[GRASS-dev] Zooming issue in gis manager
michael.barton at asu.edu
Thu Nov 2 23:19:54 EST 2006
This is very helpful, though I'll have to sort through this and follow-up
messages to see if and where they can fit into the existing algorithms.
A couple of things bother me that perhaps you could clear up. First is that
it looks like your examples below and some of the follow-ups assume that
resolution is always an integer. Although the number of rows and columns
are, of course, integers, I'm sure that ewres and nsres can take on any
float value. This makes what you are suggesting more complicated I think.
The other issue is that even when I take out the -a flag, d.rast makes some
kind of adjustments to the way that grid cells are displayed such that they
appear to be somewhat different sized when you make the resolution=1 grid
cell and when you make the resolution>>1 (original) grid cell. Maybe this is
what you are referring to in some places below and other posts.
Finally, is there any reason that I need to be concerned about the values
of GRASS_WIDTH and GRASS_HEIGHT for PPM output?
Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University
> From: Glynn Clements <glynn at gclements.plus.com>
> Date: Fri, 3 Nov 2006 00:09:20 +0000
> To: Michael Barton <michael.barton at asu.edu>
> Cc: grass-dev <grass-dev at grass.itc.it>
> Subject: Re: FW: [GRASS-dev] Zooming issue in gis manager
> Michael Barton wrote:
>> The original post here didn't make it past the list file size filter. So
>> I've reduced it to one graphic that should make it. I've also done a few
>> more tests.
>> In addition to what I mention below, g.region run without the -a flag
>> creates rounding errors in resolution--minimal but annoying.
>> Without the -a flag, the number of cells displayed accurately reflects the
>> actual display region setting. So if you save the display region to the WIND
>> file, and reset the display to match the new current region (i.e., WIND
>> file), it looks the same as before. But, as noted below, it does this by
>> shrinking the cells a bit to make them squeeze into the PPM output created
>> by the PNG driver (could the X-driver be doing this differently???).
> There is no difference between the X and PNG drivers.
> However, using direct rendering can potentially produce different
> results to using a driver.
> Each display frame can have a geographic region associated with it. If
> the current frame has a region, this is used in place of the current
> region (whether from the WIND file, $WIND_OVERRIDE or $GRASS_REGION).
> If you draw something on a monitor, change the current region, then
> draw something else without running d.erase, it will use the region
> stored on the frame rather than the current region.
> This can't happen with direct rendering, as nothing persists between
>> With the -a flag, the number of rows or columns displayed is one or two less
>> than the number in the display region. This is not noticeable (and maybe
>> doesn't even happen) when there are a lot of cells in the display, but can
>> be noticeable when there are very few cells in the display. The result is
>> that if you copy the display region to the WIND file and display it back
>> again, you see more cells than you started with. This is the "correct"
>> number of rows and columns, but it not what was displayed. However, there
>> are no rounding errors in the resolution as reported by g.region.
> "g.region -a" expands the region so that the bounds are aligned to a
> grid whose spacing is determine by the existing resolution and whose
> alignment is such that the origin of the coordinate system falls on a
> grid point (an intersection between four cells).
>> Is this all happening in g.region?
>> The display driver? Or both? As I say
>> below, I can't think of how gis.m could compensate for this. In the region
>> computation, it would involve 'rounding' to the nearest resolution value.
>> But how can you round to the nearest 0.0001543 degrees? ...or even the
>> nearest 25m? There is no 'starting place' to round from, since the region
>> boundary can potentially be anywhere theoretically.
> First, you have to choose some point which will not change as a result
> of the adjustment, e.g. the centre or one of the four corners.
> If you pick the south-west corner, then you adjust the north and east
> bounds so that the height and width of the region are multiples of the
> resolution, i.e.
> width = east - west
> height = north - south
> width = (width / ewres + 0.5) * ewres # round to nearest integer multiple of
> height = (height / nsres + 0.5) * nsres # round to nearest integer multiple of
> east = west + width
> north = south + height
> The same applies for any other corner, except that either or both of
> the last two lines would be replaced by:
> west = east - width
> south = north - height
> To keep the centre fixed, the last two lines would be replaced by:
> cx = (east + west ) / 2
> cy = (north + south) / 2
> east = cx + width / 2
> west = cx - width / 2
> north = cy + height / 2
> south = cy - height / 2
> The end result will be similar to using "g.region -a", except that:
> 1. The grid is anchored at either the centre or a corner of the
> region, not the origin of the coordinate system.
> 2. Each bound is aligned to the nearest grid line, rather than always
> rounding outwards.
> When panning, you might want to align the region to the previous grid.
> If (px, py) is some point which is aligned to the previous grid (e.g.
> one corner of a region which was so aligned), then you can align with:
> east = px + ((east - px) / ewres) * ewres
> west = px + ((west - px) / ewres) * ewres
> north = py + ((north - py) / nsres) * nsres
> south = py + ((south - py) / nsres) * nsres
>> I'm going to put the -a flag back into gis.m to keep the resolution rounding
>> errors out. If you want to set region with precision, especially in a
>> close-up view with few cells showing in a raster, you need to use g.region.
>> If you want to display a region in a close-up view of a raster, where the
>> cell sizes are large compared with the display and pixel size, you'll need
>> to use explore mode for now at least. As documented below and in the
>> attached graphic, this does a good job of making a display with accurately
>> rendered raster cells. Vectors either don't face these issues or have
>> different ones, as they are not behaving parallel to rasters in this.
> I think the main point is that you need to keep gis.m's internal
> regions valid at all times. Whenever a new region is created due to
> zooming or panning, it should be adjusted (so that its size is an
> integer multiple of the resolution) as soon as it is created, rather
> than storing an invalid region and relying upon it being coerced into
> compliance when passed to GRASS (G__read_Cell_head_array() calls
> G_adjust_Cell_head(), which affects anything which reads a region).
> E.g. if the user marks a rectangle on a map, it should ideally be
> adjusted as it is being entered, so that WYSIWYG.
> Glynn Clements <glynn at gclements.plus.com>
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