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      1 // Copyright 2011 Google Inc. All Rights Reserved.
      2 //
      3 // Use of this source code is governed by a BSD-style license
      4 // that can be found in the COPYING file in the root of the source
      5 // tree. An additional intellectual property rights grant can be found
      6 // in the file PATENTS. All contributing project authors may
      7 // be found in the AUTHORS file in the root of the source tree.
      8 // -----------------------------------------------------------------------------
      9 //
     10 // Alpha-plane decompression.
     11 //
     12 // Author: Skal (pascal.massimino (at) gmail.com)
     13 
     14 #include <stdlib.h>
     15 #include "./vp8i.h"
     16 #include "./vp8li.h"
     17 #include "../utils/filters.h"
     18 #include "../utils/quant_levels_dec.h"
     19 #include "webp/format_constants.h"
     20 
     21 #if defined(__cplusplus) || defined(c_plusplus)
     22 extern "C" {
     23 #endif
     24 
     25 //------------------------------------------------------------------------------
     26 // Decodes the compressed data 'data' of size 'data_size' into the 'output'.
     27 // The 'output' buffer should be pre-allocated and must be of the same
     28 // dimension 'height'x'width', as that of the image.
     29 //
     30 // Returns 1 on successfully decoding the compressed alpha and
     31 //         0 if either:
     32 //           error in bit-stream header (invalid compression mode or filter), or
     33 //           error returned by appropriate compression method.
     34 
     35 static int DecodeAlpha(const uint8_t* data, size_t data_size,
     36                        int width, int height, uint8_t* output) {
     37   WEBP_FILTER_TYPE filter;
     38   int pre_processing;
     39   int rsrv;
     40   int ok = 0;
     41   int method;
     42   const uint8_t* const alpha_data = data + ALPHA_HEADER_LEN;
     43   const size_t alpha_data_size = data_size - ALPHA_HEADER_LEN;
     44 
     45   assert(width > 0 && height > 0);
     46   assert(data != NULL && output != NULL);
     47 
     48   if (data_size <= ALPHA_HEADER_LEN) {
     49     return 0;
     50   }
     51 
     52   method = (data[0] >> 0) & 0x03;
     53   filter = (data[0] >> 2) & 0x03;
     54   pre_processing = (data[0] >> 4) & 0x03;
     55   rsrv = (data[0] >> 6) & 0x03;
     56   if (method < ALPHA_NO_COMPRESSION ||
     57       method > ALPHA_LOSSLESS_COMPRESSION ||
     58       filter >= WEBP_FILTER_LAST ||
     59       pre_processing > ALPHA_PREPROCESSED_LEVELS ||
     60       rsrv != 0) {
     61     return 0;
     62   }
     63 
     64   if (method == ALPHA_NO_COMPRESSION) {
     65     const size_t alpha_decoded_size = height * width;
     66     ok = (alpha_data_size >= alpha_decoded_size);
     67     if (ok) memcpy(output, alpha_data, alpha_decoded_size);
     68   } else {
     69     ok = VP8LDecodeAlphaImageStream(width, height, alpha_data, alpha_data_size,
     70                                     output);
     71   }
     72 
     73   if (ok) {
     74     WebPUnfilterFunc unfilter_func = WebPUnfilters[filter];
     75     if (unfilter_func != NULL) {
     76       // TODO(vikas): Implement on-the-fly decoding & filter mechanism to decode
     77       // and apply filter per image-row.
     78       unfilter_func(width, height, width, output);
     79     }
     80     if (pre_processing == ALPHA_PREPROCESSED_LEVELS) {
     81       ok = DequantizeLevels(output, width, height);
     82     }
     83   }
     84 
     85   return ok;
     86 }
     87 
     88 //------------------------------------------------------------------------------
     89 
     90 const uint8_t* VP8DecompressAlphaRows(VP8Decoder* const dec,
     91                                       int row, int num_rows) {
     92   const int width = dec->pic_hdr_.width_;
     93   const int height = dec->pic_hdr_.height_;
     94 
     95   if (row < 0 || num_rows < 0 || row + num_rows > height) {
     96     return NULL;    // sanity check.
     97   }
     98 
     99   if (row == 0) {
    100     // Decode everything during the first call.
    101     assert(!dec->is_alpha_decoded_);
    102     if (!DecodeAlpha(dec->alpha_data_, (size_t)dec->alpha_data_size_,
    103                      width, height, dec->alpha_plane_)) {
    104       return NULL;  // Error.
    105     }
    106     dec->is_alpha_decoded_ = 1;
    107   }
    108 
    109   // Return a pointer to the current decoded row.
    110   return dec->alpha_plane_ + row * width;
    111 }
    112 
    113 #if defined(__cplusplus) || defined(c_plusplus)
    114 }    // extern "C"
    115 #endif
    116