use header-less lossless bitstream for alpha channel

This saves ~26 bytes of headers.
* introduce new VP8LDecodeAlphaImageStream() for decoding
* use VP8LEncodeStream() for encoding
* refactor code a bit

still TODO: make the alpha-quality/enc-method user-configurable

Change-Id: I23e599bebe335cfb5868e746e076c3358ef12e71

src/dec/alpha.c

@@ -11,7 +11,7 @@
 
 #include <stdlib.h>
 #include "./vp8i.h"
-#include "../webp/decode.h"
+#include "./vp8li.h"
 #include "../utils/filters.h"
 #include "../utils/quant_levels.h"
 
@@ -79,26 +79,12 @@ static int DecodeAlpha(const uint8_t* data, size_t data_size,
     ok = (data_size >= decoded_size);
     decoded_data = (uint8_t*)data + ALPHA_HEADER_LEN;
   } else {
-    size_t i;
-    int w, h;
-    uint32_t* const output =
-        (uint32_t*)WebPDecodeRGBA(data + ALPHA_HEADER_LEN,
-                                  data_size - ALPHA_HEADER_LEN,
-                                  &w, &h);
-    if (w != width || h != height || output == NULL) {
-      free(output);
-      return 0;
-    }
     decoded_data = (uint8_t*)malloc(decoded_size);
-    if (decoded_data == NULL) {
-      free(output);
-      return 0;
-    }
-    for (i = 0; i < decoded_size; ++i) {
-      decoded_data[i] = (output[i] >> 8) & 0xff;
-    }
-    free(output);
-    ok = 1;
+    if (decoded_data == NULL) return 0;
+    ok = VP8LDecodeAlphaImageStream(width, height,
+                                    data + ALPHA_HEADER_LEN,
+                                    data_size - ALPHA_HEADER_LEN,
+                                    decoded_data);
   }
 
   if (ok) {
@@ -119,10 +105,10 @@ static int DecodeAlpha(const uint8_t* data, size_t data_size,
       // Construct raw_data (height x stride) from alpha data (height x width).
       CopyPlane(decoded_data, width, output, stride, width, height);
     }
-  }
     if (pre_processing == ALPHA_PREPROCESSED_LEVELS) {
       ok = DequantizeLevels(decoded_data, width, height);
     }
+  }
 
  Error:
   if (method != ALPHA_NO_COMPRESSION) {

src/dec/vp8l.c

@@ -455,7 +455,7 @@ static int EmitRows(WEBP_CSP_MODE colorspace,
 // Note that 'pixel_stride' is in units of 'uint32_t' (and not 'bytes).
 // Returns true if the crop window is not empty.
 static int SetCropWindow(VP8Io* const io, int y_start, int y_end,
-                         uint32_t** const in_data, int pixel_stride) {
+                         const uint32_t** const in_data, int pixel_stride) {
   assert(y_start < y_end);
   assert(io->crop_left < io->crop_right);
   if (y_end > io->crop_bottom) {
@@ -497,52 +497,59 @@ static WEBP_INLINE HTreeGroup* GetHtreeGroupForPos(VP8LMetadata* const hdr,
 
 typedef void (*ProcessRowsFunc)(VP8LDecoder* const dec, int row);
 
-// Processes (transforms, scales & color-converts) the rows decoded after the
-// last call.
-static void ProcessRows(VP8LDecoder* const dec, int row) {
+static void ApplyTransforms(VP8LDecoder* const dec, int num_rows,
+                            const uint32_t* const rows) {
   int n = dec->next_transform_;
-  VP8Io* const io = dec->io_;
-  WebPDecParams* const params = (WebPDecParams*)io->opaque;
-  const WebPDecBuffer* const output = params->output;
-  const int argb_offset = dec->width_ * dec->last_row_;
-  const int num_rows = row - dec->last_row_;
   const int cache_pixs = dec->width_ * num_rows;
   uint32_t* rows_data = dec->argb_cache_;
-  int num_rows_out = num_rows;  // Default.
-
-  if (num_rows <= 0) return;  // Nothing to be done.
+  const int start_row = dec->last_row_;
+  const int end_row = start_row + num_rows;
 
   // Inverse transforms.
   // TODO: most transforms only need to operate on the cropped region only.
-  memcpy(rows_data, dec->argb_ + argb_offset, cache_pixs * sizeof(*rows_data));
+  memcpy(rows_data, rows, cache_pixs * sizeof(*rows_data));
   while (n-- > 0) {
     VP8LTransform* const transform = &dec->transforms_[n];
-    VP8LInverseTransform(transform, dec->last_row_, row,
-                         dec->argb_ + argb_offset, rows_data);
+    VP8LInverseTransform(transform, start_row, end_row, rows, rows_data);
   }
+}
+
+// Processes (transforms, scales & color-converts) the rows decoded after the
+// last call.
+static void ProcessRows(VP8LDecoder* const dec, int row) {
+  const uint32_t* const rows = dec->argb_ + dec->width_ * dec->last_row_;
+  const int num_rows = row - dec->last_row_;
+
+  if (num_rows <= 0) return;  // Nothing to be done.
+  ApplyTransforms(dec, num_rows, rows);
 
   // Emit output.
   {
+    VP8Io* const io = dec->io_;
+    const uint32_t* rows_data = dec->argb_cache_;
+    if (!SetCropWindow(io, dec->last_row_, row, &rows_data, io->width)) {
+      // Nothing to output (this time).
+    } else {
+      WebPDecParams* const params = (WebPDecParams*)io->opaque;
+      const WebPDecBuffer* const output = params->output;
       const WebPRGBABuffer* const buf = &output->u.RGBA;
       uint8_t* const rgba = buf->rgba + dec->last_out_row_ * buf->stride;
-    const WEBP_CSP_MODE colorspace = output->colorspace;
-    if (!SetCropWindow(io, dec->last_row_, row, &rows_data, io->width)) {
-      num_rows_out = 0;  // Nothing to output.
-    } else {
       const int in_stride = io->width * sizeof(*rows_data);
-      num_rows_out = io->use_scaling ?
+      const WEBP_CSP_MODE colorspace = output->colorspace;
+      const int num_rows_out = io->use_scaling ?
           EmitRescaledRows(dec, colorspace, rows_data, in_stride, io->mb_h,
                            rgba, buf->stride) :
           EmitRows(colorspace, rows_data, in_stride, io->mb_w, io->mb_h,
                    rgba, buf->stride);
+      // Update 'last_out_row_'.
+      dec->last_out_row_ += num_rows_out;
+      assert(dec->last_out_row_ <= output->height);
     }
   }
 
-  // Update 'last_row' and 'last_out_row'.
+  // Update 'last_row_'.
   dec->last_row_ = row;
-  assert(dec->last_row_ <= io->height);
-  dec->last_out_row_ += num_rows_out;
-  assert(dec->last_out_row_ <= output->height);
+  assert(dec->last_row_ <= dec->height_);
 }
 
 static int DecodeImageData(VP8LDecoder* const dec,
@@ -940,6 +947,63 @@ static int AllocateARGBBuffers(VP8LDecoder* const dec, int final_width) {
   return 1;
 }
 
+//------------------------------------------------------------------------------
+// Special row-processing that only stores the alpha data.
+
+static void ExtractAlphaRows(VP8LDecoder* const dec, int row) {
+  const int num_rows = row - dec->last_row_;
+  const int cache_pixs = dec->width_ * num_rows;
+  const int argb_offset = dec->width_ * dec->last_row_;
+  const uint32_t* const in = dec->argb_ + argb_offset;
+
+  if (num_rows <= 0) return;  // Nothing to be done.
+  ApplyTransforms(dec, num_rows, in);
+
+  // Extract alpha (which is stored in the green plane).
+  {
+    uint8_t* const dst = (uint8_t*)dec->io_->opaque + argb_offset;
+    const uint32_t* const src = dec->argb_cache_;
+    int i;
+    for (i = 0; i < cache_pixs; ++i) dst[i] = (src[i] >> 8) & 0xff;
+  }
+
+  dec->last_row_ = dec->last_out_row_ = row;
+}
+
+int VP8LDecodeAlphaImageStream(int width, int height, const uint8_t* const data,
+                               const size_t data_size, uint8_t* const output) {
+  VP8Io io;
+  int ok = 0;
+  VP8LDecoder* const dec = VP8LNew();
+  if (dec == NULL) return 0;
+
+  dec->width_ = width;
+  dec->height_ = height;
+  dec->io_ = &io;
+
+  VP8InitIo(&io);
+  WebPInitCustomIo(NULL, &io);    // Just a sanity Init. io won't be used.
+  io.opaque = output;
+
+  dec->status_ = VP8_STATUS_OK;
+  VP8LInitBitReader(&dec->br_, data, data_size);
+
+  dec->action_ = READ_HDR;
+  if (!DecodeImageStream(width, height, 1, dec, NULL)) goto Err;
+
+  // Allocate output (note that dec->width_ may have changed here).
+  if (!AllocateARGBBuffers(dec, width)) goto Err;
+
+  // Decode (with special row processing).
+  dec->action_ = READ_DATA;
+  ok = DecodeImageData(dec, dec->argb_, dec->width_, dec->height_,
+                       ExtractAlphaRows);
+
+ Err:
+  VP8LDelete(dec);
+  return ok;
+}
+
 //------------------------------------------------------------------------------
 
 int VP8LDecodeHeader(VP8LDecoder* const dec, VP8Io* const io) {

src/dec/vp8li.h

@@ -106,6 +106,13 @@ int VP8LGetInfo(const uint8_t* data,
                 size_t data_size,    // data available so far
                 int *width, int *height);
 
+
+// Decodes a raw image stream (without header) and store the alpha data
+// into *output, which must be of size width x height. Returns false in case
+// of error.
+int VP8LDecodeAlphaImageStream(int width, int height, const uint8_t* const data,
+                               const size_t data_size, uint8_t* const output);
+
 // Allocates and initialize a new lossless decoder instance.
 VP8LDecoder* VP8LNew(void);
 

src/enc/alpha.c

@@ -54,16 +54,6 @@ extern "C" {
 #ifdef USE_LOSSLESS_ENCODER
 
 #include "../enc/vp8li.h"
-#include "../webp/encode.h"
-
-static int MyWriter(const uint8_t* data, size_t data_size,
-                    const WebPPicture* const picture) {
-  VP8BitWriter* const bw = (VP8BitWriter*)picture->custom_ptr;
-  if (data_size > 0) {
-    VP8BitWriterAppend(bw, data, data_size);
-  }
-  return !bw->error_;
-}
 
 static int EncodeLossless(const uint8_t* data, int width, int height,
                           VP8BitWriter* const bw) {
@@ -71,6 +61,7 @@ static int EncodeLossless(const uint8_t* data, int width, int height,
   int ok = 0;
   WebPConfig config;
   WebPPicture picture;
+  VP8LBitWriter tmp_bw;
 
   WebPPictureInit(&picture);
   picture.width = width;
@@ -78,6 +69,7 @@ static int EncodeLossless(const uint8_t* data, int width, int height,
   picture.use_argb_input = 1;
   if (!WebPPictureAlloc(&picture)) return 0;
 
+  // Transfer the alpha values to the green channel.
   {
     int i, j;
     uint32_t* dst = picture.argb;
@@ -90,16 +82,21 @@ static int EncodeLossless(const uint8_t* data, int width, int height,
       dst += picture.argb_stride;
     }
   }
-  picture.writer = MyWriter;
-  picture.custom_ptr = (void*)bw;
 
   WebPConfigInit(&config);
   config.lossless = 1;
   config.method = 2;
-  config.quality = 100;
+  config.quality = 100;   // TODO(skal): make it adjustable.
 
-  ok = WebPEncode(&config, &picture);
+  VP8LBitWriterInit(&tmp_bw, (width * height) >> 3);
+  ok = (VP8LEncodeStream(&config, &picture, &tmp_bw) == VP8_ENC_OK);
   WebPPictureFree(&picture);
+  if (ok) {
+    const uint8_t* const data = VP8LBitWriterFinish(&tmp_bw);
+    const size_t data_size = VP8LBitWriterNumBytes(&tmp_bw);
+    VP8BitWriterAppend(bw, data, data_size);
+  }
+  VP8LBitWriterDestroy(&tmp_bw);
   return ok && !bw->error_;
 }