Ensure 16-bit+alpha input images work with vips_premultiply #301

Improves SVG support as *magick serves these as 16-bit

Add automated tests for SVG and 16-bit+alpha PNG inputs

src/pipeline.cc

@@ -437,17 +437,22 @@ class PipelineWorker : public AsyncWorker {
       image = transformed;
     }
 
+    // Calculate maximum alpha value based on input image pixel depth
+    double maxAlpha = (image->BandFmt == VIPS_FORMAT_USHORT) ? 65535.0 : 255.0;
+
     // Flatten image to remove alpha channel
     if (baton->flatten && HasAlpha(image)) {
+      // Scale up 8-bit values to match 16-bit input image
+      double multiplier = (image->Type == VIPS_INTERPRETATION_RGB16) ? 256.0 : 1.0;
       // Background colour
       VipsArrayDouble *background = vips_array_double_newv(
         3, // Ignore alpha channel as we're about to remove it
-        baton->background[0],
-        baton->background[1],
-        baton->background[2]
+        baton->background[0] * multiplier,
+        baton->background[1] * multiplier,
+        baton->background[2] * multiplier
       );
       VipsImage *flattened;
-      if (vips_flatten(image, &flattened, "background", background, nullptr)) {
+      if (vips_flatten(image, &flattened, "background", background, "max_alpha", maxAlpha, nullptr)) {
         vips_area_unref(reinterpret_cast<VipsArea*>(background));
         return Error();
       }
@@ -526,7 +531,7 @@ class PipelineWorker : public AsyncWorker {
     // See: http://entropymine.com/imageworsener/resizealpha/
     if (shouldPremultiplyAlpha) {
       VipsImage *imagePremultiplied;
-      if (vips_premultiply(image, &imagePremultiplied, nullptr)) {
+      if (vips_premultiply(image, &imagePremultiplied, "max_alpha", maxAlpha, nullptr)) {
         (baton->err).append("Failed to premultiply alpha channel.");
         return Error();
       }
@@ -792,7 +797,7 @@ class PipelineWorker : public AsyncWorker {
     // Reverse premultiplication after all transformations:
     if (shouldPremultiplyAlpha) {
       VipsImage *imageUnpremultiplied;
-      if (vips_unpremultiply(image, &imageUnpremultiplied, nullptr)) {
+      if (vips_unpremultiply(image, &imageUnpremultiplied, "max_alpha", maxAlpha, nullptr)) {
         (baton->err).append("Failed to unpremultiply alpha channel");
         return Error();
       }
@@ -820,7 +825,24 @@ class PipelineWorker : public AsyncWorker {
     }
 
     // Convert image to sRGB, if not already
-    if (image->Type != VIPS_INTERPRETATION_sRGB) {
+    if (image->Type == VIPS_INTERPRETATION_RGB16) {
+      // Ensure 16-bit integer
+      VipsImage *ushort;
+      if (vips_cast_ushort(image, &ushort, nullptr)) {
+        return Error();
+      }
+      vips_object_local(hook, ushort);
+      image = ushort;
+      // Fast conversion to 8-bit integer by discarding least-significant byte
+      VipsImage *msb;
+      if (vips_msb(image, &msb, nullptr)) {
+        return Error();
+      }
+      vips_object_local(hook, msb);
+      image = msb;
+      // Explicitly set to sRGB
+      image->Type = VIPS_INTERPRETATION_sRGB;
+    } else if (image->Type != VIPS_INTERPRETATION_sRGB) {
       // Switch interpretation to sRGB
       VipsImage *rgb;
       if (vips_colourspace(image, &rgb, VIPS_INTERPRETATION_sRGB, nullptr)) {

test/fixtures/index.js

@@ -71,6 +71,7 @@ module.exports = {
   inputPngWithTransparency: getPath('blackbug.png'), // public domain
   inputPngWithGreyAlpha: getPath('grey-8bit-alpha.png'),
   inputPngWithOneColor: getPath('2x2_fdcce6.png'),
+  inputPngWithTransparency16bit: getPath('tbgn2c16.png'), // http://www.schaik.com/pngsuite/tbgn2c16.png
   inputPngOverlayLayer0: getPath('alpha-layer-0-background.png'),
   inputPngOverlayLayer1: getPath('alpha-layer-1-fill.png'),
   inputPngOverlayLayer2: getPath('alpha-layer-2-ink.png'),

test/unit/alpha.js

@@ -46,6 +46,19 @@ describe('Alpha transparency', function() {
       });
   });
 
+  it('Flatten 16-bit PNG with transparency to orange', function(done) {
+    sharp(fixtures.inputPngWithTransparency16bit)
+      .flatten()
+      .background({r: 255, g: 102, b: 0})
+      .toBuffer(function(err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(true, info.size > 0);
+        assert.strictEqual(32, info.width);
+        assert.strictEqual(32, info.height);
+        fixtures.assertSimilar(fixtures.expected('flatten-rgb16-orange.jpg'), data, done);
+      });
+  });
+
   it('Do not flatten', function(done) {
     sharp(fixtures.inputPngWithTransparency)
       .flatten(false)

test/unit/io.js

@@ -638,16 +638,17 @@ describe('Input/output', function() {
       sharp(fixtures.inputSvg)
         .resize(100, 100)
         .toFormat('png')
-        .toFile(fixtures.path('output.svg.png'), function(err, info) {
+        .toBuffer(function(err, data, info) {
           if (err) {
             assert.strictEqual(0, err.message.indexOf('Input file is of an unsupported image format'));
+            done();
           } else {
             assert.strictEqual(true, info.size > 0);
             assert.strictEqual('png', info.format);
             assert.strictEqual(100, info.width);
             assert.strictEqual(100, info.height);
+            fixtures.assertSimilar(fixtures.expected('svg.png'), data, done);
           }
-          done();
         });
     });
   }