Fixes CGImage extension method for colour space converting copying (#23)

* Fixes colour space conversion with an Accelerate (vImage) backed implementation

* Only convert color space if needed

* Some cleanup

Package.swift

@@ -43,6 +43,7 @@ let package = Package(
                 .process("iphone5.jpg"),
                 .process("Pixls/DNG/hdrmerge-bayer-fp16-w-pred-deflate_.dng"),
                 .process("Pixls/X3F/DP2M1726_.X3F"),
+                .process("outline-invert_2x.png")
             ]
         ),
     ]

Sources/Carpaccio/CGImage+Extensions.swift

@@ -9,6 +9,7 @@
 import Foundation
 import CoreGraphics
 import ImageIO
+import Accelerate
 
 #if os(iOS)
 import MobileCoreServices
@@ -106,7 +107,7 @@ public extension CGImage {
             throw CGImageExtensionError.failedToLoadCGImage
         }
 
-        if let colorSpace = colorSpace {
+        if let colorSpace = colorSpace, colorSpace != cgImage.colorSpace {
             return try cgImage.convertedToColorSpace(colorSpace)
         }
         return cgImage
@@ -157,10 +158,39 @@ public extension CGImage {
         return pngData
     }
 
-    func convertedToColorSpace(_ colorSpace: CGColorSpace) throws -> CGImage {
-        guard let convertedImage = self.copy(colorSpace: colorSpace) else {
-            throw CGImageExtensionError.failedToConvertColorSpace
+    /// Return a copy of the image converted to the target color space.
+    ///
+    /// Callee is responsible for checking that the colour space of the source image is not already the target color space.
+    func convertedToColorSpace(_ colorSpace: CGColorSpace, renderingIntent: CGColorRenderingIntent = .defaultIntent)
+    throws -> CGImage {
+        precondition(self.colorSpace != colorSpace)
+        guard
+            let sourceImageFormat = vImage_CGImageFormat(cgImage: self),
+            let destinationImageFormat = vImage_CGImageFormat(
+                // TODO: Generalise this to work with arbitrary color space bit depths
+                bitsPerComponent: 8,
+                // + 1 for alpha (TODO: generalise over alpha / non-alpha channeled)
+                bitsPerPixel: 8 * (colorSpace.numberOfComponents + 1),
+                colorSpace: colorSpace,
+                bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.first.rawValue),
+                renderingIntent: renderingIntent) else {
+                    throw CGImageExtensionError.failedToConvertColorSpace
+                }
+
+        let sourceBuffer = try vImage_Buffer(cgImage: self)
+        var destinationBuffer = try vImage_Buffer(width: Int(sourceBuffer.width),
+                                                     height: Int(sourceBuffer.height),
+                                                     bitsPerPixel: destinationImageFormat.bitsPerPixel)
+
+        let converter = try vImageConverter.make(sourceFormat: sourceImageFormat,
+                                                          destinationFormat: destinationImageFormat)
+        try converter.convert(source: sourceBuffer, destination: &destinationBuffer)
+
+        defer {
+            sourceBuffer.free()
+            destinationBuffer.free()
         }
-        return convertedImage
+
+        return try destinationBuffer.createCGImage(format: destinationImageFormat)
     }
 }

Sources/Carpaccio/ImageLoader.swift

@@ -241,8 +241,11 @@ public class ImageLoader: ImageLoaderProtocol, URLBackedImageLoaderProtocol {
 
             try stopIfCancelled(cancelChecker, "Before converting color space of thumbnail image")
 
-            let image = try thumbnail.convertedToColorSpace(colorSpace)
-            return image
+            if thumbnail.colorSpace != colorSpace {
+                let image = try thumbnail.convertedToColorSpace(colorSpace)
+                return image
+            }
+            return thumbnail
         }()
 
         // Crop letterboxing out, if needed

Tests/CarpaccioTests/CarpaccioTests.swift

@@ -296,15 +296,38 @@ class CarpaccioTests: XCTestCase {
         XCTAssertEqual(map[image3], "walrus")
 
         // Simulate moving images to a Keepers subdirectory
-        let newURL1 = originalURL1.deletingLastPathComponent().appendingPathComponent("Keepers").appendingPathComponent(originalURL1.lastPathComponent)
+        let newURL1 = originalURL1
+            .deletingLastPathComponent()
+            .appendingPathComponent("Keepers")
+            .appendingPathComponent(originalURL1.lastPathComponent)
         image1.updateURL(newURL1)
-        let newURL2 = originalURL1.deletingLastPathComponent().appendingPathComponent("Keepers").appendingPathComponent(originalURL2.lastPathComponent)
+
+        let newURL2 = originalURL1
+            .deletingLastPathComponent()
+            .appendingPathComponent("Keepers")
+            .appendingPathComponent(originalURL2.lastPathComponent)
         image2.updateURL(newURL2)
-        let newURL3 = originalURL1.deletingLastPathComponent().appendingPathComponent("Keepers").appendingPathComponent(originalURL3.lastPathComponent)
+
+        let newURL3 = originalURL1.deletingLastPathComponent()
+            .appendingPathComponent("Keepers")
+            .appendingPathComponent(originalURL3.lastPathComponent)
         image3.updateURL(newURL3)
 
         XCTAssertEqual(map[image1], "cat")
         XCTAssertEqual(map[image2], "dog")
         XCTAssertEqual(map[image3], "walrus")
     }
+
+    func testColorSpaceConversion() throws {
+        guard let url = Bundle.module.url(forResource: "outline-invert_2x", withExtension: "png") else {
+            XCTAssert(false)
+            return
+        }
+
+        let dataProvider = CGDataProvider(filename: url.path)!
+        let image = CGImage(pngDataProviderSource: dataProvider, decode: nil, shouldInterpolate: true, intent: .defaultIntent)!
+        XCTAssertEqual(image.colorSpace!.name, CGColorSpace.genericGrayGamma2_2)
+        let convertedImage = try image.convertedToColorSpace(CGColorSpace(name: CGColorSpace.sRGB)!, renderingIntent: .perceptual)
+        XCTAssertEqual(convertedImage.colorSpace!.name, CGColorSpace.sRGB)
+    }
 }