Label Cache Performance

[Stephen Woodbridge]

Steve,

The hash key for say
	"Main St" => Array(label_drawn_1, label_drawn_2, ..]
We only need to remember the the items that were actually drawn on the 
map since these are the only ones that need to be compared against when 
considering a potentially new label.

Since the MINDISTANCE test is localized to a LAYER and possibly a CLASS 
modifying the key to include just the CLASS will suffice, like 
"<class_num>:<label_text>". The hash would be created for each layer and 
then disposed and recreated for the next layer. This is ok because 
MINDISTANCE does not span LAYERs unless I am mistaken.

So here is pseudo-code for how this might work. Obviously there are 
details omitted and this would be implemented to be efficient. My goal 
here is to express the concept:

for each layer {
     if MINDISTANCE {
         hash = create_hash()
     } else {
         hash = NULL;
     }
     for each label {
         if (mindistance_ok(this.label, hash) &&
             label_fits(this.label, bitmap)) {
             # assumes it will update hash
             draw_label(this.label, hash, bitmap)
         }
     }
}

mindistance_ok() returns true if hash == NULL
label_fits() compares the label polygon to the bitmap and returns true 
if it fits
draw_label() renders the label and updates the hash if it is not NULL 
and updates the bitmap.

I can expand the pseudo-code for mindistance_ok() and label_fits() is 
needed, but it seems that we should agree in principal that this will 
work for all existing cases. I have not heard any case that this can not 
be easily adapted to support.

Steve - Does this cover the case you were concerned about? If not can 
you explain the scenario a little better, because I'm missing it.

-Steve

Steve Lime wrote:
> Was thinking about this more and don't think a hash for MINDISTANCE will
> work since
> the hash keys need to be unique and label/class/layer certainly isn't.
> You might have
> 50 occurances of the same label for a series of road segments. I'm sure
> there's a 
> workaround but it would undoubtedly be complex...
> 
> Steve
> 
>>>> Stephen Woodbridge <woodbri at SWOODBRIDGE.COM> 1/3/2007 9:35:26 PM
>>>>
> Steve Lime wrote:
>> We use the loops for MINDISTANCE because we obviously are already
> doing
>> so for collisions.
>>
>> I thought about a hash too. Trouble is it's based not solely on the
>> label text, but
>> also class and layer indexes. For example you wouldn't want to ommit
> a
>> road
>> label 'Washington' because of it's proximity to 'Washington' lake.
>> Those types of
>> situations are not all that rare...
> 
> Sure, but if you do the bitmap collision detection and use the hash for
> 
> MINDISTANCE, the hash would only exist while you are processing a
> single 
> layer and then get dumped and a new hash created for the next layer. If
> 
> you need to keep hash entries unique by CLASS also, then you could 
> either create an array of hashes and index by classnum or have a single
> 
> hash and prefix ot postfix the keys with the class like
> "classnum:key".
> 
> This should be very straight forward and I think very, very fast 
> compared to the way it works now. I'm sure there are other little 
> adjustments that would need to be made, but I can't think of any show 
> stoppers.
> 
> -Steve
> 
>> Steve
>>
>>
>>>>> Stephen Woodbridge <woodbri at SWOODBRIDGE.COM> 1/3/2007 12:14:06 PM
>>>>>
>> If I were dealing with the MINDISTANCE issue in Perl I would create a
> 
>> hash where the key was the label text, because that is what
> MINDISTANCE
>> is restricting, and make the hash value an array of locations. This 
>> would much faster than any search algorithm and you would narrow the
> 
>> search to the array of placed text items of the same name.
>>
>> There is no reason this approach could not be used for the
> MINDISTANCE
>> case in mapserver. Using this and the bitmap collision detection
> should
>> remove most of the overhead of the label cache processing and make it
> 
>> very scalable with respect to number of items to be processed.
>>
>> -Steve
>>
>> Steve Lime wrote:
>>> I did think of one other gotcha. The cache is not just used for
>> physical
>>> overlap, there
>>> are other tests applied. For example, MINDISTANCE, which filters
> out
>>> like labels based
>>> on proximity. That requires direct comparison of the bounding
>> polygons.
>>> One might be
>>> able to reduce that to a test on label points (in fact that might
> be
>>> what's there now,
>>> I'd have to check) but that still requires traversing placed labels
>>> (albeit only those
>>> within a particular layer)...
>>>
>>> Steve
>>>
>>>>>> Stephen Woodbridge <woodbri at swoodbridge.com> 1/2/2007 1:27:47 PM
>>>>>>
>>> Yes, it works for all cases. You have to think about the problem as
> a
>>> raster problem not a vector or character problem.
>>>
>>> So you do this by creating an image the size of rectangular bbox of
>> the
>>> label. Then you render the the character or label polygons boxes
> into
>>> the image. The image now becomes a mask representing the pixels of
>>> where 
>>> the label will fall if it was rendered.
>>>
>>> the test: Logical AND this mask image with the map label image
> given
>>> its 
>>> position in that image. You have a collision on the first AND
> result
>>> the 
>>> it true.
>>>
>>> to update the map label image, just OR this label mask with the map
> 
>>> label image given its position in that image.
>>>
>>> if you want to test another location, ie: you are searching for an 
>>> location nearby that the label will fit, then change the position
> by
>> a
>>> pixel and repeat the test. This is more costly than just a test,
> but
>> is
>>> still pretty efficient. I don't think you would want to apply this
> to
>>> every label, but having it as an option for a layer might be 
>>> interesting. You could constrain this to say search up/down or 
>>> right/left 10 pixels and then something like lot labels could
> easily
>> be
>>> staggered. This is just an idea and it might not farm out to be
>> useful,
>>> but once the machinery is in, it falls out as a simple variation on
>> the
>>> theme.
>>>
>>> -Steve
>>>
>>> Paul Spencer wrote:
>>>> This would need to work for 'curved' labels too.  Would it work to
>> do
>>>> things per-character?
>>>>
>>>> Paul
>>>>
>>>> On 2-Jan-07, at 12:40 PM, Steve Lime wrote:
>>>>
>>>>> Remember also that you're not dealing strictly with bounding
> boxes
>>> (e.g.
>>>>> rects) but
>>>>> actual polygons whenever rotation is applied. This is so you get
>>>>> relatively tight fits
>>>>> around labels and can cram labels on a map.
>>>>>
>>>>>>>> Stephen Woodbridge <woodbri at swoodbridge.com> 1/2/2007 10:25:49
>>> AM
>>>>> Steve Lime wrote:
>>>>>> Interesting idea. No, it's not been looked at before, not by me
>>>>>> anyway. Frank might have some thoughts on performance. You'd
>> still
>>>>>> have to loop through the affected area to find collisions but I
>>>>> think
>>>>>> your correct in that the search time would basically be constant
>>>>>> (affected slightly by the size of the label).
>>>>>>
>>>>>> Wouldn't be that hard to implement. I wonder if GD holding the
>>> cache
>>>>>> image could be useful or if a custom 2-D bit array would be
>>> faster-
>>>>>> probably the latter...
>>>>> I thin a 2-D bit array would be much faster and require
>>> significantly
>>>>> less memory, but would require writing a little additional code.
>>> The
>>>>> upside to this is that the code could be optimized for this
>>> specific
>>>>> task, unless there are other potential uses for this.
>>>>>
>>>>> Another idea I had related to this scheme would be that you could
>>>>> implement a function could search for a placement position
> withing
>>> some
>>>>> number of pixels. Here the idea would be that you use the polygon
>>> to
>>>>> create a mask and see if the mask collides, if not you are done,
>>>>> otherwise you do a spiral search outwards from the anchor point
>> for
>>>>> some
>>>>> number of pixels looking for a place the label will fit. This is
>>> easy
>>>>> to
>>>>> do in some nested loops and very fast.
>>>>>
>>>>> This idea of a moving mask is fundamental to a lot of the raster
>>>>> manipulation algorithm and is easy to implement and very fast to
>>>>> execute.
>>>>>
>>>>> -Steve W
>>>>>
>>>>>> Steve
>>>>>>
>>>>>>>>> Stephen Woodbridge <woodbri at SWOODBRIDGE.COM> 1/2/2007 9:46:46
>>>>>>>>> AM
>>>>>>>>>
>>>>>> HNY All,
>>>>>>
>>>>>> Last year I spent a fair amount of time optimizing a bunch of
>>>>>> mapfiles
>>>>>>
>>>>>> for performance. One of the areas that seems to always be a
>>>>>> performance
>>>>>>
>>>>>> hog if processing the Label Cache. I have been successful are
>>>>>> thinning
>>>>>>
>>>>>> down the number of labels going into the cache, but I wondering
>> if
>>>>>> there might be a more efficient way of dealing with it. Since I
>>> have
>>>>>> been playing with rasters over the holidays I was wondering if
> it
>>>>>> might be possible to implement the label cache as a bitmap mask
>>> and
>>>>>> us a raster
>>>>>>
>>>>>> OP to see if there is a collision?
>>>>>>
>>>>>> OK, I'm sure it is possible, I guess my real question(s) are:
>>>>>>
>>>>>> Has this approach been looked at before? Any thoughts on
>>>>> performance?
>>>>>> My idea is:
>>>>>>
>>>>>> 1) create an label cache image 2) to check a label, get its bbox
>>>>>> polyon check if it overlaps and set pixels 3) polygon fill the
>>> bbox
>>>>>> of a label into the label cache image when a label is rendered
>>>>>>
>>>>>> we might be able to use a 2:1 or 4:1 reduced size label cache
>>> image
>>>>>> with some thought about how to work around the issues.
>>>>>>
>>>>>> I would think that this would be much faster than searching a
>> list
>>>>> of
>>>>>>  drawn labels and checking for intersections. And it should be
>>>>> linear
>>>>>>  with the number of labels in the cache.
>>>>>>
>>>>>> -Steve
> +-----------------------------------------------------------------+
>>>> |Paul Spencer                          pspencer at dmsolutions.ca   
> |
> +-----------------------------------------------------------------+
>>>> |Chief Technology Officer                                        
> |
>>>> |DM Solutions Group Inc                http://www.dmsolutions.ca/
> |
> +-----------------------------------------------------------------+