Skia深入分析7——区域解码

1、概述

-当图片很大时，解码速度缓慢，占用内存很高，并且，当图片超过一定尺寸时，无法做纹理上传和显示（这跟GPU能力有关，一般的GPU是8192*8192）。这时只好做下采样，但会牺牲图片显示的质量。 -对于图库等需要清晰浏览图片的应用，不可能设置一个下采样率去解决这一问题，因此，Google加入了区域解码这个功能，使我们可以从原始的图片文件中，解出一部分区域完整的图片内容。 -区域解码的难点主要在于定位像素区域所对应的文件位置，这个需要图像编码时有连续性，如果像JPEG2000标准那样做整体的小波变换就难弄了。 -目前区域解码主要实现了png、jpeg、webp类型图片的支持。本篇过一下区域解码的框架，并介绍一下最常用的jpeg格式的区域解码实现。

2、区域解码总流程

如图所示在Framework侧创建 BitmapRegionDecoder时，创建对应类型的SkImageDecoder扫描全文件，调用其onBuildTileIndex方法构建tileIndex，嵌入其关联的SkImageDecoder之中，在后续的 decodeRegion调用时，使用 SkImageDecoder的 onDecodeSubset 方法做区域解码。

3、Jpeg的区域解码

#ifdef SK_BUILD_FOR_ANDROIDbool SkJPEGImageDecoder::onBuildTileIndex(SkStreamRewindable* stream, int *width, int *height) {    SkAutoTDelete
     
       imageIndex(SkNEW_ARGS(SkJPEGImageIndex, (stream, this)));    jpeg_decompress_struct* cinfo = imageIndex->cinfo();    skjpeg_error_mgr sk_err;    set_error_mgr(cinfo, &sk_err);    // All objects need to be instantiated before this setjmp call so that    // they will be cleaned up properly if an error occurs.    if (setjmp(sk_err.fJmpBuf)) {        return false;    }    // create the cinfo used to create/build the huffmanIndex    if (!imageIndex->initializeInfoAndReadHeader()) {        return false;    }    if (!imageIndex->buildHuffmanIndex()) {        return false;    }    // destroy the cinfo used to create/build the huffman index    imageIndex->destroyInfo();    // Init decoder to image decode mode    if (!imageIndex->initializeInfoAndReadHeader()) {        return false;    }    // FIXME: This sets cinfo->out_color_space, which we may change later    // based on the config in onDecodeSubset. This should be fine, since    // jpeg_init_read_tile_scanline will check out_color_space again after    // that change (when it calls jinit_color_deconverter).    (void) this->getBitmapColorType(cinfo);    turn_off_visual_optimizations(cinfo);    // instead of jpeg_start_decompress() we start a tiled decompress    if (!imageIndex->startTileDecompress()) {        return false;    }    SkASSERT(1 == cinfo->scale_num);    fImageWidth = cinfo->output_width;    fImageHeight = cinfo->output_height;    if (width) {        *width = fImageWidth;    }    if (height) {        *height = fImageHeight;    }    SkDELETE(fImageIndex);    fImageIndex = imageIndex.detach();    return true;}bool SkJPEGImageDecoder::onDecodeSubset(SkBitmap* bm, const SkIRect& region) {    if (NULL == fImageIndex) {        return false;    }    jpeg_decompress_struct* cinfo = fImageIndex->cinfo();    SkIRect rect = SkIRect::MakeWH(fImageWidth, fImageHeight);    if (!rect.intersect(region)) {        // If the requested region is entirely outside the image return false        return false;    }    skjpeg_error_mgr errorManager;    set_error_mgr(cinfo, &errorManager);    if (setjmp(errorManager.fJmpBuf)) {        return false;    }    int requestedSampleSize = this->getSampleSize();    cinfo->scale_denom = requestedSampleSize;    set_dct_method(*this, cinfo);    const SkColorType colorType = this->getBitmapColorType(cinfo);    adjust_out_color_space_and_dither(cinfo, colorType, *this);    int startX = rect.fLeft;    int startY = rect.fTop;    int width = rect.width();    int height = rect.height();    jpeg_init_read_tile_scanline(cinfo, fImageIndex->huffmanIndex(),                                 &startX, &startY, &width, &height);    int skiaSampleSize = recompute_sampleSize(requestedSampleSize, *cinfo);    int actualSampleSize = skiaSampleSize * (DCTSIZE / cinfo->min_DCT_scaled_size);    SkScaledBitmapSampler sampler(width, height, skiaSampleSize);    SkBitmap bitmap;    // Assume an A8 bitmap is not opaque to avoid the check of each    // individual pixel. It is very unlikely to be opaque, since    // an opaque A8 bitmap would not be very interesting.    // Otherwise, a jpeg image is opaque.    bitmap.setInfo(SkImageInfo::Make(sampler.scaledWidth(), sampler.scaledHeight(), colorType,                                     kAlpha_8_SkColorType == colorType ?                                         kPremul_SkAlphaType : kOpaque_SkAlphaType));    // Check ahead of time if the swap(dest, src) is possible or not.    // If yes, then we will stick to AllocPixelRef since it's cheaper with the    // swap happening. If no, then we will use alloc to allocate pixels to    // prevent garbage collection.    int w = rect.width() / actualSampleSize;    int h = rect.height() / actualSampleSize;    bool swapOnly = (rect == region) && bm->isNull() &&                    (w == bitmap.width()) && (h == bitmap.height()) &&                    ((startX - rect.x()) / actualSampleSize == 0) &&                    ((startY - rect.y()) / actualSampleSize == 0);    if (swapOnly) {        if (!this->allocPixelRef(&bitmap, NULL)) {            return return_false(*cinfo, bitmap, "allocPixelRef");        }    } else {        if (!bitmap.allocPixels()) {            return return_false(*cinfo, bitmap, "allocPixels");        }    }    SkAutoLockPixels alp(bitmap);#ifdef ANDROID_RGB    /* short-circuit the SkScaledBitmapSampler when possible, as this gives a significant performance boost. */    if (skiaSampleSize == 1 &&        ((kN32_SkColorType == colorType && cinfo->out_color_space == JCS_RGBA_8888) ||         (kRGB_565_SkColorType == colorType && cinfo->out_color_space == JCS_RGB_565)))    {        JSAMPLE* rowptr = (JSAMPLE*)bitmap.getPixels();        INT32 const bpr = bitmap.rowBytes();        int rowTotalCount = 0;        while (rowTotalCount < height) {            int rowCount = jpeg_read_tile_scanline(cinfo,                                                   fImageIndex->huffmanIndex(),                                                   &rowptr);            // if rowCount == 0, then we didn't get a scanline, so abort.            // onDecodeSubset() relies on onBuildTileIndex(), which            // needs a complete image to succeed.            if (0 == rowCount) {                return return_false(*cinfo, bitmap, "read_scanlines");            }            if (this->shouldCancelDecode()) {                return return_false(*cinfo, bitmap, "shouldCancelDecode");            }            rowTotalCount += rowCount;            rowptr += bpr;        }        if (swapOnly) {            bm->swap(bitmap);        } else {            cropBitmap(bm, &bitmap, actualSampleSize, region.x(), region.y(),                       region.width(), region.height(), startX, startY);        }        return true;    }#endif    // check for supported formats    SkScaledBitmapSampler::SrcConfig sc;    int srcBytesPerPixel;    if (!get_src_config(*cinfo, &sc, &srcBytesPerPixel)) {        return return_false(*cinfo, *bm, "jpeg colorspace");    }    if (!sampler.begin(&bitmap, sc, *this)) {        return return_false(*cinfo, bitmap, "sampler.begin");    }    SkAutoMalloc  srcStorage(width * srcBytesPerPixel);    uint8_t* srcRow = (uint8_t*)srcStorage.get();    // Possibly skip initial rows [sampler.srcY0]    if (!skip_src_rows_tile(cinfo, fImageIndex->huffmanIndex(), srcRow, sampler.srcY0())) {        return return_false(*cinfo, bitmap, "skip rows");    }    // now loop through scanlines until y == bitmap->height() - 1    for (int y = 0;; y++) {        JSAMPLE* rowptr = (JSAMPLE*)srcRow;        int row_count = jpeg_read_tile_scanline(cinfo, fImageIndex->huffmanIndex(), &rowptr);        // if row_count == 0, then we didn't get a scanline, so abort.        // onDecodeSubset() relies on onBuildTileIndex(), which        // needs a complete image to succeed.        if (0 == row_count) {            return return_false(*cinfo, bitmap, "read_scanlines");        }        if (this->shouldCancelDecode()) {            return return_false(*cinfo, bitmap, "shouldCancelDecode");        }        if (JCS_CMYK == cinfo->out_color_space) {            convert_CMYK_to_RGB(srcRow, width);        }        sampler.next(srcRow);        if (bitmap.height() - 1 == y) {            // we're done            break;        }        if (!skip_src_rows_tile(cinfo, fImageIndex->huffmanIndex(), srcRow,                                sampler.srcDY() - 1)) {            return return_false(*cinfo, bitmap, "skip rows");        }    }    if (swapOnly) {        bm->swap(bitmap);    } else {        cropBitmap(bm, &bitmap, actualSampleSize, region.x(), region.y(),                   region.width(), region.height(), startX, startY);    }    return true;}#endif
     

在 onBuildTileIndex 时，创建了huffman_index，其中的内容主要是一系列的huffman_offset，记录每个block对应的偏移量。在解码时，先移到对应block的位置，然后解出像素。

GLOBAL(JDIMENSION)jpeg_read_tile_scanline (j_decompress_ptr cinfo, huffman_index *index,        JSAMPARRAY scanlines){  // Calculates the boundary of iMCU  int lines_per_iMCU_row = cinfo->max_v_samp_factor * DCTSIZE;  int lines_per_iMCU_col = cinfo->max_h_samp_factor * DCTSIZE;  int sample_size = DCTSIZE / cinfo->min_DCT_scaled_size;  JDIMENSION row_ctr = 0;  if (cinfo->progressive_mode) {    (*cinfo->main->process_data) (cinfo, scanlines, &row_ctr, 1);  } else {    if (cinfo->output_scanline % (lines_per_iMCU_row / sample_size) == 0) {      // Set the read head to the next iMCU row      int iMCU_row_offset = cinfo->output_scanline /            (lines_per_iMCU_row / sample_size);      int offset_data_col_position = cinfo->coef->MCU_column_left_boundary /            index->MCU_sample_size;      huffman_offset_data offset_data =          index->scan[0].offset[iMCU_row_offset][offset_data_col_position];      (*cinfo->entropy->configure_huffman_decoder) (cinfo, offset_data);    }    (*cinfo->main->process_data) (cinfo, scanlines, &row_ctr, 1);  }  cinfo->output_scanline += row_ctr;  return row_ctr;}