Threaded image-scaling sub still bogs down UI

Question

I created an image-viewing control for my boss that incorporates panning, zooming via the mouse wheel, and drawing a box to zoom to. The control needs to support very large image files (i.e. several thousand pixels on each side).

It all works, but whenever the code is scaling the image the control UI becomes unresponsive. My boss had me use threading to set the scaling code apart from the UI. The scaling code is now definitely on a separate thread but the UI is still bogged down while scaling code is running! Can anyone please help me with this??

Below is the Scaling code. Let me know if this isn't enough to help me and I'll post whatever code you need!

UPDATE: Here's the control code in its entirety. link text

Answer 1

I suspect that since the scaling is using all the CPU that the UI is bogging. Both threads are probably at the same priority.

Try lowering the priority of the scaling thread to allow the UI to be responsive.

Values for Thread Priority:

Above Normal -> Gives thread higher priority
Below Normal ->Gives thread lower priority
Normal -> Gives thread normal priority
Lowest -> Gives thread lowest priority
Highest -> Gives thread highest priority

so you would probably use:

Thread1.Priority=System.Threading.ThreadPriority.BelowNormal

Answer 2

Try using this instead.

Private Sub Worker(object o)
    Dim args as ScaleImageArguments
    args = CType(o, ScaleImageArguments) 
    ScaleImage(args)
End Sub

Private Sub RunScaleImageAsync(ByVal img As Image, ByVal scale As Double)
    System.Threading.ThreadPool.QueueUserWorkItem(Worker, _
                 New ScaleImageArguments(img.Clone, scale))
End Sub

ALTERNATIVE - Using the Async pattern.

Private Delegate Sub ScaleImageDelegate(ByRef arg As ScaleImageArguments)

Private Sub BeginScaleImage(ByRef img As Image, ByVal scale As Double)
    Dim d As ScaleImageDelegate
    d = New ScaleImageDelegate(AddressOf ScaleImage)

    d.BeginInvoke(New ScaleImageArguments(img.Clone, scale), _
                  New AsyncCallback(AddressOf EndScaleImage), d)        
End Sub

Private Sub EndScaleImage(ar As IAsyncResult)
    Dim d As ScaleImageDelegate
    d = CType(ar.AsyncState, ScaleImageDelegate)
    d.EndInvoke(ar)
End Sub

Then just call BeginScaleImage to run it asynchronously.

EDIT - Please see corrections above. The ar argument on the EndScaleImage should be declared ByRef, and also the img param of the BeginScaleImage. There is no reason why they should be passed ByVal!!

Answer 3

How is your ImageScaled() diplaying the image back to the UI?

Answer 4

It appears that there is some global lock within the GDI+ API. For test I created two threads based on following function

static void test_thread()
{
    Bitmap bmp = new Bitmap( 4000, 4000 );
    Graphics g = Graphics.FromImage( bmp );
    Brush b = Brushes.Red ;

    for ( ; ; ) {
        g.FillRectangle( b, 0, 0, bmp.Width, bmp.Height );
    }
}

If the infinite loop has ben left empty, the CPU usage was over 90% so they they utilized both cores of my CPU. With the FillRectangle present, the usage was slightly bellow 50% indicating that only one thread can run it at time.

So it is possible that any GDI+ call you do from the GUI thread while the scaling is in progress will block until the scale completes.