;
+    }
+    return false;
+}
+
+SkImageDecoder* sk_libpng_dfactory(SkStreamRewindable* stream) {
+    if (is_png(stream)) {
+        return new SkPNGImageDecoder;
+    }
+    return nullptr;
+}
+
+static SkImageDecoder::Format get_format_png(SkStreamRewindable* stream) {
+    if (is_png(stream)) {
+        return SkImageDecoder::kPNG_Format;
+    }
+    return SkImageDecoder::kUnknown_Format;
+}
+
 SkImageEncoder* sk_libpng_efactory(SkImageEncoder::Type t) {
     return (SkImageEncoder::kPNG_Type == t) ? new SkPNGImageEncoder : nullptr;
 }
 
+static SkImageDecoder_DecodeReg gDReg(sk_libpng_dfactory);
+static SkImageDecoder_FormatReg gFormatReg(get_format_png);
 static SkImageEncoder_EncodeReg gEReg(sk_libpng_efactory);