Perl Mapscript Problem

[Sean Gillies]

On Sep 15, 2005, at 9:27 AM, Stephen Woodbridge wrote:

> Sean Gillies wrote:
>> On Sep 13, 2005, at 7:09 PM, Stephen Woodbridge wrote:
>>
>>> Hi All,
>>>
>>> I had dynamic symbol manipulation working on MS 4.01 but can not get 
>>> it
>>> to work on 4.4.1 or 4.6.1. In the following code snippets every think
>>> works with the default symbol that is defined in symbolset file, but 
>>> the
>>> code does not let me change the imagepath on the fly.
>>>
>>> Also, I can not get the label on the umarker layer to show unless I
>>> specify     LABELCACHE FALSE    and umarker layer is the last in the
>>> mapfile.
>>>
>>> Any ideas or help would be appreciated.
>>>
>>> -Steve W.
>>>
>>
>> Steve,
>>
>> Perhaps mutating the imagepath has no effect due to caching of symbol
>> images. About the labelcache, I would guess that the label is losing 
>> the
>> battle against other in the map.
>
> Sean,
>
> Thank you for your response. You are probably correct on the labelcache
> issue now that I think about it. While it is the last layer and should
> take priority in the cache, it is being drawn after all the other 
> labels
> in the cache are drawn so the only way to get it on top is to not use
> the cache as it is too late in the process.
>
>> This is how I do dynamic symbols
>>
>>     # create a new symbol
>>     new_symbol = mapscript.symbolObj('symbol_a', 
>> '/path/to/symbol_a.png')
>>
>>     # add a *copy* of the symbol to the map symbolset and get the 
>> index
>>     new_symbol_index = map.symbolset.appendSymbol(new_symbol)
>>
>> See http://zcologia.com/mapserver/mapscript#symbolobj and
>> http://zcologia.com/mapserver/mapscript#symbolsetobj or the same docs 
>> on
>> the new mapserver site.
>>
>> The symbolObj also has a method to set symbol imagery directly. See
>> http://zcologia.com/mapserver/imagery-howto#images-and-symbols.
>
> I will look at this, but my concern is that we generate a large number
> of symbols (actually numbered dots on the map) and I'm concerned we 
> will
> exceed the hardcoded limit on symbols in map.h
>

Numbered dots? I have an application that requires numbered symbols: 
highway shields with the highway number. I'm simply using an annotation 
type layer with an unnumbered shield image as a symbol and then using 
the class label to render the highway number over the top. Works 
perfectly. I don't know if this method is something you can use, but it 
would be faster as well as simpler.

> I think that the mapscript API needs to fix the cached image if the
> image file is changed. This is a bug. What good is it to change the
> symbol name if it does not change the symbol also? Would you concur on 
> this?
>

Remember that mapscript is basically a mapfile as an object tree. In 
theory, you can freely modify attributes of the objects, like a 
symbol's imagepath. In practice, much of MapServer's code is still 
based on the assumption that a mapfile is read only once at the very 
beginning and afterwards remains immutable during execution. A similar 
issue is arising in a discussion about layer feature source plugins. 
Many people, it turns out, have been mutating layer.connectiontype in 
their scripts.

IMO, the right solution might be to implement a method for symbolObj 
that loads imagery from disk and updates the cache, and to make 
symbolObj.imagepath immutable. Explicit is better.

> Thank you for your suggestions, I will see if I can get them to work.
>
> -Steve
>

You're welcome, Steve. I hope my highway shield suggestion is useful. 
That would be the most efficient, and worth some data preprocessing.

Sean

--
Sean Gillies
sgillies at frii dot com
http://zcologia.com