src/common.cc

@@ -951,6 +951,14 @@ namespace sharp {
     return interpretation == VIPS_INTERPRETATION_RGB16 || interpretation == VIPS_INTERPRETATION_GREY16;
   }
 
+  /*
+    Return the image alpha maximum. Useful for combining alpha bands. scRGB
+    images are 0 - 1 for image data, but the alpha is 0 - 255.
+  */
+  double MaximumImageAlpha(VipsInterpretation const interpretation) {
+    return Is16Bit(interpretation) ? 65535.0 : 255.0;
+  }
+
   /*
     Convert RGBA value to another colourspace
   */
@@ -995,14 +1003,16 @@ namespace sharp {
     }
     // Add alpha channel(s) to alphaColour colour
     if (colour[3] < 255.0 || image.has_alpha()) {
-      int extraBands = image.bands() > 4 ? image.bands() - 3 : 1;
-      alphaColour.insert(alphaColour.end(), extraBands, colour[3] * multiplier);
+      do {
+        alphaColour.push_back(colour[3] * multiplier);
+      } while (alphaColour.size() < static_cast<size_t>(image.bands()));
     }
     // Ensure alphaColour colour uses correct colourspace
     alphaColour = sharp::GetRgbaAsColourspace(alphaColour, image.interpretation(), premultiply);
     // Add non-transparent alpha channel, if required
     if (colour[3] < 255.0 && !image.has_alpha()) {
-      image = image.bandjoin_const({ 255 * multiplier });
+      image = image.bandjoin(
+        VImage::new_matrix(image.width(), image.height()).new_from_image(255 * multiplier).cast(image.format()));
     }
     return std::make_tuple(image, alphaColour);
   }
@@ -1022,7 +1032,9 @@ namespace sharp {
   */
   VImage EnsureAlpha(VImage image, double const value) {
     if (!image.has_alpha()) {
-      image = image.bandjoin_const({ value * vips_interpretation_max_alpha(image.interpretation()) });
+      std::vector<double> alpha;
+      alpha.push_back(value * sharp::MaximumImageAlpha(image.interpretation()));
+      image = image.bandjoin_const(alpha);
     }
     return image;
   }

src/common.h

@@ -357,6 +357,12 @@ namespace sharp {
   */
   bool Is16Bit(VipsInterpretation const interpretation);
 
+  /*
+    Return the image alpha maximum. Useful for combining alpha bands. scRGB
+    images are 0 - 1 for image data, but the alpha is 0 - 255.
+  */
+  double MaximumImageAlpha(VipsInterpretation const interpretation);
+
   /*
     Convert RGBA value to another colourspace
   */

src/stats.cc

@@ -60,7 +60,7 @@ class StatsWorker : public Napi::AsyncWorker {
         // Image is not opaque when alpha layer is present and contains a non-mamixa value
         if (image.has_alpha()) {
           double const minAlpha = static_cast<double>(stats.getpoint(STAT_MIN_INDEX, bands).front());
-          if (minAlpha != vips_interpretation_max_alpha(image.interpretation())) {
+          if (minAlpha != sharp::MaximumImageAlpha(image.interpretation())) {
             baton->isOpaque = false;
           }
         }