Skip normalise operation for images with one colour

It didn't play nicely with premultiplication

src/composite.c

@@ -128,11 +128,6 @@ int Composite(VipsObject *context, VipsImage *srcPremultiplied, VipsImage *dstPr
 int Premultiply(VipsObject *context, VipsImage *image, VipsImage **out) {
   VipsImage *imagePremultiplied;
 
-  // Trivial case: Copy images without alpha channel:
-  if (!HasAlpha(image)) {
-    return vips_image_write(image, *out);
-  }
-
 #if (VIPS_MAJOR_VERSION >= 9 || (VIPS_MAJOR_VERSION >= 8 && VIPS_MINOR_VERSION >= 1))
 
   if (vips_premultiply(image, &imagePremultiplied, NULL))
@@ -140,9 +135,6 @@ int Premultiply(VipsObject *context, VipsImage *image, VipsImage **out) {
 
 #else
 
-  if (image->Bands != 4)
-    return -1;
-
   VipsImage *imageRGB;
   if (vips_extract_band(image, &imageRGB, 0, "n", NUM_COLOR_BANDS, NULL))
     return -1;
@@ -181,11 +173,6 @@ int Premultiply(VipsObject *context, VipsImage *image, VipsImage **out) {
 int Unpremultiply(VipsObject *context, VipsImage *image, VipsImage **out) {
   VipsImage *imageUnpremultiplied;
 
-  // Trivial case: Copy images without alpha channel:
-  if (!HasAlpha(image)) {
-    return vips_image_write(image, *out);
-  }
-
 #if (VIPS_MAJOR_VERSION >= 9 || (VIPS_MAJOR_VERSION >= 8 && VIPS_MINOR_VERSION >= 1))
 
   if (vips_unpremultiply(image, &imageUnpremultiplied, NULL))
@@ -193,9 +180,6 @@ int Unpremultiply(VipsObject *context, VipsImage *image, VipsImage **out) {
 
 #else
 
-  if (image->Bands != 4)
-    return -1;
-
   VipsImage *imageRGBPremultipliedTransformed;
   if (vips_extract_band(image, &imageRGBPremultipliedTransformed, 0, "n", NUM_COLOR_BANDS, NULL))
     return -1;
@@ -219,7 +203,6 @@ int Unpremultiply(VipsObject *context, VipsImage *image, VipsImage **out) {
   if (vips_bandjoin2(imageRGBUnpremultipliedTransformed, imageAlphaTransformed, &imageUnpremultiplied, NULL))
     return -1;
 
-
 #endif
 
   // Return a reference to the unpremultiplied output image:

src/resize.cc

@@ -763,17 +763,10 @@ class ResizeWorker : public NanAsyncWorker {
         return Error();
       }
       vips_object_local(hook, stats);
+
       double min = *VIPS_MATRIX(stats, 0, 0);
       double max = *VIPS_MATRIX(stats, 1, 0);
-
+      if (min != max) {
-      VipsImage *normalized;
-      if (min == max) {
-        // Range of zero: create black image
-        if (vips_black(&normalized, image->Xsize, image->Ysize, "bands", 1, NULL )) {
-          return Error();
-        }
-        vips_object_local(hook, normalized);
-      } else {
         double f = 100.0 / (max - min);
         double a = -(min * f);
 
@@ -788,27 +781,29 @@ class ResizeWorker : public NanAsyncWorker {
           return Error();
         }
         vips_object_local(hook, normalizedLab);
+
+        VipsImage *normalized;
         if (vips_colourspace(normalizedLab, &normalized, typeBeforeNormalize, NULL)) {
           return Error();
         }
         vips_object_local(hook, normalized);
-      }
 
-      if (HasAlpha(image)) {
+        if (HasAlpha(image)) {
-        VipsImage *alpha;
+          VipsImage *alpha;
-        if (vips_extract_band(image, &alpha, image->Bands - 1, "n", 1, NULL)) {
+          if (vips_extract_band(image, &alpha, image->Bands - 1, "n", 1, NULL)) {
-          return Error();
+            return Error();
-        }
+          }
-        vips_object_local(hook, alpha);
+          vips_object_local(hook, alpha);
 
-        VipsImage *normalizedAlpha;
+          VipsImage *normalizedAlpha;
-        if (vips_bandjoin2(normalized, alpha, &normalizedAlpha, NULL)) {
+          if (vips_bandjoin2(normalized, alpha, &normalizedAlpha, NULL)) {
-          return Error();
+            return Error();
+          }
+          vips_object_local(hook, normalizedAlpha);
+          image = normalizedAlpha;
+        } else {
+          image = normalized;
         }
-        vips_object_local(hook, normalizedAlpha);
-        image = normalizedAlpha;
-      } else {
-        image = normalized;
       }
     }
 #endif

test/unit/normalize.js

@@ -98,31 +98,15 @@ describe('Normalization', function () {
         });
     });
 
-    it('returns a black image for images with only one color', function (done) {
+    it('does not alter images with only one color', function (done) {
+      var output = fixtures.path('output.unmodified-png-with-one-color.png');
       sharp(fixtures.inputPngWithOneColor)
         .normalize()
-        .toBuffer()
+        .toFile(output, function(err, info) {
-        .bind({})
+          if (err) done(err);
-        .then(function (imageData) {
+          fixtures.assertMaxColourDistance(output, fixtures.inputPngWithOneColor, 0);
-          this.imageData = imageData;
+          done();
-          return sharp(imageData)
+        });
-            .metadata();
-        })
-        .then(function (metadata) {
-          assert.strictEqual(false, metadata.hasAlpha);
-          assert.strictEqual(3, metadata.channels);
-          assert.strictEqual('srgb', metadata.space);
-        })
-        .then(function () {
-          return sharp(this.imageData)
-            .raw()
-            .toBuffer();
-        })
-        .then(function (rawData) {
-          var blackBuffer = new Buffer([0,0,0, 0,0,0, 0,0,0, 0,0,0]);
-          assert.strictEqual(blackBuffer.toString(), rawData.toString());
-        })
-        .finally(done);
     });
 
   }