diff --git a/source/scale.cc b/source/scale.cc
index ade50df2c..29424ae71 100644
--- a/source/scale.cc
+++ b/source/scale.cc
@@ -1219,6 +1219,11 @@ static int ScalePlaneBilinearUp(int src_width,
                                 const uint8_t* src_ptr,
                                 uint8_t* dst_ptr,
                                 enum FilterMode filtering) {
+  assert(src_width > 0);
+  assert(src_height > 0);
+  assert(dst_width > 0);
+  assert(dst_height > 0);
+
   int j;
   // Initial source x/y coordinate and step values as 16.16 fixed point.
   int x = 0;
@@ -1234,6 +1239,7 @@ static int ScalePlaneBilinearUp(int src_width,
       filtering ? ScaleFilterCols_C : ScaleCols_C;
   ScaleSlope(src_width, src_height, dst_width, dst_height, filtering, &x, &y,
              &dx, &dy);
+  assert(dy <= 65536);
   src_width = Abs(src_width);
 
 #if defined(HAS_INTERPOLATEROW_AVX2)
@@ -1327,22 +1333,27 @@ static int ScalePlaneBilinearUp(int src_width,
       src += src_stride;
     }
 
+    // 2-row rolling buffer:
+    // rowptr and (rowptr + rowstride) hold the scaled rows for yi and yi + 1.
+    // Because dy <= 65536 (dy <= 1.0 in 16.16), yi advances in unit steps.
+    // When yi != lasty:
+    // 1. Scale the next source row into the older buffer (rowptr).
+    // 2. Swap buffer pointers (rowptr += rowstride; rowstride = -rowstride;)
+    //    so rowptr points to yi and (rowptr + rowstride) points to yi + 1.
+    // 3. Advance src by 1 row if row yi + 2 exists ((y + 65536) < max_y),
+    //    otherwise clamp src at (src_height - 1) to avoid reading out of bounds.
     for (j = 0; j < dst_height; ++j) {
+      if (y > max_y) {
+        y = max_y;
+      }
       yi = y >> 16;
       if (yi != lasty) {
-        if (y > max_y) {
-          y = max_y;
-          yi = y >> 16;
-          src = src_ptr + yi * src_stride;
-        }
-        if (yi != lasty) {
-          ScaleFilterCols(rowptr, src, dst_width, x, dx);
-          rowptr += rowstride;
-          rowstride = -rowstride;
-          lasty = yi;
-          if ((y + 65536) < max_y) {
-            src += src_stride;
-          }
+        ScaleFilterCols(rowptr, src, dst_width, x, dx);
+        rowptr += rowstride;
+        rowstride = -rowstride;
+        lasty = yi;
+        if ((y + 65536) < max_y) {
+          src += src_stride;
         }
       }
       if (filtering == kFilterLinear) {
@@ -1705,6 +1716,11 @@ static int ScalePlaneBilinearUp_16(int src_width,
                                    const uint16_t* src_ptr,
                                    uint16_t* dst_ptr,
                                    enum FilterMode filtering) {
+  assert(src_width > 0);
+  assert(src_height > 0);
+  assert(dst_width > 0);
+  assert(dst_height > 0);
+
   int j;
   // Initial source x/y coordinate and step values as 16.16 fixed point.
   int x = 0;
@@ -1720,6 +1736,7 @@ static int ScalePlaneBilinearUp_16(int src_width,
       filtering ? ScaleFilterCols_16_C : ScaleCols_16_C;
   ScaleSlope(src_width, src_height, dst_width, dst_height, filtering, &x, &y,
              &dx, &dy);
+  assert(dy <= 65536);
   src_width = Abs(src_width);
 
 #if defined(HAS_INTERPOLATEROW_16_SSSE3)
@@ -1768,6 +1785,7 @@ static int ScalePlaneBilinearUp_16(int src_width,
     }
 #endif
   }
+
   if (y > max_y) {
     y = max_y;
   }
@@ -1793,22 +1811,27 @@ static int ScalePlaneBilinearUp_16(int src_width,
       src += src_stride;
     }
 
+    // 2-row rolling buffer:
+    // rowptr and (rowptr + rowstride) hold the scaled rows for yi and yi + 1.
+    // Because dy <= 65536 (dy <= 1.0 in 16.16), yi advances in unit steps.
+    // When yi != lasty:
+    // 1. Scale the next source row into the older buffer (rowptr).
+    // 2. Swap buffer pointers (rowptr += rowstride; rowstride = -rowstride;)
+    //    so rowptr points to yi and (rowptr + rowstride) points to yi + 1.
+    // 3. Advance src by 1 row if row yi + 2 exists ((y + 65536) < max_y),
+    //    otherwise clamp src at (src_height - 1) to avoid reading out of bounds.
     for (j = 0; j < dst_height; ++j) {
+      if (y > max_y) {
+        y = max_y;
+      }
       yi = y >> 16;
       if (yi != lasty) {
-        if (y > max_y) {
-          y = max_y;
-          yi = y >> 16;
-          src = src_ptr + yi * src_stride;
-        }
-        if (yi != lasty) {
-          ScaleFilterCols(rowptr, src, dst_width, x, dx);
-          rowptr += rowstride;
-          rowstride = -rowstride;
-          lasty = yi;
-          if ((y + 65536) < max_y) {
-            src += src_stride;
-          }
+        ScaleFilterCols(rowptr, src, dst_width, x, dx);
+        rowptr += rowstride;
+        rowstride = -rowstride;
+        lasty = yi;
+        if ((y + 65536) < max_y) {
+          src += src_stride;
         }
       }
       if (filtering == kFilterLinear) {
diff --git a/source/scale_argb.cc b/source/scale_argb.cc
index 9c2ac29c9..8dda11797 100644
--- a/source/scale_argb.cc
+++ b/source/scale_argb.cc
@@ -441,6 +441,12 @@ static int ScaleARGBBilinearUp(int src_width,
                                int y,
                                int dy,
                                enum FilterMode filtering) {
+  assert(src_width > 0);
+  assert(src_height > 0);
+  assert(dst_width > 0);
+  assert(dst_height > 0);
+  assert(dy <= 65536);
+
   int j;
   void (*InterpolateRow)(uint8_t* dst_argb, const uint8_t* src_argb,
                          ptrdiff_t src_stride, int dst_width,
@@ -576,6 +582,15 @@ static int ScaleARGBBilinearUp(int src_width,
       src += src_stride;
     }
 
+    // 2-row rolling buffer:
+    // rowptr and (rowptr + rowstride) hold the scaled rows for yi and yi + 1.
+    // Because dy <= 65536 (dy <= 1.0 in 16.16), yi advances in unit steps.
+    // When yi != lasty:
+    // 1. Scale the next source row into the older buffer (rowptr).
+    // 2. Swap buffer pointers (rowptr += rowstride; rowstride = -rowstride;)
+    //    so rowptr points to yi and (rowptr + rowstride) points to yi + 1.
+    // 3. Advance src by 1 row if row yi + 2 exists ((y64 + 65536) < max_y),
+    //    otherwise clamp src at (src_height - 1) to avoid reading out of bounds.
     for (j = 0; j < dst_height; ++j) {
       if (y64 > max_y) {
         y64 = max_y;
diff --git a/source/scale_uv.cc b/source/scale_uv.cc
index e20e708be..c0dc4b84c 100644
--- a/source/scale_uv.cc
+++ b/source/scale_uv.cc
@@ -463,6 +463,12 @@ static int ScaleUVBilinearUp(int src_width,
                              int y,
                              int dy,
                              enum FilterMode filtering) {
+  assert(src_width > 0);
+  assert(src_height > 0);
+  assert(dst_width > 0);
+  assert(dst_height > 0);
+  assert(dy <= 65536);
+
   int j;
   void (*InterpolateRow)(uint8_t* dst_uv, const uint8_t* src_uv,
                          ptrdiff_t src_stride, int dst_width,
@@ -575,22 +581,27 @@ static int ScaleUVBilinearUp(int src_width,
       src += src_stride;
     }
 
+    // 2-row rolling buffer:
+    // rowptr and (rowptr + rowstride) hold the scaled rows for yi and yi + 1.
+    // Because dy <= 65536 (dy <= 1.0 in 16.16), yi advances in unit steps.
+    // When yi != lasty:
+    // 1. Scale the next source row into the older buffer (rowptr).
+    // 2. Swap buffer pointers (rowptr += rowstride; rowstride = -rowstride;)
+    //    so rowptr points to yi and (rowptr + rowstride) points to yi + 1.
+    // 3. Advance src by 1 row if row yi + 2 exists ((y + 65536) < max_y),
+    //    otherwise clamp src at (src_height - 1) to avoid reading out of bounds.
     for (j = 0; j < dst_height; ++j) {
+      if (y > max_y) {
+        y = max_y;
+      }
       yi = y >> 16;
       if (yi != lasty) {
-        if (y > max_y) {
-          y = max_y;
-          yi = y >> 16;
-          src = src_uv + yi * src_stride;
-        }
-        if (yi != lasty) {
-          ScaleUVFilterCols(rowptr, src, dst_width, x, dx);
-          rowptr += rowstride;
-          rowstride = -rowstride;
-          lasty = yi;
-          if ((y + 65536) < max_y) {
-            src += src_stride;
-          }
+        ScaleUVFilterCols(rowptr, src, dst_width, x, dx);
+        rowptr += rowstride;
+        rowstride = -rowstride;
+        lasty = yi;
+        if ((y + 65536) < max_y) {
+          src += src_stride;
         }
       }
       if (filtering == kFilterLinear) {