On OS X, take repeated screenshots / screengrabs of a certain window, scale, and save as RGBA constantly.

grabscreen.m

//
//  main.m
//  grabscreen
//
//  Created by Peter Cooper on 09/10/2016.
//  Copyright © 2016 Peter Cooper. All rights reserved.
//

#import <Cocoa/Cocoa.h>
#include <ApplicationServices/ApplicationServices.h>
#import <CoreGraphics/CoreGraphics.h>
#import <QuartzCore/QuartzCore.h>

int main(int argc, const char * argv[]) {
    int destinationWidth = 1280;
    int destinationHeight = 748;

    NSString *path = @"/tmp/xbox.rgba";
    NSURL *fileURL = [NSURL fileURLWithPath:path];

    int windowId = 11367;
    if (argc > 1)
        windowId = atoi(argv[1]);
    
    while(1) {
        NSLog(@"grabbing frame");
        CGImageRef image = CGWindowListCreateImage(CGRectNull, kCGWindowListOptionIncludingWindow, windowId, kCGWindowImageBoundsIgnoreFraming);

        CGContextRef context = CGBitmapContextCreate(NULL, destinationWidth, destinationHeight, 8, 0, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaPremultipliedFirst);
            
        CGContextDrawImage(context, CGRectMake(0, 0, destinationWidth, destinationHeight), image);
            
        char *data = CGBitmapContextGetData (context);

        int size = destinationWidth * destinationHeight * 4;
                
        // switch ARGB to RGBA (with full opacity)
        for (int p = 0; p <= size; p += 4) {
            data[p] = data[p + 1];
            data[p + 1] = data[p + 2];
            data[p + 2] = data[p + 3];
            data[p + 3] = 0xFF;
        }
        
        NSData* d2 = [NSData dataWithBytes:(const void *)data length: size];
            
        CGContextRelease(context);
            
        [d2 writeToURL: fileURL atomically: NO];
        
        CFRelease(image);  // this took a while to realize, lol
        
        [NSThread sleepForTimeInterval:0.05f];
    }
    
    return 0;
}


// odds and ends!

//NSString *path = [NSString stringWithFormat:@"%@/%@", NSHomeDirectory() , @"lincolnhack/a.png"];
// CFArrayRef windowList = CGWindowListCopyWindowInfo(kCGWindowListOptionOnScreenOnly, kCGNullWindowID);

// -- this will write a PNG instead (now unused)
//CGImageRef imgRef = CGBitmapContextCreateImage(context);
//CGImageDestinationRef dr = CGImageDestinationCreateWithURL((__bridge CFURLRef)fileURL, kUTTypePNG , 1, NULL);
//CGImageDestinationAddImage(dr, imgRef, NULL);
//CGImageDestinationFinalize(dr);
//CFRelease(dr);