Tinting a grayscale NSImage (or CIImage)

Question

I have a grayscale image which I want to use for drawing Cocoa controls. The image has various levels of gray. Where it is darkest, I want it to draw a specified tint color dark

Answer 1

Swift 5 version that also handles the alpha component of the tint color.

I use this to support dark mode with multiple icon states by converting template icons to different colors and transparency levels. For example, you could pass NSColor(white: 0, alpha: 0.5) to get a dimmed version of an icon for light mode, and NSColor(white: 1, alpha: 0.5) to get a dimmed version for dark mode.

func tintedImage(_ image: NSImage, color: NSColor) -> NSImage {
    let newImage = NSImage(size: image.size)
    newImage.lockFocus()

    // Draw with specified transparency
    let imageRect = NSRect(origin: .zero, size: image.size)
    image.draw(in: imageRect, from: imageRect, operation: .sourceOver, fraction: color.alphaComponent)

    // Tint with color
    color.withAlphaComponent(1).set()
    imageRect.fill(using: .sourceAtop)

    newImage.unlockFocus()
    return newImage
}