I have submitted a bug (#84145) and a couple of patches about pixel
rounding when drawing straight lines. The debate in the bugzilla
comments seem to be about to turn into a design argument about
subpixel positioning, which I think it would be more convienient to
have through a mailing list than a bug-tracking system.

The current status of the Gimp:

1. The position for a mouse button press is converted to a
   floating-point value i such a way that the FP coordinates
   refer to the upper-left corner of the area on the image
   that is covered by the actual display pixel the mouse is
   pointing to (with the displayed zoom and all that).

2. The paint tools (pencil, paintbrush, eraser, & friends)
   simply use these floating-point coordinates for the center
   of the brush.

3. Consequence of (1) and (2): When one paints a single click
   with the paintbrush and a 1x1 brush in 1:1 zoom mode, what one
   actually gets thanks to brush subsampling is a block of four
   image pixels each being painted by 25%.

4. The same floating-point coordinates are used as endpoints of the
   lines that are drawn when Shift is pressed.

5. For the preview line, the endpoints are rounded to nearest pixel
   centers (which can be easily seen in zooms greater than about
   500%), but this rounding is not performed when the paintcore
   actually draws the line.
   (There's another bug with the preview lines, but that is on the
   order of one display pixel, not an image pixel).

6. As a consequence of (1) and (4), SW-NE lines drawn in 1:1 zoom
   will get displaced by one pixel to the NW relative to the ideal
   line between of the centers the user actually clicks on.

7. As a consequence of (1) and (5), the user does not always get the
   line he is hoping for when he tries to draw a line in highly zoomed
   mode (because the preview line does not look like the subpixel
   position of the mouse is important when the button is clicked).

Here are some opinions about this:

a. The current behavior is clearly not internally inconsistent.
   Something has to be done. We need some kind of consensus about what
   "something" actually is, though.

b. I think of (1) as a bug in and of itself. The FP-coordinates that a
   mouse position is translated into should be the coordinates of the
   image point beneath the *center* of the display pixel. Would
   anything break if this is fixed generally? I.e., does anyone depend
   on the ability to point the mouse exactly on an image-pixel
   boundary?

c. The painting tools that do not subsample their brush will instead
   round the brush center to the nearest pixel center each time a
   brush "dab" is drawn. There seems to be little argument that the
   also ought to round the endpoints of a straight line to pixel
   centers before computing the intermediate points on the line.

d. Apparently there are people who want to be able to draw straight
   lines between non-pixel-centers with the tools that do subsample
   brushes.

e. On the other hand, I personally think it is also important to be
   able to draw anti-aliased lines between image-pixel centers without
   having to succumb to finger cramps because the *display* pixel
   smack in the middle of the desired image-pixel must be hit
   precisely in with a pointing device. Some mice are a pain to
   position more precisely than 2-3 pixels, and exactly a zoom mode is
   what one would hope could mitigate that for precision editing.

f. Possible ways to reconcile (d) with (e) would include
    - Adding a tool option that says whether line endpoints (and even
      non-shift keypresses?) get rounded to pixel centers or not.
      But down that way lies madness, or what?
    - Defining a modifier key (meta?) to mean "round mouse positions
      to image-pixel center". Which would mean that to draw a
      15-degree-constrained straight line between pixel centers one
      would need holding down *three* modifiers.
   Both these designs seem to be somewhat lacking in elegance.

Thoughts, anyone?

-- 
Henning Makholm                            "What a hideous colour khaki is."
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