diff --git a/src/dec/alpha.c b/src/dec/alpha.c
index 752ea665..d91345db 100644
--- a/src/dec/alpha.c
+++ b/src/dec/alpha.c
@@ -78,7 +78,7 @@ static int DecodeAlpha(const uint8_t* data, size_t data_size,
       unfilter_func(width, height, width, 0, height, output);
     }
     if (pre_processing == ALPHA_PREPROCESSED_LEVELS) {
-      ok = DequantizeLevels(output, width, height);
+      ok = DequantizeLevels(output, width, height, 0, height);
     }
   }
 
diff --git a/src/utils/quant_levels_dec.c b/src/utils/quant_levels_dec.c
index d93594b3..2574e08e 100644
--- a/src/utils/quant_levels_dec.c
+++ b/src/utils/quant_levels_dec.c
@@ -17,11 +17,12 @@
 extern "C" {
 #endif
 
-int DequantizeLevels(uint8_t* const data, int width, int height) {
-  if (data == NULL || width <= 0 || height <= 0) return 0;
-  (void)data;
-  (void)width;
-  (void)height;
+int DequantizeLevels(uint8_t* const data, int width, int height,
+                     int row, int num_rows) {
+  if (data == NULL || width <= 0 || height <= 0 || row < 0 || num_rows < 0 ||
+      row + num_rows > height) {
+    return 0;
+  }
   return 1;
 }
 
diff --git a/src/utils/quant_levels_dec.h b/src/utils/quant_levels_dec.h
index 58910676..a707224d 100644
--- a/src/utils/quant_levels_dec.h
+++ b/src/utils/quant_levels_dec.h
@@ -20,10 +20,12 @@
 extern "C" {
 #endif
 
-// Apply post-processing to input 'data' of size 'width'x'height' assuming
-// that the source was quantized to a reduced number of levels.
+// Apply post-processing to input 'data' of size 'width'x'height' assuming that
+// the source was quantized to a reduced number of levels. The post-processing
+// will be applied to 'num_rows' rows of 'data' starting from 'row'.
 // Returns false in case of error (data is NULL, invalid parameters, ...).
-int DequantizeLevels(uint8_t* const data, int width, int height);
+int DequantizeLevels(uint8_t* const data, int width, int height,
+                     int row, int num_rows);
 
 #if defined(__cplusplus) || defined(c_plusplus)
 }    // extern "C"