Processing (the Java visualization language): spinning rectangle leaving odd fading trail effect with rectMode(CENTER) but not rectMode(CORNER) -

I am practicing with objects, as if they move. At the moment, I will rotate clockwise in the middle of a rectangular canvas and on the canvas I am depicting a low-viscosity rectangle, which produces a good fading mark.

Trouble: Fixed Trail Only when I use the Recode (Corner) is only completely Fade Fade. [I should mention that the recordmod default was not mentioned in the corner.] It is undesirable, as is the rectangle about a corner, instead the center.
I would like to use the Ricotode (CNTR), but the speed trail completely fades with this command; Instead, the trails form 3/4 of a circle of solid, completely opaque color.

I tried to include a screenshot of strange 3/4 speed trail circle, but I am told that I lack the required reputation.

It is very strange that it seems like the bug.
Any thoughts?

  Integer size size = 150; Float q = 0; Zero setup () {size (500, 500); Smooth (); NoStroke (); Background (0); } Zero draw () {noStroke (); Fill (0, 15); // creates low-opacity layers on object ret (0, 0, width, height); // pushmatrix (); RectMode (middle); // toggle and to see the theme Translation (width / 2, height / 2); (Q) rotate; NoStroke (); Filling (255,255); Rect (0,0, size, size, size); Q + = 0.02; // popMatrix (); // Touching Push & Popmatrix - No Fate}  

I always hesitate to add a bug Accusing the language :) Try the following code and I think you will see why this behavior happens (this is yours, only I have changed the color of translucent rectangle). If you want more explanation, leave a comment and I will be happy to oblige.

  shape size = 150; Float q = 0; Zero setup () {size (500, 500); Smooth (); NoStroke (); Background (0); } Zero draw () {noStroke (); Fill (100, 15); // creates low-opacity layers on object ret (0, 0, width, height); // pushmatrix (); RectMode (middle); // toggle and to see the theme Translation (width / 2, height / 2); (Q) rotate; NoStroke (); Filling (255,255); Rect (0,0, size, size, size); Q + = 0.02; // popMatrix (); // triggering push and popmatrix - no fate}