second commit

GreenshotMac/Core/ImageExporter.swift

@@ -76,6 +76,38 @@ enum ImageExporter {
         return url
     }
 
+    /// Reads a single pixel from an image. `point` is in image points with a top-left
+    /// origin; `scale` converts those points to pixels.
+    static func color(at point: CGPoint, in image: CGImage, scale: CGFloat) -> NSColor? {
+        let x = Int((point.x * scale).rounded(.down))
+        let y = Int((point.y * scale).rounded(.down))
+        guard x >= 0, y >= 0, x < image.width, y < image.height else { return nil }
+
+        var pixel: [UInt8] = [0, 0, 0, 0]
+        guard let context = CGContext(data: &pixel,
+                                      width: 1,
+                                      height: 1,
+                                      bitsPerComponent: 8,
+                                      bytesPerRow: 4,
+                                      space: CGColorSpace(name: CGColorSpace.sRGB) ?? CGColorSpaceCreateDeviceRGB(),
+                                      bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)
+        else { return nil }
+
+        // Draw the image shifted so the pixel of interest lands in the 1x1 context.
+        context.draw(image,
+                     in: CGRect(x: -CGFloat(x),
+                                y: -CGFloat(image.height - 1 - y),
+                                width: CGFloat(image.width),
+                                height: CGFloat(image.height)))
+
+        let alpha = CGFloat(pixel[3]) / 255
+        guard alpha > 0 else { return NSColor.clear }
+        return NSColor(srgbRed: CGFloat(pixel[0]) / 255 / alpha,
+                       green: CGFloat(pixel[1]) / 255 / alpha,
+                       blue: CGFloat(pixel[2]) / 255 / alpha,
+                       alpha: 1)
+    }
+
     static func format(forPathExtension pathExtension: String) -> OutputFormat {
         switch pathExtension.lowercased() {
         case "jpg", "jpeg": return .jpg