API review request: ImageView.viewport behavior

Jim Graham james.graham at oracle.com
Mon May 14 03:25:30 PDT 2012

We have these two requests to clarify and fix the behavior of the 
viewport on an ImageView.  They don't involve any new API, just 
questions and suggestions on changing the behavior of the existing property:


The history of this bug is that the code to implement viewports was 
ported from the Java2D source base where it had an integer "draw sub 
image" method.  The 2D method interpreted the subimage parameters as 
defining a piece of the image "a subimage" that behaved as if it were 
extracted and isolated and then rendered as its own image.

The FX implementation uses floating point values to define the viewport 
and describes it merely as defining "the portion of the image which 
falls within the viewport [is] displayed" and elsewhere as achieving a 
"zoom effect" if you narrow the viewport without narrowing the fit 

The distinction of whether the viewport/subimage is considered in 
isolation only matters if you are using image smoothing, which is on by 
default in FX and off by default in AWT.  If you are smoothing, then 
special math is used for the border pixels because they have no adjacent 
pixels to smooth against.

In 1.3 and 2.0 the math was broken in the FX version because it was 
ported over from the 2D version which had a different rendering model. 
We fixed the bugs in 2.1 and brought it into compliance with "what it 
seemed to be trying to do" which was to isolate the image defined by the 

The problem was, when we "fixed those bugs" we realized that there 
wasn't really any good definition of what the "subimage" was when you 
are using floating point coordinates since it is hard to create an image 
from fractions of the border pixels.

So, we filed the above bugs to clarify this and to change the 
implementation of viewports to be more like what you get with "texture 
mapping" where they simply define the points on the image that map to 
the corners of the ImageView's output.

If there are no objections, I'd like to remove the synthetic "edge 
conditions" on the ImageView viewport so that it simply acts like a clip 
affecting what is seen from the original image - a sort of "narrowing 
lens" if you will...


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