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Look at result of this script:

 canvas .c -bg white
 grid .c
 set x1 20
 set x2 22
 set y2 105
 for {set f 0} {$f<50} {incr f} {
     set y1 [expr {$y2-0.05*$f}]
     .c create rectangle $x1 $y1 $x2 $y2 -fill black
     incr x1 2
     incr x2 2
 }

On Windows XP I see that at left side of figure bottom margin is one pixel lower than at right side. But it shouldn't happen as y2 is the same (105) for all rectangles. What do you think?

Doktor J
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Yaroslav Schekin
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  • It's the same on OSX, and with both 8.5 and 8.6. – Donal Fellows Sep 09 '11 at 07:25
  • Thanks for testing! But is it a bug? Should I report it to Tk tracker? – Yaroslav Schekin Sep 09 '11 at 07:47
  • I don't know yet. :-) I do suspect that the issue is related to what happens when two coordinates round to the same value. – Donal Fellows Sep 09 '11 at 07:55
  • See https://codegolf.stackexchange.com/questions/85141/draw-the-national-flag-of-iceland/141479#comment346671_131490 and https://stackoverflow.com/a/4311134/383779 — The problem is the canvas **`highlightthickness`** attribute is by default set to **2** instead of **0** – sergiol Nov 02 '17 at 22:27

1 Answers1

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I think it has to do with the effort of TK to draw a rectangle of a least 1 pixel in size.

In the code I can see, that y2 is incremeted by 1 if it's equal to y1 after rounding to short integer.

Logging your creation statements one can see, that the pixel jump occurs between f=10 and f=11. That is the point where y1 and y2 become unequal and no adjust takes place:

f=10 .c create rectangle 40 104.5 42 105 -fill black
   rounded:   y1=105 y2=105
   adjusted:  y1=105 y2=106
f=11 .c create rectangle 42 104.45 44 105 -fill black
   rounded:   y1=104 y2=105
   no adjustment

That explains the pixel jump.

IMO you should file a bug on this.

schmitzu
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  • Correct. In tk/generic/tkRectOval.c::ComputeRectOvalBbox we find "Special note: the rectangle is always drawn at least 1x1 in size, so round up the upper coordinates to be at least 1 unit greater than the lower ones." A number of your earlier blocks have the same y1 and y2 so y2 gets increments to 106. The Tk canvas does not do sub-pixel drawing. You may specify coords differently but all the drawing code calculates up to pixels. – patthoyts Sep 09 '11 at 12:21
  • Understood, but I think it's wrong, coords must get converted so that line is drawn at y=105, not 106. And again, look at the result of the script execution --- it's plain wrong, IMHO. – Yaroslav Schekin Sep 09 '11 at 13:38
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    “Must get converted”? How do we know your way is the best (out of a set of arbitrary choices)? – Donal Fellows Sep 09 '11 at 22:28
  • IMHO, it's trivial. Here is the model to demonstrate Tk's behaviour. Green rectangles are actual coords provided, colored rectangles show how Tk draws them: – Yaroslav Schekin Sep 10 '11 at 00:33
  • `canvas .c -bg white -highlightthickness 0 grid .c for {set f 0} {$f<500} {incr f 20} { .c create line $f 0 $f 2000 -width 0 -fill grey .c create line 0 $f 2000 $f -width 0 -fill grey } set x1 20 set x2 40 set y2 200 for {set f 1} {$f<20} {incr f} { set y1 [expr {$y2-5*$f}] if {($y2-$y1)<20} { set a [.c create rectangle $x1 $y2 $x2 [expr {$y2+20}] -fill red -width 0] } else { set a [.c create rectangle $x1 [expr {round($y1/20.0)*20}] $x2 $y2 -fill lightblue -width 0] } .c lower $a .c create rectangle $x1 $y1 $x2 $y2 -outline darkgreen incr x1 20 incr x2 20 }` – Yaroslav Schekin Sep 10 '11 at 00:34