Continually scrolling dynamic bitmap in Core Graphics - swift

Question

I have been struggling with a performance related issue for a simple Cocoa based visualization program, that is continually drawing a heat map as a bitmap and displaing the bitmap on screen. Broadly, the heat map is a continually scrolling source of realtime information. At each redraw, new pixels are added at the right side of the bitmap and then the bitmap is shifted to the left, creating an on-going flow of information.
My current approach is implemented in a custom NVSView, with a circular buffer for the underlying pixels. The image is actually drawn rotated 90 degrees, so adding new data is simply a matter of appending the buffer (adding rows to the image). I then use a CGDataProviderCreateWithData to create a CoreGraphics image and draw that to a rotated context. (The code is below, although is less important to the question.)
My hope is to figure out a more performant way of achieving this. It seems like redrawing the full bitmap each time is excessive, and in my attempts, it uses a surprisingly high amount of CPU (~20-30%). I feel like there should be a way to somehow instruct the GPU to cache, but shift the existing pixels and then append new data.
I am considering an approach that will use two CGBitmapContextCreate, one context for what is currently on the screen and one context for modification. Prior pixels would be copied from one context to the other context, but I am not sure that will improve performance significantly. Before I proceed too far down that rabbit hole, are there better ways to handle such updating?
Here is the relevant code from my current implementation, although I think I am more in need of higher level guidance:
class PlotView: NSView
{
private let bytesPerPixel = 4
private let height = 512
private let bufferLength = 5242880
private var buffer: TPCircularBuffer
// ... Other code that appends data to buffer and set needsDisplay ...
override func drawRect(dirtyRect: NSRect) {
super.drawRect(dirtyRect)
// get context
guard let context = NSGraphicsContext.currentContext() else {
return
}
// get colorspace
guard let colorSpace = CGColorSpaceCreateDeviceRGB() else {
return
}
// number of sampels to draw
let maxSamples = Int(frame.width) * height
let maxBytes = maxSamples * bytesPerPixel
// bytes for reading
var availableBytes: Int32 = 0
let bufferStart = UnsafeMutablePointer<UInt8>(TPCircularBufferTail(&buffer, &availableBytes))
let samplesStart: UnsafeMutablePointer<UInt8>
let samplesCount: Int
let samplesBytes: Int
// buffer management
if Int(availableBytes) > maxBytes {
// advance buffer start
let bufferOffset = Int(availableBytes) - maxBytes
// set sample start
samplesStart = bufferStart.advancedBy(bufferOffset)
// consume values
TPCircularBufferConsume(&buffer, Int32(bufferOffset))
// number of samples
samplesCount = maxSamples
samplesBytes = maxBytes
}
else {
// set to start
samplesStart = bufferStart
// number of samples
samplesBytes = Int(availableBytes)
samplesCount = samplesBytes / bytesPerPixel
}
// get dimensions
let rows = height
let columns = samplesCount / rows
// bitmap info
let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.NoneSkipFirst.rawValue)
// counts
let bytesPerComponent = sizeof(UInt8)
let bytesPerPixel = sizeof(UInt8) * bytesPerPixel
// prepare image
let provider = CGDataProviderCreateWithData(nil, samplesStart, samplesBytes, nil)
let image = CGImageCreate(rows, columns, 8 * bytesPerComponent, 8 * bytesPerPixel, rows * bytesPerPixel, colorSpace, bitmapInfo, provider, nil, false, .RenderingIntentDefault)
// make rotated size
let rotatedSize = CGSize(width: frame.width, height: frame.height)
let rotatedCenterX = rotatedSize.width / 2, rotatedCenterY = rotatedSize.height / 2
// rotate, from: http://stackoverflow.com/questions/10544887/rotating-a-cgimage
CGContextTranslateCTM(context.CGContext, rotatedCenterX, rotatedCenterY)
CGContextRotateCTM(context.CGContext, CGFloat(0 - M_PI_2))
CGContextScaleCTM(context.CGContext, 1, -1)
CGContextTranslateCTM(context.CGContext, -rotatedCenterY, -rotatedCenterX)
// draw
CGContextDrawImage(context.CGContext, NSRect(origin: NSPoint(x: rotatedSize.height - CGFloat(rows), y: 0), size: NSSize(width: rows, height: columns)), image)
}
}