[GRASS-dev] Zooming issue in gis manager

Wed Nov 1 00:39:20 EST 2006

Before we make these changes, I'd like to reiterate again what is going on
with a bit of visual display to better show the issues.

1. There is not a systematic, major inaccuracy from zooming the display in
the GIS Manager.

2. This inaccuracy reported only occurs with interactive zooming when
displaying a raster map so that the raster grid squares are much larger than
the pixel size. That is, when you can see only a few grid squares in the
display.

3. It affects the number of grid cells displayed, not the actual
computational or display regions.

4. It is primarily (or likely entirely) a function of using g.region -a to
align the display region to the raster grid at the current resolution. I
originally did NOT use this flag, because I was concerned about potentially
uncontrolled changes to zoom precision, but added this to the zooming a few
months back at the request of others.

Please note the screen shots links below to help document this issue. These
pretty much reconstruct the original steps to show the problem, but without
region alignment.

First, I took out the -a switch from the g.region command in mapcanvas.tcl

Then I interactively zoomed (zoom box) the spearfish erosion map to show 7
rows by 13 columns

This is shown in 
<http://www.public.asu.edu/~cmbarton/files/grass_screenshots/display1.jpg>

I then selected "save current region to match display" to make the WIND file
match the display region set interactively with the zoom.

I next selected "display region settings" from the menu

This can be seen in
<http://www.public.asu.edu/~cmbarton/files/grass_screenshots/output.jpg>

Next, I switched to query mode and clicked the NW and SE corners of the
display

This can also be seen in output.jpg. Note that the xy values match those in
the region extents output (within the small error of a mouse click)

Finally, I selected "zoom to current region" to redraw the screen and make
the display match the WIND file

This can be seen in
<http://www.public.asu.edu/~cmbarton/files/grass_screenshots/display2.jpg>

Note that display1.jpg and display2.jpg are identical.

*****************
The reported problem with displaying grid cells at high magnification is
primarily or entirely a function of how g.region -a aligns the region with
the raster grid cell boundaries. I don't know how this works internally and
someone with C programming experience will have to deal with it if we want
it to work differently

So, what do we want to do? We can either have

1. All zooming is aligned with the cell borders, matching the current
resolution, with the possible result of a bit of rounding error in the
display when zoomed in to a point where the cells are very large compared to
pixel size--due to the way that g.region -a works.

OR

2. Zooming is NOT aligned with the cell borders and the current resolution.
In this case, what you see is very close to the actual region extents when
zoomed in to a point where the cells are very large compared to pixel size.
This is how I originally had it.

Note that if you are NOT zoomed in to a point where grid cells look very
large compared with pixels (i.e., most the the time in most maps), then the
displays appear identical whether the region is aligned to the cell borders
or not.

We cannot have it both ways given how g.region -a works. That is, the
underlying zoom math is correct and the same in both cases, but can either
zoom with g.region -a or with g.region (without the -a flag).

Michael
__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

> From: Hamish <hamish_nospam at yahoo.com>
> Date: Tue, 31 Oct 2006 15:13:43 +1300
> To: Maciej Sieczka <tutey at o2.pl>
> Cc: <mlennert at club.worldonline.be>, <michael.barton at asu.edu>,
> <grass-dev at grass.itc.it>
> Subject: Re: [GRASS-dev] d.m/d.gis mysteries..
> 
> Maciej Sieczka wrote:
>> I, personally, would even prefer d.m as the default GUI back,
> 
> 
> with a mind to making long term users happy, and making the wxpython
> development easier, I'd like to extend the GRASS_GUI variable.
> 
> currently it is both a startup shell variable and a GRASS gisenv
> variable. (if it exists, the former sets the latter at startup)
> 
> currently you can do
> 
> GRASS_GUI=text grass63
>   or
> grass63 -text
> 
> to start with the command line one
> 
> GRASS_GUI=tcltk grass63
>   or
> grass63 -gui
>   or
> grass63 -tcltk
> 
> to start with the Tcl/Tk startup + gis.m
> 
> 
> 
> with this patch you can also do
> 
> grass63 -oldgui   # for d.m
> 
> or
> 
> GRASS_GUI=d.m grass63
> 
> or
> 
> GRASS_GUI=gis.m grass63
> 
> or even
> 
> # rename it to whatever the wxpython GUI will be called
> GRASS_GUI=wxpython grass63
> 
> when wx becomes the default just switch "grass63 -gui" to that target.
> 
> 
> grass remembers the last one you used and uses that as the default.
> 
> 
> WRT, d.zoom & gis.m zooming- the x monitors and d.zoom have had more
> than a decade of heavy use to become mature. gis.m has had barely 9
> months of developer testing to become mature. And even then, most of the
> long-time developers who hold the bulk  of the "institutional knowledge"
> only work from the command line.... it is really great that it has come
> so far so fast.
> 
> the zooming stuff is tricky business- even after all this time d.zoom
> still has some problems and is far from elegant. accurate rendering &
> point placement is the bread and butter of a GIS though, so it'd be nice
> to have everyone happy with gis.m in the long term. Folks will be
> picking points off the screen and writing down coordinates for their
> field sites. All the time spent now perfecting the design choices will
> pay off well, I am sure. Even if only for when we do the wxPython
> version we can pick the best choices the first time.
> 
> 
> choices are good,
> Hamish

__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

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