add jxlsave tests

and revise colour encoding set

see https://github.com/libvips/libvips/issues/2872

libvips/foreign/jxlsave.c

@@ -34,8 +34,8 @@
  */
 
 /*
- */
 #define DEBUG
+ */
 
 #ifdef HAVE_CONFIG_H
 #include <config.h>
@@ -360,11 +360,7 @@ vips_foreign_save_jxl_build( VipsObject *object )
 		return( -1 );
 	}
 
-	/* Set the colour encoding. Either ICC based, or sRGB. 
+	/* Set any ICC profile.
-	 *
-	 * libjxl will use linear 0 - 1 by default for float, so we
-	 * don't need to call JxlColorEncodingSetToLinearSRGB() or
-	 * JxlColorEncodingSetToSRGB().
 	 */
 	if( vips_image_get_typeof( in, VIPS_META_ICC_NAME ) ) {
 		const void *data;
@@ -384,6 +380,34 @@ vips_foreign_save_jxl_build( VipsObject *object )
 			return( -1 );
 		}
 	}
+	else {
+		/* If there's no ICC profile, we must set the colour encoding
+		 * ourselves.
+		 */
+		if( in->Type == VIPS_INTERPRETATION_scRGB ) {
+#ifdef DEBUG
+			printf( "setting scRGB colourspace\n" );
+#endif /*DEBUG*/
+
+			JxlColorEncodingSetToLinearSRGB( &jxl->color_encoding,
+				jxl->format.num_channels < 3 );
+		}
+		else {
+#ifdef DEBUG
+			printf( "setting sRGB colourspace\n" );
+#endif /*DEBUG*/
+
+			JxlColorEncodingSetToSRGB( &jxl->color_encoding,
+				jxl->format.num_channels < 3 );
+		}
+
+		if( JxlEncoderSetColorEncoding( jxl->encoder, 
+			&jxl->color_encoding ) ) {
+			vips_foreign_save_jxl_error( jxl,
+				"JxlEncoderSetColorEncoding" );
+			return( -1 );
+		}
+	}
 
 	/* Render the entire image in memory. libjxl seems to be missing
 	 * tile-based write at the moment.

test/test-suite/test_foreign.py

@@ -1375,6 +1375,51 @@ class TestForeign:
         # im2 = pyvips.Image.new_from_buffer(buf, "")
         # assert (im == im2).min() == 255
 
+    @skip_if_no("jxlsave")
+    def test_jxlsave(self):
+        # save and load with an icc profile
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              self.colour, 120)
+
+        # with no icc profile
+        no_profile = self.colour.copy()
+        no_profile.remove("icc-profile-data")
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              no_profile, 120)
+
+        # scrgb mode
+        scrgb = self.colour.colourspace("scrgb")
+        no_profile.remove("icc-profile-data")
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              scrgb, 120)
+
+        # scrgb mode, no profile
+        scrgb_no_profile = scrgb.copy()
+        scrgb_no_profile.remove("icc-profile-data")
+        no_profile.remove("icc-profile-data")
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              scrgb_no_profile, 120)
+
+        # 16-bit mode
+        rgb16 = self.colour.colourspace("rgb16")
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              rgb16, 30000)
+
+        # repeat for lossless mode
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              self.colour, 0, lossless=True)
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              no_profile, 0, lossless=True)
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              scrgb, 0, lossless=True)
+        self.save_load_buffer("jxlsave_buffer", "jxlload_buffer",
+                              scrgb_no_profile, 0, lossless=True)
+
+        # lossy should be much smaller than lossless
+        lossy = self.colour.jxlsave_buffer()
+        lossless = self.colour.jxlsave_buffer(lossless=True)
+        assert len(lossy) < len(lossless) / 5
+
     @skip_if_no("gifsave")
     def test_gifsave(self):
         # Animated GIF round trip