[Qgis-developer] QgsRubberBand 1px shift

[Radim Blazek]

On Mon, May 2, 2011 at 3:04 PM, Martin Dobias <wonder.sk at gmail.com> wrote:
>> // use the same rounding as in qrasterizer.cpp (6 bit fixed point)
>> static const qreal aliasedCoordinateDelta = 0.5 - 0.015625;
>
> As far as I understand this, Qt treats pixels to be squares (x,y) to
> (x+1,y+1), where x and y are integers. Therefore a horizontal line
> (0,1) - (10,1) is located exactly between the row 0 and row 1. Indeed
> when rendering with antialiasing turned on, and 1pixel pen width the
> line spans two rows, each with 50% opacity. When rendering without
> antialiasing, Qt probably wants to avoid rounding errors and shifts
> everything by 0.5px, therefore the same line (0,1) - (10,1) fits
> exactly the pixels on row 1.

What 'rounding errors'? The problem is, that using
  p.drawLine(QPointF(0,0.2), QPointF(10,0.2))
in your script without antialiasing will render the line on the second
row! Which I believe, is far from any expected result.

> Shouldn't we actually use the antialiasing flag from map canvas
> instead of hardcoding antialiasing on? Like that the offset in
> rendered map and canvas items should be equal.

Better, but currently QgsMapCanvas sets antialiasing directly on
QgsMapCanvasMap which has no QgsMapCanvasMap::antiAliasing() method so
it has to be added somewhere.

Radim