Re: y-axis reversed again

[Hans-Bernhard B. <br...@ph...>]

On Mon, 14 Jun 2004, Petr Mikulik wrote:

> There seems to be again the bug of "y-axis gets reversed" in "set pm3d map"
> splots. Now, every 2nd splot zoomed by mouse has reversed y-axis. Can you
> please have a look to this?

Here's the start of an analysis of this.  Brace yourself, for this is
really all a rather despicable mess.  

The core of the problem is that we still have two different possible
results of the 'set yrange' command regarding reverted axes:

	set yrange [10:5] 
	set yrange [5:10] reverse

are (nowadays) supposed to do mean the same thing, but in the reality
of our code, they don't:

gnuplot> set yrange [1:0]
[...]
(gdb) p axis_array[FIRST_Y_AXIS ]
$1 = {autoscale = AUTOSCALE_BOTH, set_autoscale = AUTOSCALE_NONE,
  range_flags = 2, range_is_reverted = 0 '\0', min = -10, max = 10,
  set_min = 1, set_max = 0, writeback_min = -10, writeback_max = 10,
[...]

gnuplot> set yrange [0:1] reverse
[...]
(gdb) p axis_array[FIRST_Y_AXIS ]
$3 = {autoscale = AUTOSCALE_BOTH, set_autoscale = AUTOSCALE_NONE,
  range_flags = 2, range_is_reverted = 0 '\0', min = -10, max = 10,
  set_min = 0, set_max = 1, writeback_min = -10, writeback_max = 10,

The difference is that variant 1 has set_min=1, set_max = 0, whereas
variant 2 has it the opposite way around.  Both of them have the
RANGE_REVERSE flag set (range_flags == 2).  It's this RANGE_REVERSE flag
alone that 'set view map' will modify.  It doesn't touch any of the axis
range endpoints themselves (and it shouldn't).  

RANGE_REVERSE is then consulted by the CHECK_REVERSE() macro to decide
whether to exchange axis->min and axis->max, and set the
axis->range_is_reverted flag, which in turn is read by
axis_revert_and_unlog_range() to decide whether to exchange axis->min and
axis->max *again*.  Sounds insane, doesn't it?

The error at hand seems to happen because the mouse zoom generates a
command as in variant 1, as can be seen in verbose mode.  E.g. in a fresh
gnuplot session:

gnuplot> set view map
gnuplot> sp x*y
gnuplot> communication commands will be echoed.
starting zoom region.
set xr[-6.93319487712:6.80166147456]; set yr[7.03235403496:-8.13313499442]
zoom region finished.
gnuplot> show yr

        set yrange [ 7.03235 : -8.13313 ] noreverse nowriteback

This state was reached by a combination of the 'backward' set yrange
issued by the zoom with the action of splot_map_activate(), which flips
the RANGE_REVERSE flag off again after 'set yrange' turned it on.

I suspect it may take a thorough re-think and rewrite of how 'set view
map' works before this is fixed for good.

-- 
Hans-Bernhard Broeker (br...@ph...)
Even if all the snow were burnt, ashes would remain.