Excerpt from circa-2015 implementation of early rendering compositor (no layers)

RLCompositor.m

// Objective-C snippet circa 2015

CMTime frameTime = CMTimeMake(timescale / self.frameRate, timescale);
NSUInteger frameStride = imageFilePaths.count;
NSUInteger maxIndex = frameStride - 1;
NSUInteger loopStride = frameStride * ((self.mirrorSequence) ? 2 : 1);

NSMutableArray *pixelBuffers = [NSMutableArray arrayWithCapacity:imageFilePaths.count];
NSUInteger frameCount = loopStride * self.loopCount;
for (int i = 0; i < frameStride; ++i) {
    NSString *imageFileName = imageFilePaths[i];
    NSImage *image = [[NSImage alloc] initWithContentsOfFile:
                      [rootDirectory stringByAppendingPathComponent:imageFileName]];

    size_t bitsPerComponent = 8;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
    CGBitmapInfo bitmapInfo = (CGBitmapInfo)kCGImageAlphaNoneSkipFirst;

    CVPixelBufferRef pixelBuffer = nil;
    CVPixelBufferPoolCreatePixelBuffer(kCFAllocatorDefault,
                                       adaptor.pixelBufferPool,
                                       &pixelBuffer);
    assert(pixelBuffer);
    CVPixelBufferLockBaseAddress(pixelBuffer, 0);
    void *pixelData = CVPixelBufferGetBaseAddress(pixelBuffer);
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(pixelBuffer);
    CGContextRef cgContext = CGBitmapContextCreate(pixelData,
                                                   self.frameWidth,
                                                   self.frameHeight,
                                                   bitsPerComponent,
                                                   bytesPerRow,
                                                   colorSpace,
                                                   bitmapInfo);
    assert(cgContext);

    NSGraphicsContext *gfxContext = 
        [NSGraphicsContext graphicsContextWithGraphicsPort:cgContext flipped:NO];
    [NSGraphicsContext saveGraphicsState];
    [NSGraphicsContext setCurrentContext:gfxContext];
    [image drawInRect:NSMakeRect(0, 0, self.frameWidth, self.frameHeight)
             fromRect:NSZeroRect
            operation:NSCompositeCopy
             fraction:1.0];
    [NSGraphicsContext restoreGraphicsState];

    CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);
    [pixelBuffers addObject:(__bridge id _Nonnull)(pixelBuffer)];
}