public Rect GetRequiredSourceRectangle(
	ICanvasResourceCreatorWithDpi resourceCreator,
	Rect outputRectangle,
	ICanvasEffect sourceEffect,
	uint sourceIndex,
	Rect sourceBounds
)

CanvasBitmap myBitmap = ...;

var blurEffect = new GaussianBlurEffect { Source = myBitmap };
var desaturatedBlur = new SaturationEffect { Source = blurEffect };

Rect outputRegionThatWillBeDrawn = ...;

Rect neededPartOfTheBitmap = desaturatedBlur.GetRequiredSourceRectangle(
                                                  drawingSession, 
                                                  outputRegionThatWillBeDrawn, 
                                                  blurEffect, 
                                                  0, 
                                                  myBitmap.Bounds);

// We now only need to make sure the 'outputRegionThatWillBeDrawn'
// region of 'myBitmap' is valid, because we know this draw call
// will not access any other parts of the bitmap:

drawingSession.DrawImage(desaturatedBlur, position, outputRegionThatWillBeDrawn);