[Qt-interest] Map to view from inside QGraphicsItem::paint()

[Josiah Bryan]

Josiah Bryan wrote:
> Hi All -
>
> Is there any way for me to be able to map my bounding rect inside a 
> QGraphicsItem::paint() call to the actual pixel size on the screen?
>
> This is for the purpose of scaling large pixmaps to the "right" size. If 
> my item is scaled up, and I only render the pixmap at the size of my 
> item, the final pixmap will implicitly be scaled up by the view/scene 
> scaling. I want to cache the scaled pixmap size that is the actual 
> screen size and render a good quality pixmap at the same time.
>   
Maybe I should explain more:

I've got a large jpeg ( > 3MB on disk) that I'm rendering.

Since I'm rendering it at a scaled down size of its original (around 
1024x768 instead of the 3000px or whatever the orig width is), I don't 
want the QPainter to have to scale the 3000px -> 1024px every time it 
paints. So I want to draw the scaled version to a pixmap and cache the 
pixmap. Great. Works fine.

Problem is that if I do that cached pixmap version, the final output 
looks "block" / pixelated in the final QGraphicsView (since the view may 
be scaled to fit the target output screen, in this case a 1400x900 at 
the moment), so the final scene is scaled up a bit. Which results in 
blocky pixels when using the cached pixmap version.

But if I just pass the original 3000px pixmap to the painter and let it 
do the scaling on the fly, the resulting image is much clearer. (I'm 
using the painter->drawPixmap(QRect,QPixmap) form of the function to do 
the scaling).

And the sizes are the same - e.g. the size of the QRect that I use when 
I give it the 3000px image is the same rect size that I use to draw the 
cached pixmap - so the only explanation I can see for the differing 
visual outputs is the view transformations.

Does anyone have any ideas how to get around that and render "good 
looking" pixmaps while retaining the speed advantage of caching the 
scaled pixmaps?

Thanks!
-josiah