https://github.com/ImageMagick/ImageMagick/issues/1740

MagickCore/quantize.c

@@ -2347,8 +2347,8 @@ MagickExport MagickBooleanType PosterizeImage(Image *image,const size_t levels,
   const DitherMethod dither_method,ExceptionInfo *exception)
 {
 #define PosterizeImageTag  "Posterize/Image"
-#define PosterizePixel(pixel) (Quantum) (QuantumRange*(MagickRound( \
-  QuantumScale*pixel*(levels-1)))/MagickMax((ssize_t) levels-1,1))
+#define PosterizePixel(pixel) ClampToQuantum((MagickRealType) QuantumRange*( \
+  MagickRound(QuantumScale*pixel*(levels-1)))/MagickMax((ssize_t) levels-1,1))
 
   CacheView
     *image_view;
@@ -3282,17 +3282,21 @@ extern "C" {
 
 static int IntensityCompare(const void *x,const void *y)
 {
+  double
+    intensity;
+
   PixelInfo
     *color_1,
     *color_2;
 
-  ssize_t
-    intensity;
-
   color_1=(PixelInfo *) x;
   color_2=(PixelInfo *) y;
-  intensity=(ssize_t) (GetPixelInfoIntensity((const Image *) NULL,color_1)-
-    GetPixelInfoIntensity((const Image *) NULL,color_2));
+  intensity=GetPixelInfoIntensity((const Image *) NULL,color_1)-
+    GetPixelInfoIntensity((const Image *) NULL,color_2);
+  if (intensity > (double) INT_MAX)
+    intensity=(double) INT_MAX;
+  if (intensity < (double) INT_MIN)
+    intensity=(double) INT_MIN;
   return((int) intensity);
 }