Simplify ICC processing when retaining input profiles #4468

Takes advantage of libvips' improved ICC handling

docs/src/content/docs/api-output.md

@@ -201,7 +201,7 @@ const dataWithMergedExif = await sharp(inputWithExif)
 
 Keep ICC profile from the input image in the output image.
 
-For non-RGB output use [toColourspace](/api-colour/#tocolourspace).
+When input and output colour spaces differ, use with [toColourspace](/api-colour/#tocolourspace) and optionally [pipelineColourspace](/api-colour/#pipelinecolourspace).
 
 
 **Since**: 0.33.0  
@@ -211,6 +211,14 @@ const outputWithIccProfile = await sharp(inputWithIccProfile)
   .keepIccProfile()
   .toBuffer();
 ```
+**Example**  
+```js
+const cmykOutputWithIccProfile = await sharp(cmykInputWithIccProfile)
+  .pipelineColourspace('cmyk')
+  .toColourspace('cmyk')
+  .keepIccProfile()
+  .toBuffer();
+```
 
 
 ## withIccProfile

docs/src/content/docs/changelog/v0.34.5.md

@@ -16,3 +16,6 @@ slug: changelog/v0.34.5
 
 * Improve error messaging when only warnings issued.
   [#4465](https://github.com/lovell/sharp/issues/4465)
+
+* Simplify ICC processing when retaining input profiles.
+  [#4468](https://github.com/lovell/sharp/issues/4468)

lib/output.js

@@ -256,7 +256,7 @@ function withExifMerge (exif) {
 /**
  * Keep ICC profile from the input image in the output image.
  *
- * For non-RGB output use {@link /api-colour/#tocolourspace toColourspace}.
+ * When input and output colour spaces differ, use with {@link /api-colour/#tocolourspace toColourspace} and optionally {@link /api-colour/#pipelinecolourspace pipelineColourspace}.
  *
  * @since 0.33.0
  *
@@ -265,6 +265,13 @@ function withExifMerge (exif) {
  *   .keepIccProfile()
  *   .toBuffer();
  *
+ * @example
+ * const cmykOutputWithIccProfile = await sharp(cmykInputWithIccProfile)
+ *   .pipelineColourspace('cmyk')
+ *   .toColourspace('cmyk')
+ *   .keepIccProfile()
+ *   .toBuffer();
+ *
  * @returns {Sharp}
  */
 function keepIccProfile () {

src/pipeline.cc

@@ -797,20 +797,14 @@ class PipelineWorker : public Napi::AsyncWorker {
         image = sharp::EnsureAlpha(image, baton->ensureAlpha);
       }
 
-      // Convert image to sRGB, if not already
+      // Ensure output colour space
       if (sharp::Is16Bit(image.interpretation())) {
         image = image.cast(VIPS_FORMAT_USHORT);
       }
       if (image.interpretation() != baton->colourspace) {
-        // Convert colourspace, pass the current known interpretation so libvips doesn't have to guess
         image = image.colourspace(baton->colourspace, VImage::option()->set("source_space", image.interpretation()));
-        // Transform colours from embedded profile to output profile
-        if ((baton->keepMetadata & VIPS_FOREIGN_KEEP_ICC) && baton->colourspacePipeline != VIPS_INTERPRETATION_CMYK &&
-          baton->withIccProfile.empty() && sharp::HasProfile(image)) {
-          image = image.icc_transform(processingProfile, VImage::option()
-            ->set("embedded", true)
-            ->set("depth", sharp::Is16Bit(image.interpretation()) ? 16 : 8)
-            ->set("intent", VIPS_INTENT_PERCEPTUAL));
+        if (inputProfile.first && baton->withIccProfile.empty()) {
+          image = sharp::SetProfile(image, inputProfile);
         }
       }
 
@@ -845,8 +839,6 @@ class PipelineWorker : public Napi::AsyncWorker {
         } catch(...) {
           sharp::VipsWarningCallback(nullptr, G_LOG_LEVEL_WARNING, "Invalid profile", nullptr);
         }
-      } else if (baton->keepMetadata & VIPS_FOREIGN_KEEP_ICC) {
-        image = sharp::SetProfile(image, inputProfile);
       }
 
       // Negate the colours in the image

test/unit/metadata.js

@@ -651,16 +651,27 @@ describe('Image metadata', () => {
     assert.strictEqual(description, 'Generic RGB Profile');
   });
 
-  it('keep existing ICC profile, ignore colourspace conversion', async () => {
+  it('keep existing CMYK input profile for CMYK output', async () => {
+    const data = await sharp(fixtures.inputJpgWithCmykProfile)
+      .keepIccProfile()
+      .toColourspace('cmyk')
+      .toBuffer();
+
+    const metadata = await sharp(data).metadata();
+    assert.strictEqual(metadata.channels, 4);
+    const { description } = icc.parse(metadata.icc);
+    assert.strictEqual(description, 'U.S. Web Coated (SWOP) v2');
+  });
+
+  it('transform with but discard existing RGB input profile for CMYK output', async () => {
     const data = await sharp(fixtures.inputJpgWithExif)
       .keepIccProfile()
       .toColourspace('cmyk')
       .toBuffer();
 
     const metadata = await sharp(data).metadata();
-    assert.strictEqual(metadata.channels, 3);
-    const { description } = icc.parse(metadata.icc);
-    assert.strictEqual(description, 'Generic RGB Profile');
+    assert.strictEqual(metadata.channels, 4);
+    assert.strictEqual(metadata.icc, undefined);
   });
 
   it('keep existing ICC profile, avoid colour transform', async () => {