CODEWITHSUNDEEP
iosswiftcore-graphicscgcontextdrawimage
Philip De Vries
Philip De Vries

Reputation: 415

Why does my CGImage not display correctly after setNeedsDisplay

I'm drawing a colorWheel in my app. I do this by generating a CGImage in a separate function and using CGContextDrawImage in drawRect to put it onscreen.

On the initial presentation it looks fine, but after I call setNeedsDisplay (or setNeedsDisplayInRect) the image turns black/distorted. I must be doing something stupid, but can't see what.

Cool Wheel Cool Wheel

DrawRect code looks like:

override func drawRect(rect: CGRect) {

    if let context = UIGraphicsGetCurrentContext() {

        let wheelFrame = CGRectMake(0,0,circleRadius*2,circleRadius*2)
        CGContextSaveGState(context)

            //create clipping mask for circular wheel
        CGContextAddEllipseInRect(context, wheelFrame)
        CGContextClip(context)

            //draw the wheel
        if colorWheelImage != nil { CGContextDrawImage(context, CGRectMake(0,0,circleRadius*2,circleRadius*2), colorWheelImage) }
        CGContextRestoreGState(context)

            //draw a selector element
        self.drawSliderElement(context)
    }
}

CGimage generation function:

func generateColorWheelImage() {

    let width       = Int(circleRadius*2)
    let height      = Int(circleRadius*2)

    var pixels = [PixelData]()

    for yIndex in 0..<width {
        for xIndex in 0..<height {
            pixels.append(colorAtPoint(CGPoint(x: xIndex, y: yIndex)))
        }
    }

    let rgbColorSpace   = CGColorSpaceCreateDeviceRGB()
    let bitmapInfo      = CGBitmapInfo.ByteOrderDefault

    let bitsPerComponent: Int   = 8
    let bitsPerPixel: Int       = 24

    let renderingIntent = CGColorRenderingIntent.RenderingIntentDefault

    assert(pixels.count == Int(width * height))

    var data = pixels

    let providerRef = CGDataProviderCreateWithData(nil, data, data.count * sizeof(PixelData), nil)

    colorWheelImage = CGImageCreate(width, height, bitsPerComponent, bitsPerPixel, width * Int(sizeof(PixelData)), rgbColorSpace, bitmapInfo, providerRef, nil, true,renderingIntent)

}

Upvotes: 2

Views: 129

Answers (1)

Philip De Vries
Philip De Vries

Reputation: 415

I finally found the answer (here: https://stackoverflow.com/a/10798750/5233176) to this problem when I started running the code on ipad rather than simulator and received a BAD_ACCESS_ERROR, rather seeing a distorted image.

As the answer explains: '[The] CMSampleBuffer is used directly to create a CGImageRef, so [the] CGImageRef will become invalid once the buffer is released.'

Hence the problem was with this line:

    let providerRef = CGDataProviderCreateWithData(nil, data, data.count * sizeof(PixelData), nil)

And the problem can be fixed by using a copy of the buffer, like so:

    let providerData    = NSData(bytes: data, length: data.count * sizeof(PixelData))
    let provider        = CGDataProviderCreateWithCFData(providerData)

Upvotes: 0

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