add WebPDispatchAlphaToGreen() to dsp

SSE2 version is 2.1x faster

This is used to transfer the alpha plane to green channel before lossless compression.

Change-Id: I01d9df0051c183b1ff5d6eb69961d4f43e33141a

src/dsp/alpha_processing.c

@@ -303,6 +303,19 @@ static int DispatchAlpha(const uint8_t* alpha, int alpha_stride,
   return (alpha_mask != 0xff);
 }
 
+static void DispatchAlphaToGreen(const uint8_t* alpha, int alpha_stride,
+                                 int width, int height,
+                                 uint32_t* dst, int dst_stride) {
+  int i, j;
+  for (j = 0; j < height; ++j) {
+    for (i = 0; i < width; ++i) {
+      dst[i] = alpha[i] << 8;  // leave A/R/B channels zero'd.
+    }
+    alpha += alpha_stride;
+    dst += dst_stride;
+  }
+}
+
 static int ExtractAlpha(const uint8_t* argb, int argb_stride,
                         int width, int height,
                         uint8_t* alpha, int alpha_stride) {
@@ -324,6 +337,7 @@ static int ExtractAlpha(const uint8_t* argb, int argb_stride,
 void (*WebPApplyAlphaMultiply)(uint8_t*, int, int, int, int);
 void (*WebPApplyAlphaMultiply4444)(uint8_t*, int, int, int);
 int (*WebPDispatchAlpha)(const uint8_t*, int, int, int, uint8_t*, int);
+void (*WebPDispatchAlphaToGreen)(const uint8_t*, int, int, int, uint32_t*, int);
 int (*WebPExtractAlpha)(const uint8_t*, int, int, int, uint8_t*, int);
 
 //------------------------------------------------------------------------------
@@ -339,6 +353,7 @@ void WebPInitAlphaProcessing(void) {
   WebPApplyAlphaMultiply = ApplyAlphaMultiply;
   WebPApplyAlphaMultiply4444 = ApplyAlphaMultiply_16b;
   WebPDispatchAlpha = DispatchAlpha;
+  WebPDispatchAlphaToGreen = DispatchAlphaToGreen;
   WebPExtractAlpha = ExtractAlpha;
 
   // If defined, use CPUInfo() to overwrite some pointers with faster versions.

src/dsp/alpha_processing_sse2.c

@@ -72,6 +72,32 @@ static int DispatchAlpha(const uint8_t* alpha, int alpha_stride,
   return (alpha_and != 0xff);
 }
 
+static void DispatchAlphaToGreen(const uint8_t* alpha, int alpha_stride,
+                                 int width, int height,
+                                 uint32_t* dst, int dst_stride) {
+  int i, j;
+  const __m128i zero = _mm_setzero_si128();
+  const int limit = width & ~15;
+  for (j = 0; j < height; ++j) {
+    for (i = 0; i < limit; i += 16) {   // process 16 alpha bytes
+      const __m128i a0 = _mm_loadu_si128((__m128i*)&alpha[i]);
+      const __m128i a1 = _mm_unpacklo_epi8(zero, a0);  // note the 'zero' first!
+      const __m128i b1 = _mm_unpackhi_epi8(zero, a0);
+      const __m128i a2_lo = _mm_unpacklo_epi16(a1, zero);
+      const __m128i b2_lo = _mm_unpacklo_epi16(b1, zero);
+      const __m128i a2_hi = _mm_unpackhi_epi16(a1, zero);
+      const __m128i b2_hi = _mm_unpackhi_epi16(b1, zero);
+      _mm_storeu_si128((__m128i*)&dst[i +  0], a2_lo);
+      _mm_storeu_si128((__m128i*)&dst[i +  4], a2_hi);
+      _mm_storeu_si128((__m128i*)&dst[i +  8], b2_lo);
+      _mm_storeu_si128((__m128i*)&dst[i + 12], b2_hi);
+    }
+    for (; i < width; ++i) dst[i] = alpha[i] << 8;
+    alpha += alpha_stride;
+    dst += dst_stride;
+  }
+}
+
 static int ExtractAlpha(const uint8_t* argb, int argb_stride,
                         int width, int height,
                         uint8_t* alpha, int alpha_stride) {
@@ -264,6 +290,7 @@ void WebPInitAlphaProcessingSSE2(void) {
   WebPMultRow = MultRow;
   WebPApplyAlphaMultiply = ApplyAlphaMultiply;
   WebPDispatchAlpha = DispatchAlpha;
+  WebPDispatchAlphaToGreen = DispatchAlphaToGreen;
   WebPExtractAlpha = ExtractAlpha;
 #endif
 }

src/dsp/dsp.h

@@ -274,6 +274,12 @@ extern int (*WebPDispatchAlpha)(const uint8_t* alpha, int alpha_stride,
                                 int width, int height,
                                 uint8_t* dst, int dst_stride);
 
+// Transfer packed 8b alpha[] values to green channel in dst[], zero'ing the
+// A/R/B values. 'dst_stride' is the stride for dst[] in uint32_t units.
+extern void (*WebPDispatchAlphaToGreen)(const uint8_t* alpha, int alpha_stride,
+                                        int width, int height,
+                                        uint32_t* dst, int dst_stride);
+
 // Extract the alpha values from 32b values in argb[] and pack them into alpha[]
 // (this is the opposite of WebPDispatchAlpha).
 // Returns true if there's only trivial 0xff alpha values.

src/enc/alpha.c

@@ -61,18 +61,8 @@ static int EncodeLossless(const uint8_t* const data, int width, int height,
   if (!WebPPictureAlloc(&picture)) return 0;
 
   // Transfer the alpha values to the green channel.
-  {
+  WebPDispatchAlphaToGreen(data, width, picture.width, picture.height,
-    int i, j;
+                           picture.argb, picture.argb_stride);
-    uint32_t* dst = picture.argb;
-    const uint8_t* src = data;
-    for (j = 0; j < picture.height; ++j) {
-      for (i = 0; i < picture.width; ++i) {
-        dst[i] = src[i] << 8;  // we leave A/R/B channels zero'd.
-      }
-      src += width;
-      dst += picture.argb_stride;
-    }
-  }
 
   WebPConfigInit(&config);
   config.lossless = 1;
@@ -376,6 +366,7 @@ static int CompressAlphaJob(VP8Encoder* const enc, void* dummy) {
 }
 
 void VP8EncInitAlpha(VP8Encoder* const enc) {
+  WebPInitAlphaProcessing();
   enc->has_alpha_ = WebPPictureHasTransparency(enc->pic_);
   enc->alpha_data_ = NULL;
   enc->alpha_data_size_ = 0;
@@ -430,4 +421,3 @@ int VP8EncDeleteAlpha(VP8Encoder* const enc) {
   enc->has_alpha_ = 0;
   return ok;
 }
-