Improved near-lossless mode.

Compared to previous mode it gives another 10-30% improvement in compression keeping comparable PSNR on corresponding quality settings.

Still protected by the WEBP_EXPERIMENTAL_FEATURES flag.

Change-Id: I4821815b9a508f4f38c98821acaddb74c73c60ac

src/enc/near_lossless.c

@@ -16,8 +16,9 @@
 
 #include <stdlib.h>
 
-#include "./vp8enci.h"
+#include "../dsp/lossless.h"
 #include "../utils/utils.h"
+#include "./vp8enci.h"
 
 #ifdef WEBP_EXPERIMENTAL_FEATURES
 // Computes quantized pixel value and distance from original value.
@@ -28,12 +29,13 @@ static void GetValAndDistance(int a, int initial, int bits,
   *distance = 2 * abs(a - *val);
 }
 
-// Quantizes values {a, a+(1<<bits), a-(1<<bits)} and returns the nearest one.
-static int FindClosestDiscretized(int a, int bits) {
-  int best_val, min_distance, i;
-  GetValAndDistance(a, a, bits, &best_val, &min_distance);
+// Quantizes values {a, a+(1<<bits), a-(1<<bits)}, checks if in [min, max] range
+// and returns the nearest one.
+static int FindClosestDiscretized(int a, int bits, int min, int max) {
+  int best_val = a, i;
+  int min_distance = 256;
 
-  for (i = -1; i <= 1; i += 2) {
+  for (i = -1; i <= 1; ++i) {
     int val = a + i * (1 << bits);
     int candidate, distance;
     if (val < 0) {
@@ -42,11 +44,13 @@ static int FindClosestDiscretized(int a, int bits) {
       val = 255;
     }
     GetValAndDistance(a, val, bits, &candidate, &distance);
+    if (i != 0) {
       ++distance;
+    }
     // Smallest distance but favor i == 0 over i == -1 and i == 1
     // since that keeps the overall intensity more constant in the
     // images.
-    if (distance < min_distance) {
+    if (distance < min_distance && candidate >= min && candidate <= max) {
       min_distance = distance;
       best_val = candidate;
     }
@@ -54,12 +58,34 @@ static int FindClosestDiscretized(int a, int bits) {
   return best_val;
 }
 
+// Discretizes value (actual - predicted) in the way that actual pixel value
+// stays within error bounds.
+static WEBP_INLINE uint32_t DiscretizedResidual(uint32_t actual,
+                                                uint32_t predicted,
+                                                int limit_bits) {
+  const uint32_t res = (actual - predicted) & 0xff;
+  uint32_t min, max;
+  if (actual < predicted) {
+    min = 256 - predicted;
+    max = 255;
+  } else {
+    min = 0;
+    max = 255 - predicted;
+  }
+  return FindClosestDiscretized(res, limit_bits, min, max);
+}
+
 // Applies FindClosestDiscretized to all channels of pixel.
-static uint32_t ClosestDiscretizedArgb(uint32_t a, int bits) {
-  return (FindClosestDiscretized(a >> 24, bits) << 24) |
-         (FindClosestDiscretized((a >> 16) & 0xff, bits) << 16) |
-         (FindClosestDiscretized((a >> 8) & 0xff, bits) << 8) |
-         (FindClosestDiscretized(a & 0xff, bits));
+static uint32_t ClosestDiscretizedArgb(uint32_t a, int bits,
+                                       uint32_t min, uint32_t max) {
+  return (FindClosestDiscretized(a >> 24, bits, min >> 24, max >> 24) << 24) |
+         (FindClosestDiscretized((a >> 16) & 0xff, bits,
+                                 (min >> 16) & 0xff,
+                                 (max >> 16) & 0xff) << 16) |
+         (FindClosestDiscretized((a >> 8) & 0xff, bits,
+                                 (min >> 8) & 0xff,
+                                 (max >> 8) & 0xff) << 8) |
+         (FindClosestDiscretized(a & 0xff, bits, min & 0xff, max & 0xff));
 }
 
 // Checks if distance between corresponding channel values of pixels a and b
@@ -102,12 +128,16 @@ static void NearLossless(int xsize, int ysize, uint32_t* argb,
         smooth_area = 0;
       }
       if (!smooth_area) {
-        argb[ix] = ClosestDiscretizedArgb(argb[ix], limit_bits);
+        argb[ix] = ClosestDiscretizedArgb(argb[ix], limit_bits, 0, 0xffffffff);
       }
     }
   }
 }
-#endif
+
+static int QualityToLimitBits(int quality) {
+  return 5 - (quality + 12) / 25;
+}
+#endif  // WEBP_EXPERIMENTAL_FEATURES
 
 // TODO(akramarz): optimize memory to O(xsize)
 int VP8ApplyNearLossless(int xsize, int ysize, uint32_t* argb, int quality) {
@@ -122,7 +152,7 @@ int VP8ApplyNearLossless(int xsize, int ysize, uint32_t* argb, int quality) {
       (uint32_t *)WebPSafeMalloc(xsize * ysize, sizeof(*copy_buffer));
   // quality mapping 0..12 -> 5
   //                 13..100 -> 4..1
-  const int limit_bits = 5 - (quality + 12) / 25;
+  const int limit_bits = QualityToLimitBits(quality);
   assert(argb != NULL);
   assert(limit_bits >= 0);
   assert(limit_bits < 31);
@@ -133,6 +163,97 @@ int VP8ApplyNearLossless(int xsize, int ysize, uint32_t* argb, int quality) {
     NearLossless(xsize, ysize, argb, i, copy_buffer);
   }
   WebPSafeFree(copy_buffer);
-#endif
+#endif  // WEBP_EXPERIMENTAL_FEATURES
   return 1;
 }
+
+#ifdef WEBP_EXPERIMENTAL_FEATURES
+
+// In-place sum of each component with mod 256.
+// This probably should go somewhere else (lossless.h?). This is just copy-paste
+// from lossless.c.
+static WEBP_INLINE void AddPixelsEq(uint32_t* a, uint32_t b) {
+  const uint32_t alpha_and_green = (*a & 0xff00ff00u) + (b & 0xff00ff00u);
+  const uint32_t red_and_blue = (*a & 0x00ff00ffu) + (b & 0x00ff00ffu);
+  *a = (alpha_and_green & 0xff00ff00u) | (red_and_blue & 0x00ff00ffu);
+}
+
+void VP8ApplyNearLosslessPredict(int xsize, int ysize, int pred_bits,
+                                 const uint32_t* argb_orig,
+                                 uint32_t* argb, uint32_t* argb_scratch,
+                                 const uint32_t* const transform_data,
+                                 int quality, int subtract_green) {
+  const int tiles_per_row = VP8LSubSampleSize(xsize, pred_bits);
+  uint32_t* const upper_row = argb_scratch;
+  const int limit_bits = QualityToLimitBits(quality);
+
+  int y;
+  for (y = 0; y < ysize; ++y) {
+    int x;
+    uint32_t curr_pix = 0, prev_pix = 0;
+    for (x = 0; x < xsize; ++x) {
+      const int tile_idx = (y >> pred_bits) * tiles_per_row + (x >> pred_bits);
+      const int pred = (transform_data[tile_idx] >> 8) & 0xf;
+      const VP8LPredictorFunc pred_func = VP8LPredictors[pred];
+      uint32_t predict, rb_shift = 0, delta_g = 0;
+      if (y == 0) {
+        predict = (x == 0) ? ARGB_BLACK : prev_pix;  // Left.
+      } else if (x == 0) {
+        predict = upper_row[x];  // Top.
+      } else {
+        predict = pred_func(prev_pix, upper_row + x);
+      }
+
+      // Discretize all residuals keeping the original pixel values in error
+      // bounds.
+      curr_pix = argb_orig[x];
+      {
+        const uint32_t a = curr_pix >> 24;
+        const uint32_t a_pred = predict >> 24;
+        const uint32_t a_res = DiscretizedResidual(a, a_pred, limit_bits);
+        curr_pix = (curr_pix & 0x00ffffff) | a_res << 24;
+      }
+
+      {
+        const uint32_t g = (curr_pix >> 8) & 0xff;
+        const uint32_t g_pred = (predict >> 8) & 0xff;
+        const uint32_t g_res = DiscretizedResidual(g, g_pred, limit_bits);
+        // In case subtract-green transform is used, we need to shift
+        // red and blue later.
+        if (subtract_green) {
+          delta_g = (g_pred + g_res - g) & 0xff;
+          rb_shift = g;
+        }
+        curr_pix = (curr_pix & 0xffff00ff) | (g_res << 8);
+      }
+
+      {
+        const uint32_t r = ((curr_pix >> 16) + rb_shift) & 0xff;
+        const uint32_t r_pred = ((predict >> 16) + rb_shift + delta_g) & 0xff;
+        const uint32_t r_res = DiscretizedResidual(r, r_pred, limit_bits);
+        curr_pix = (curr_pix & 0xff00ffff) | (r_res << 16);
+      }
+
+      {
+        const uint32_t b = (curr_pix + rb_shift) & 0xff;
+        const uint32_t b_pred = (predict + rb_shift + delta_g) & 0xff;
+        const uint32_t b_res = DiscretizedResidual(b, b_pred, limit_bits);
+        curr_pix = (curr_pix & 0xffffff00) | b_res;
+      }
+
+      // Change pixel value.
+      argb[x] = curr_pix;
+      curr_pix = predict;
+      AddPixelsEq(&curr_pix, argb[x]);
+      // Copy previous pixel to upper row.
+      if(x > 0) {
+        upper_row[x - 1] = prev_pix;
+      }
+      prev_pix = curr_pix;
+    }
+    argb += xsize;
+    argb_orig += xsize;
+    upper_row[xsize - 1] = curr_pix;
+  }
+}
+#endif  // WEBP_EXPERIMENTAL_FEATURES

src/enc/vp8enci.h

@@ -572,7 +572,12 @@ int WebPPictureAllocYUVA(WebPPicture* const picture, int width, int height);
   // in near_lossless.c
 // Near lossless preprocessing in RGB color-space.
 int VP8ApplyNearLossless(int xsize, int ysize, uint32_t* argb, int quality);
-
+// Near lossless adjustment for predictors.
+void VP8ApplyNearLosslessPredict(int xsize, int ysize, int pred_bits,
+                                 const uint32_t* argb_orig,
+                                 uint32_t* argb, uint32_t* argb_scratch,
+                                 const uint32_t* const transform_data,
+                                 int quality, int subtract_green);
 //------------------------------------------------------------------------------
 
 #ifdef __cplusplus

src/enc/vp8l.c

@@ -1214,6 +1214,7 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
   const int height = picture->height;
   VP8LEncoder* const enc = VP8LEncoderNew(config, picture);
   const size_t byte_position = VP8LBitWriterNumBytes(bw);
+  const int use_near_lossless = !enc->use_palette_ && config->near_lossless;
   int hdr_size = 0;
   int data_size = 0;
 
@@ -1230,6 +1231,13 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
     goto Error;
   }
 
+  // If no prediction transform just apply near-lossless preprocessing.
+  if (!enc->use_predict_ && use_near_lossless &&
+      !VP8ApplyNearLossless(width, height, picture->argb,
+                            config->near_lossless)) {
+    goto Error;
+  }
+
   if (enc->use_palette_) {
     err = EncodePalette(bw, enc);
     if (err != VP8_ENC_OK) goto Error;
@@ -1256,8 +1264,39 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
   }
 
   if (enc->use_predict_) {
+    uint32_t* copy_buffer = NULL;
+#ifdef WEBP_EXPERIMENTAL_FEATURES
+    if (use_near_lossless) {
+      // Copy image to temporary buffer.
+      int y;
+      copy_buffer = WebPSafeMalloc(height * enc->current_width_,
+                                   sizeof(*copy_buffer));
+      if (copy_buffer == NULL) {
+        err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+        goto Error;
+      }
+      for (y = 0; y < height; ++y) {
+        memcpy(copy_buffer + y * enc->current_width_,
+               enc->argb_ + y * enc->current_width_,
+               enc->current_width_ * sizeof(*enc->argb_));
+      }
+    }
+#endif  // WEBP_EXPERIMENTAL_FEATURES
     err = ApplyPredictFilter(enc, enc->current_width_, height, quality, bw);
-    if (err != VP8_ENC_OK) goto Error;
+    if (err != VP8_ENC_OK) {
+      WebPSafeFree(copy_buffer);
+      goto Error;
+    }
+#ifdef WEBP_EXPERIMENTAL_FEATURES
+    if (use_near_lossless) {
+      VP8ApplyNearLosslessPredict(enc->current_width_, height,
+                                  enc->transform_bits_, copy_buffer, enc->argb_,
+                                  enc->argb_scratch_, enc->transform_data_,
+                                  config->near_lossless,
+                                  enc->use_subtract_green_);
+      WebPSafeFree(copy_buffer);
+    }
+#endif  // WEBP_EXPERIMENTAL_FEATURES
   }
 
   if (enc->use_cross_color_) {

src/enc/webpenc.c

@@ -368,11 +368,6 @@ int WebPEncode(const WebPConfig* config, WebPPicture* pic) {
     }
     ok &= DeleteVP8Encoder(enc);  // must always be called, even if !ok
   } else {
-    if (config->near_lossless > 0 &&
-        !VP8ApplyNearLossless(pic->width, pic->height,
-                              pic->argb, config->near_lossless)) {
-      return 0;
-    }
     // Make sure we have ARGB samples.
     if (pic->argb == NULL && !WebPPictureYUVAToARGB(pic)) {
       return 0;