From c9ee5d274d72c4919929c8a6ac30580baf238bd2 Mon Sep 17 00:00:00 2001
From: Hermet Park <hermet@lottiefiles.com>
Date: Thu, 18 Jan 2024 18:53:27 +0900
Subject: [PATCH] sw_engine: tweak the image down-scaler.

Adjust the sampling count according to the scale ratio.

This significantly improves performance
while making it hard to recognize any loss in image quality.

Lottie example performance has improved by 15%.
---
 src/renderer/sw_engine/tvgSwRaster.cpp | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/src/renderer/sw_engine/tvgSwRaster.cpp b/src/renderer/sw_engine/tvgSwRaster.cpp
index fd45ecb9..049318c7 100644
--- a/src/renderer/sw_engine/tvgSwRaster.cpp
+++ b/src/renderer/sw_engine/tvgSwRaster.cpp
@@ -257,8 +257,8 @@ static uint32_t _interpUpScaler(const uint32_t *img, TVG_UNUSED uint32_t stride,
     auto ry2 = ry + 1;
     if (ry2 >= h) ry2 = h - 1;
 
-    auto dx = static_cast<size_t>((sx - rx) * 255.0f);
-    auto dy = static_cast<size_t>((sy - ry) * 255.0f);
+    auto dx = static_cast<uint8_t>((sx - rx) * 255.0f);
+    auto dy = static_cast<uint8_t>((sy - ry) * 255.0f);
 
     auto c1 = img[rx + ry * w];
     auto c2 = img[rx2 + ry * w];
@@ -281,21 +281,23 @@ static uint32_t _interpDownScaler(const uint32_t *img, uint32_t stride, uint32_t
     int32_t maxx = (int32_t)sx + n;
     if (maxx >= (int32_t)w) maxx = w;
 
+    int32_t inc = (n / 2) + 1;
+    n = 0;
+
     auto src = img + minx + miny * stride;
 
-    for (auto y = miny; y < maxy; ++y) {
+    for (auto y = miny; y < maxy; y += inc) {
         auto p = src;
-        for (auto x = minx; x < maxx; ++x, ++p) {
-            c[0] += *p >> 24;
-            c[1] += (*p >> 16) & 0xff;
-            c[2] += (*p >> 8) & 0xff;
-            c[3] += *p & 0xff;
+        for (auto x = minx; x < maxx; x += inc, p += inc) {
+            c[0] += A(*p);
+            c[1] += C1(*p);
+            c[2] += C2(*p);
+            c[3] += C3(*p);
+            ++n;
         }
-        src += stride;
+        src += (stride * inc);
     }
 
-    n = (maxy - miny) * (maxx - minx);
-
     c[0] /= n;
     c[1] /= n;
     c[2] /= n;