[OpenJDK 2D-Dev] HiDPI support issues on Windows

Jim Graham james.graham at oracle.com
Sun Oct 2 19:10:39 UTC 2016

After looking into the code in RepaintManager and re-reading Alexander's 
message again I can see how it describes what is going on more clearly.

Fixing the rounding errors doesn't necessarily require avoiding use of 
the intermediate image for damage repair, you just have to make sure 
that you use the incoming xywh as suggestions for what needs to be 
redrawn, but instead determine exact pixels that you will repaint 
(usually floor,floor,ceil,ceil to "round out" the area), and then use 
those pixel-precise locations instead of passing along the integers that 
came from the repaint requests and hoping for the right rounding.  The 
problem is that a number of the interfaces used by the RepaintManager 
take integers and hide a scale from the caller so we need to either work 
around their implicit scale, or possible create internal variants that 
let us work in pixels.

In other words, the typical boilerplate for intermediate image damage 
repair would be:

// repainting x,y,w,h
img = make image (w,h)
g = img.getGraphics()
destination.drawImage(img, x,y)

but that boilerplate only works if x,y are exact pixel coordinates, but 
since it is all being doing on a scaled graphics then x,y will transform 
to arbitrary not-necessarily-integer locations and then all bets are off.

Fixing this could either rely on using float interfaces wherever 
available, or by undoing all of the implicit scales and working in 
pixels, but being aware of the scale that is required for the 
destination.  Something like one of these boilerplates instead:

// repainting x,y,w,h integers using floats
float pixelx1 = floor(x * scaleX)
float pixely1 = floor(y * scaleY)
float pixelx2 = ceil((x+w) * scaleX)
float pixely2 = ceil((y+h) * scaleY)
int pixelw = (int) (pixelx2 - pixelx1)
int pixelh = (int) (pixely2 - pixely1)
// Note that the code currently asks the destination to make
// a compatible image of a virtual pixel size that is then
// scaled to match.  A "make me an image of this many pixels"
// might be less cumbersome.
img = make image (ceil(pixelw / scaleX),
                   ceil(pixelh / scaleY))
g = img.getGraphics() // will be scaled already
// The following will use the translate(double, double) method
g.setClip(new Rectangle2D.Double(pixel* / scale*))
g.translate(-pixelx1 / scaleX, -pixely1 / scaleY)
// Since there is no drawImage(img, float x, float y)...
destination.translate(pixelx1 / scaleX, pixely1 / scaleY)
destination.drawImage(img, 0, 0)
// (restore transforms where needed)

That version uses floating point interfaces in a number of key places 
(notably translate() calls are available as either int or double in the 
Graphics and have to use the setClip(Shape) method to specify a floating 
point rectangle), but a down side is that using those interfaces means 
that you have a value that you know is at a pixel boundary and you pass 
it in as "number / scale" only to have the code in the Graphics 
immediately apply that scale and you end up with the final result of 
"number / scale * scale" which might incur round-off errors and end up 
being slightly off of a pixel.

In another approach, you could also kill all of the transforms and do it 
more directly in pixels as in the following:

// repainting x,y,w,h integers using unscaled operations
// Some parts more cumbersome to undo the implicit scaling
// but it won't suffer from round-off errors when constantly
// scaling and unscaling through the various interfaces
// that have transforms built in
int pixelx1 = (int) floor(x * scaleX)
int pixely1 = (int) floor(y * scaleY)
int pixelx2 = (int) ceil((x+w) * scaleX)
int pixely2 = (int) ceil((y+h) * scaleY)
int pixelw = pixelx2 - pixelx1;
int pixelh = pixely2 - pixely1;
// Not sure if there is a mechanism for this since I think
// all of the interfaces to get a compatible image are
// designed to assume that the caller is not scale-aware.
img = make pixel-sized image (pixelw, pixelh)
g = img.getGraphics()
// assuming that g would be unscaled in this case, but
// if g is scaled, then g.setTransform(IDENTITY)
// translate by an integer amount, and then scale
g.setClip(pixelx1, pixely1, pixelw, pixelh)
g.translate(pixelx1, pixely1)
g.scale(scaleX, scaleY);
destinationg.drawImage(img, pixelx1, pixely1)
// (restore transforms where needed)


On 9/30/2016 1:30 PM, Jim Graham wrote:
> On 9/30/16 3:22 AM, Alexandr Scherbatiy wrote:
>> The problem is that the RepaintManager draws a region to a buffered
>> image at first and draws the image after that to the
>> window.
>> Suppose the image has int coordinates and size (x, y, w, h) in the
>> user space. It should be drawn into the region with
>> coordinates (x, y, x+width, y+height) = (x1, y1, x2, y2).
>> If floating point UI scale is used (like 1.5) the region coordinates
>> are converted to values (1.5 * x1, 1.5 * y1, 1.5 *
>> x2, 1.5 * y2) in the dev space.
>> Now these coordinates need to be rounded and the process really
>> depends on the evenness or oddness of the start and end
>> coordinates. They both can be rounded to one side or to opposite.
>> Depending on this some lines near the drawn image
>> region can be not filled or just wrongly filled.
> The repaint manager should compute the nearest pixel bounds outside of
> the scaled repaint area, and then adjust the rendering to repaint all of
> those pixels.  You don't "round" here, you "floor,floor,ceil,ceil" (and
> then worry how to present the clip region to the app so it can do the
> right thing - probably by clipping to a Rect2D.Float() and letting the
> integer g.getClipBounds() further round out the coordinates which means
> extra paint calls, but at least you'll repaint all the dirty pixels and
> they will be blitted to the right destination pixels if the code that
> sends them to the screen is aware of the full resolution...)
>                 ...jim

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