Added simple inner-shadow drawing support

Bitmap.podspec

@@ -1,7 +1,7 @@
 Pod::Spec.new do |s|
 
 s.name                       = 'Bitmap'
-s.version                    = '1.1.0'
+s.version                    = '1.2.0'
 s.summary                    = 'A Swift-y convenience for loading, saving and manipulating bitmap images.'
 s.homepage                   = 'https://github.com/dagronf/Bitmap'
 s.license                    = { :type => 'MIT', :file => 'LICENSE' }

Sources/Bitmap/drawing/Bitmap+Shadow.swift

@@ -36,7 +36,11 @@ public extension Bitmap {
 			self.color = color
 		}
 	}
+}
+
+// MARK: - Shadows
 
+public extension Bitmap {
 	/// Apply a shadow to a drawing block
 	/// - Parameters:
 	///   - shadow: The shadow style
@@ -48,3 +52,41 @@ public extension Bitmap {
 		}
 	}
 }
+
+// MARK: - Inner shadows
+
+public extension Bitmap {
+	/// Draw a path using an inner shadow
+	/// - Parameters:
+	///   - path: The path
+	///   - fillColor: The color to fill the path, or nil for no color
+	///   - shadow: The shadow definition
+	func drawInnerShadow(_ path: CGPath, fillColor: CGColor? = nil, shadow: Bitmap.Shadow) {
+		self.draw { ctx in
+			if let fillColor = fillColor {
+				ctx.setFillColor(fillColor)
+				ctx.addPath(path)
+				ctx.fillPath()
+			}
+
+			ctx.drawInnerShadow(
+				in: path,
+				shadowColor: shadow.color,
+				offset: shadow.offset,
+				blurRadius: shadow.blur
+			)
+		}
+	}
+
+	/// Create a new bitmap by drawing a path using an inner shadow
+	/// - Parameters:
+	///   - path: The path
+	///   - fillColor: The color to fill the path, or nil for no color
+	///   - shadow: The shadow definition
+	/// - Returns: A new bitmap
+	func drawingInnerShadow(_ path: CGPath, fillColor: CGColor? = nil, shadow: Bitmap.Shadow) throws -> Bitmap {
+		let copy = try self.copy()
+		copy.drawInnerShadow(path, fillColor: fillColor, shadow: shadow)
+		return copy
+	}
+}

Sources/Bitmap/utils/CGContext+extensions.swift

@@ -27,4 +27,11 @@ extension CGContext {
 		defer { self.restoreGState() }
 		try drawBlock(self)
 	}
+
+	/// Wrap the drawing commands in `block` within a transparency layer
+	@inlinable func usingTransparencyLayer(auxiliaryInfo: CFDictionary? = nil, _ block: () -> Void) {
+		self.beginTransparencyLayer(auxiliaryInfo: auxiliaryInfo)
+		defer { self.endTransparencyLayer() }
+		block()
+	}
 }

Sources/Bitmap/utils/CGContext+innerShadow.swift

@@ -0,0 +1,60 @@
+//
+//  Copyright © 2024 Darren Ford. All rights reserved.
+//
+//  MIT License
+//
+//  Permission is hereby granted, free of charge, to any person obtaining a copy
+//  of this software and associated documentation files (the "Software"), to deal
+//  in the Software without restriction, including without limitation the rights
+//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+//  copies of the Software, and to permit persons to whom the Software is
+//  furnished to do so, subject to the following conditions:
+//
+//  The above copyright notice and this permission notice shall be included in all
+//  copies or substantial portions of the Software.
+//
+//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+//  SOFTWARE.
+
+import CoreGraphics
+import Foundation
+
+public extension CGContext {
+	/// Draw an inner shadow in the path
+	/// - Parameters:
+	///   - path: The path to apply the inner shadow to
+	///   - shadowColor: Specifies the color of the shadow, which may contain a non-opaque alpha value. If NULL, then shadowing is disabled.
+	///   - offset: Specifies a translation in base-space.
+	///   - blurRadius: A non-negative number specifying the amount of blur.
+	///
+	/// **Inner Shadows in Quartz: Helftone**
+	/// [Blog Article](https://blog.helftone.com/demystifying-inner-shadows-in-quartz/)
+	/// [(Archived)](https://web.archive.org/web/20221206132428/https://blog.helftone.com/demystifying-inner-shadows-in-quartz/)
+	func drawInnerShadow(in path: CGPath, shadowColor: CGColor?, offset: CGSize, blurRadius: CGFloat) {
+		guard
+			let shadowColor = shadowColor,
+			let opaqueShadowColor = shadowColor.copy(alpha: 1.0)
+		else {
+			// No shadow color specified, therefore no shadow.
+			return
+		}
+
+		self.savingGState { ctx in
+			ctx.addPath(path)
+			ctx.clip()
+			ctx.setAlpha(shadowColor.alpha)
+			ctx.usingTransparencyLayer {
+				ctx.setShadow(offset: offset, blur: blurRadius, color: opaqueShadowColor)
+				ctx.setBlendMode(.sourceOut)
+				ctx.setFillColor(opaqueShadowColor)
+				ctx.addPath(path)
+				ctx.fillPath()
+			}
+		}
+	}
+}

Tests/BitmapTests/BitmapTests.swift

@@ -255,20 +255,35 @@ final class BitmapTests: XCTestCase {
 	func testShadow() throws {
 		markdown.h2("Shadow drawing")
 
-		let bitmap = try Bitmap(width: 255, height: 255)
-		bitmap.applyingShadow(Bitmap.Shadow()) { bitmap in
-			bitmap.fill(CGRect(x: 10, y: 10, width: 100, height: 100).path, .init(gray: 0.5, alpha: 1))
+		do {
+			let bitmap = try Bitmap(width: 255, height: 255)
+			bitmap.applyingShadow(Bitmap.Shadow()) { bitmap in
+				bitmap.fill(CGRect(x: 10, y: 10, width: 100, height: 100).path, .init(gray: 0.5, alpha: 1))
+			}
+
+			bitmap.applyingShadow(Bitmap.Shadow(offset: CGSize(width: -3, height: 3), color: CGColor.blue)) { bitmap in
+				bitmap.stroke(
+					CGRect(x: 110, y: 110, width: 100, height: 100).path,
+					Bitmap.Stroke(color: CGColor.red, lineWidth: 2)
+				)
+			}
+
+			let image = try XCTUnwrap(bitmap.cgImage)
+			try markdown.image(image, linked: true)
 		}
 
-		bitmap.applyingShadow(Bitmap.Shadow(offset: CGSize(width: -3, height: 3), color: CGColor.blue)) { bitmap in
-			bitmap.stroke(
-				CGRect(x: 110, y: 110, width: 100, height: 100).path,
-				Bitmap.Stroke(color: CGColor.red, lineWidth: 2)
-			)
+		markdown.br()
+
+		do {
+			let bitmap = try Bitmap(width: 300, height: 300, backgroundColor: CGColor.white)
+			let path = CGPath(roundedRect: CGRect(x: 20, y: 20, width: 260, height: 260), cornerWidth: 10, cornerHeight: 10, transform: nil)
+			let shadow = Bitmap.Shadow(offset: .init(width: 4, height: -4), blur: 20, color: .blue)
+			let b = try bitmap.drawingInnerShadow(path, fillColor: CGColor(red: 1, green: 1, blue: 0.4, alpha: 1), shadow: shadow)
+
+			let image = try XCTUnwrap(b.cgImage)
+			try markdown.image(image, width: 150, linked: true)
 		}
 
-		let image = try XCTUnwrap(bitmap.cgImage)
-		try markdown.image(image, linked: true)
 		markdown.br()
 	}