mv SharpYuvEstimate420Risk to extras/

There's no need for this in libsharpyuv currently. This avoids an ABI
bump and library size increase.

Change-Id: I3e4c79052fdb4d628a2d36491547233dd0b6dc34

extras/extras.h

@@ -17,9 +17,10 @@
 extern "C" {
 #endif
 
+#include "sharpyuv/sharpyuv.h"
 #include "webp/encode.h"
 
-#define WEBP_EXTRAS_ABI_VERSION 0x0002    // MAJOR(8b) + MINOR(8b)
+#define WEBP_EXTRAS_ABI_VERSION 0x0003    // MAJOR(8b) + MINOR(8b)
 
 //------------------------------------------------------------------------------
 
@@ -70,6 +71,38 @@ WEBP_EXTERN int VP8EstimateQuality(const uint8_t* const data, size_t size);
 
 //------------------------------------------------------------------------------
 
+// Computes a score between 0 and 100 which represents the risk of having visual
+// quality loss from converting an RGB image to YUV420.
+// A low score, typically < 40, means there is a low risk of artifacts from
+// chroma subsampling and a simple averaging algorithm can be used instead of
+// the more expensive SharpYuvConvert function.
+// A medium score, typically >= 40 and < 70, means that simple chroma
+// subsampling will produce artifacts and it may be advisable to use the more
+// costly SharpYuvConvert for YUV420 conversion.
+// A high score, typically >= 70, means there is a very high risk of artifacts
+// from chroma subsampling even with SharpYuvConvert, and best results might be
+// achieved by using YUV444.
+// If not using SharpYuvConvert, a threshold of about 50 can be used to decide
+// between (simple averaging) 420 and 444.
+// r_ptr, g_ptr, b_ptr: pointers to the source r, g and b channels. Should point
+//     to uint8_t buffers if rgb_bit_depth is 8, or uint16_t buffers otherwise.
+// rgb_step: distance in bytes between two horizontally adjacent pixels on the
+//     r, g and b channels. If rgb_bit_depth is > 8, it should be a
+//     multiple of 2.
+// rgb_stride: distance in bytes between two vertically adjacent pixels on the
+//     r, g, and b channels. If rgb_bit_depth is > 8, it should be a
+//     multiple of 2.
+// rgb_bit_depth: number of bits for each r/g/b value. Only a value of 8 is
+//     currently supported.
+// width, height: width and height of the image in pixels
+// Returns 0 on failure.
+WEBP_EXTERN int SharpYuvEstimate420Risk(
+    const void* r_ptr, const void* g_ptr, const void* b_ptr, int rgb_step,
+    int rgb_stride, int rgb_bit_depth, int width, int height,
+    const SharpYuvOptions* options, float* score);
+
+//------------------------------------------------------------------------------
+
 #ifdef __cplusplus
 }    // extern "C"
 #endif