Read Pixel Under Cursor Mac

main.m

#import <Foundation/Foundation.h>
#import <Cocoa/Cocoa.h>

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        
        // Grab the current mouse location.
        
        CGPoint mouseLoc = CGEventGetLocation(CGEventCreate(NULL));
        
        
//        NSPoint mouseLoc = [NSEvent mouseLocation];
//        CGPoint mouseLocation = CGEventGetLocation(CGEventCreate(NULL));
        
        
        // Grab the display for said mouse location.
        uint32_t count = 0;
        CGDirectDisplayID displayForPoint;
        if (CGGetDisplaysWithPoint(mouseLoc, 1, &displayForPoint, &count) != kCGErrorSuccess)
        {
            NSLog(@"Oops.");
            return 0;
        }
        
        // Grab the color on said display at said mouse location.
        CGImageRef image = CGDisplayCreateImageForRect(displayForPoint, CGRectMake(mouseLoc.x, mouseLoc.y - 2, 1, 1));
        NSBitmapImageRep* bitmap = [[NSBitmapImageRep alloc] initWithCGImage:image];
        CGImageRelease(image);
        NSColor* color = [bitmap colorAtX:0 y:0];

        NSLog(@"%@", color);
//        [bitmap release];
    }
    return 0;
}

readpixel.swift

//: Playground - noun: a place where people can play

import Cocoa

var mouseLoc = CGEventGetLocation(CGEventCreate(nil).takeRetainedValue())


var display = UnsafeMutablePointer<CGDirectDisplayID>.alloc(1)
var count = UnsafeMutablePointer<UInt32>.alloc(1);
CGGetDisplaysWithPoint(mouseLoc, 1, display, count)

let windowList = CGWindowListCopyWindowInfo(kCGWindowListOptionOnScreenOnly | kCGWindowListExcludeDesktopElements, CGWindowID.allZeros)


//CGWindowListCreateImage(screenBounds: CGRect, listOption: CGWindowListOption, <#windowID: CGWindowID#>, <#imageOption: CGWindowImageOption#>)


var image = CGDisplayCreateImageForRect(display.memory, CGRectMake(mouseLoc.x, mouseLoc.y, 100, 100)).takeRetainedValue()

//var image = CGWindowListCreateImage(CGRectInfinite, nil, <#windowID: CGWindowID#>, <#imageOption: CGWindowImageOption#>)


let dest = CGImageDestinationCreateWithURL(NSURL(fileURLWithPath: "merp.png"), kUTTypePNG, 1, nil)
CGImageDestinationAddImage(dest, image, nil)
CGImageDestinationFinalize(dest)

//https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSBitmapImageRep_Class/index.html#//apple_ref/occ/instm/NSBitmapImageRep/initWithCGImage:
var bitmap = NSBitmapImageRep(CGImage: image)
bitmap.colorAtX(0, y: 0)
display.dealloc(1)
count.dealloc(1)

//setBrightnessLevel(0.9)

//http://bl.ocks.org/mbostock/981b42034400e48ac637
//var image = CGDisplayCreateImageForRect(
//http://www.alecjacobson.com/weblog/?p=1127

private func setBrightnessLevel(level: Float) {
    var iterator: io_iterator_t = 0
    
    let result = IOServiceGetMatchingServices(kIOMasterPortDefault,
        IOServiceMatching("IODisplayConnect").takeUnretainedValue(),
        &iterator)
    
    if result == kIOReturnSuccess {
        var service: io_object_t = 1
        
        for ;; {
            service = IOIteratorNext(iterator)
            
            if service == 0 {
                break
            }
            
            IODisplaySetFloatParameter(service, 0, kIODisplayBrightnessKey, level)
            IOObjectRelease(service)
        }
    }
}

hi https://github.com/osnr/pixels-under-mouse

Update for Swift 5.8

import AppKit

class Agent {

  func readPixelUnderMouse() throws -> NSColor {
    var mouseLoc = NSEvent.mouseLocation
    // Cocoa and Core Graphics (Quartz) use different coordinate systems. So need to flip y.
    mouseLoc.y = NSHeight(NSScreen.screens[0].frame) - mouseLoc.y
    print(mouseLoc)

    let display = UnsafeMutablePointer<CGDirectDisplayID>.allocate(capacity: 1)
    let count = UnsafeMutablePointer<UInt32>.allocate(capacity: 1)
    CGGetDisplaysWithPoint(mouseLoc, 1, display, count)

    guard let image = CGDisplayCreateImage(display.pointee, rect: CGRectMake(mouseLoc.x, mouseLoc.y, 1, 1)) else { throw NSError() }

    let bitmap = NSBitmapImageRep(cgImage: image)
    guard let color = bitmap.colorAt(x: 0, y: 0) else { throw NSError() }
    display.deallocate()
    count.deallocate()

    return color
  }

}