Re: Slow Image scrolling

[Fred Kiefer]

HiIan,

Ian Jones wrote:

> I have a problem with scrolling large image attachments. I regularly 
> recieve quite large image files attached to my email, someone recently 
> sent a 270kb .jpg attachment and even on my dual celeron 500 it kept 
> stopping trying to scroll through it and my CPU usage was up to 80%. I 
> have talked to some other people about this in the #GNUstep IRC channel 
> who have experienced the same problem with ImageViewer and Ink.app and 
> various image types.
> Does anyone have any ideas as to what is causing this and how it could 
> be improved?


The problem here is that NSImageView relies on the NSControl method 
drawRect: when doing a redraw. This basically ignores the given 
rectangle and redraws the full cell. That way an image embedded into a 
scroll view will always draw the full contents, even if only part is 
visible. And to make things worse, the scroll view will also attempt to 
copy parts of the image over, while you scroll. NSImageView and 
NSImageCell are just not designed to be but into a scroll view (The same 
is true for all other control and cell subclasses).

What can be done about this? Not much, as long as you are using a 
NSImageView. But you could write another subclass of NSView, that would 
implement drawRect: in a more clever way and put that into the scroll 
view. As long as you dont need a border or image scaling, this should be 
fairly simple usinge [NSImage compositeToPoint:fromRect:operation:].

I hope this helps

Fred