Tk canvas: why are some lines a pixel too short?

[Pat Wallace]

My Tk application draws its own characters, using lines. I have noticed
that some characters look OK when drawn big but go ugly when redrawn
smaller - so it's not the font to blame. For example my uppercase X seems
to have one few pixel fewer lit up on the two right-hand ends compared
with the left-hand ends.

The following Tcl script demonstrates the kind of thing I mean. It draws
lines radially outwards from a center point, every 10 degrees, constant
radius:

package require Tk
canvas .c -width 100 -height 100
pack .c
set x 50
set y 50
set r 4.5
for {set i 0} {$i < 36} {incr i} {
set t [expr $i*0.174532925]
.c create line $x $y [expr $x+$r*sin($t)] [expr $y+$r*cos($t)]
}

You can get different effects by changing x, y and r. With the values
used above, the supposedly symmetrical pattern has a distinctly flat
lower-right quadrant. (You may need to use a magnifying glass.) Using
deliberately oddball values, such as x=y=50.1, or r=4.7, have an influence
on the pixel pattern, as you'd expect, but the unwanted asymmetry is
frequently seen.

What is the cause, and can I somehow stop it happening? n.b. (i) my
application doesn't know about the mapping from world coordinates to pixel
coordinates, and doesn't want to know, and (ii) I have tried changing the
capstyle to "projecting" etc., but on 1-pixel-wide lines I don't think
this has any effect.

It is as if the line drawing is truncating coordinates rather than
rounding.

Patrick Wallace
____________________________________________________________________________
Space Science & Technology Dept
Rutherford Appleton Laboratory
Chilton, Didcot,
Oxon OX11 0QX, UK
____________________________________________________________________________

[mauric...@gmail.com]

try: canvas ... -bd 0 -highlightt 0

[suchenwi]

The problem can be easily demonstrated by the following "X":
pack [canvas .c]
.c create line 10 10 20 30
.c create line 10 30 20 10

End points of the two lines seem to be truncated by one pixel.

[Pat Wallace]

In article <1152785401.0...@s13g2000cwa.googlegroups.com>,

Excellent: much better than my long-winded example.

By the way, I found one workaround for my particular application - plot
the line twice, in the two directions.

Patrick Wallace
________________________________________________________________________

[Geoff Battye]

Use the -capstyle option to control how the ends of lines are drawn.
E.g.

pack [canvas .c]
.c create line 10 10 20 30 -capstyle round
.c create line 10 30 20 10 -capstyle round

Geoff.

[Pat Wallace]

In article <e95afm$7s5$1...@gemini.csx.cam.ac.uk>,

Hm. Interesting: I had already tried capstyle projecting and that
appeared to do nothing. But capstyle round does make a difference.

There's still an asymmetry, though. Try this:

package require Tk
canvas .c -width 100 -height 100 -bd 0 -highlightt 0

pack .c
set x 50
set y 50

set r 4.9

for {set i 0} {$i < 36} {incr i} {
set t [expr $i*0.174532925]

.c create line $x $y [expr $x+$r*sin($t)] [expr $y+$r*cos($t)] \
-capstyle round
}

Patrick Wallace
_______________________________________________________________________

[Geoff Battye]

Copying and pasting your code straight into tclsh gives me symmetric
output. I've grabbed a snapshot and zoomed in to double check.

Geoff.

[Pat Wallace]

In article <e95f6u$j5i$1...@gemini.csx.cam.ac.uk>,
Geoff Battye <ge...@mrc-lmb.cam.ac.uk> wrote:
<snip>

>Copying and pasting your code straight into tclsh gives me symmetric
>output. I've grabbed a snapshot and zoomed in to double check.

The results must be dependent on screen resolution and perhaps software
version. I've tried an ancient wish on an Alpha Tru64 system and a
fairly recent tclsh under MS Windows; both give a result that contains a
departure from symmetry of just one pixel, but it's there. And adding the
-capstyle round doesn't fix the character-drawing problem that I started
with.

Patrick Wallace
__________________________________________________________________________

[Christian Gollwitzer]

Pat Wallace wrote:
> My Tk application draws its own characters, using lines. I have noticed
> that some characters look OK when drawn big but go ugly when redrawn
> smaller - so it's not the font to blame.

You know that rendering fonts at small sizes is a very advanced art?
Todays outline fonts (like Type1, Truetype, Opentype) build on a complex
magic called "Hinting", which may even be a font program, to do
acceptable scan-conversion on small sizes. Rendering each character
individual by drawing lines will definitely have severe limits.

Read e.g. this:

http://fontforge.sourceforge.net/overview.html#Hints

and for TrueType
http://www.microsoft.com/typography/TrueTypeHintingIntro.mspx

Christian