A PDF version of this manual is available here.

Filters in Aladin

Thomas Boch

September 19th, 2006
Version 1.16


1  Introduction - What can I do with these filters

Filters are a new feature in Aladin allowing one to customize the display of catalogue planes in Aladin.

Basically (without filters), a catalogue plane :

Thanks to the filters feature, it is possible to :

Notice that the position of visible symbols will of course remain their sky position.

Briefly, filters allow you not only to select sources on the basis of constraints, but also to customize the way a catalog plane is displayed, using the catalogue contents.

Figure examplePic  is an example of what you can do with filters : we have here splitted sources into 3 classes according to the value of their photographic B magnitude.


2  Syntax

A Filter consists of a list of Constraints and associated Actions.
Example of filter :

Constraint 1 { Action 1
              Action 2 }
Constraint 2 { Action 3 }

A Constraint is a set of Conditions combined by logical operators (AND, OR).
Example of constraint : ( Condition 1 && Condition 2 ) || Condition 3

A Condition applies to Parameters (or combination of parameters).
Examples of conditions :

Parameter >= Value
Parameter 1 - Parameter 2 = Value

Parameters are specified via the corresponding UCDs or column names. Examples of parameters :
via UCD : $[PHOT_JHN_V]
via column name : ${Vmag}

A Value can be a numeric value or a string value.

The global structure is quite similar to a switch structure in C which would have a break statement at the end of each case. It means that if a source verifies one constraint, the program won't run into following cases for this source.

Note : for a given constraint, the list of actions immediately follows the constraint and must be embraced by brackets {}. Each action has to be separated either by a carriage return or by a semicolon.


Figure algorithmPic  illustrates how a filter is applied to a set of sources. If a source verifies the first constraint, then each corresponding action is made and the next source is processed. If it doesn't, we check whether the source verifies the next constraint, and so on until there are no more blocks. If the source verifies no constraint at all, it is not drawn.

2.1  Pointing out columns

In constraints and in the parameters of actions, UCDs and column names have to be pointed out in a specific way.

The syntax is :

UCD names are case insensitive (in fact, they are automatically converted to uppercase) whereas column names are case sensitive.

The star * and the question mark ? can be used as wildcards in UCDs or column names. For instance, $[PHOT*] will point out the first column name being tagged by a UCD beginning by PHOT.


You can put comments into your filters. Each line beginning with "#" will be ignored.
E.g :
# This is a comment 

2.3  Syntax of arithmetic expressions using UCDs/columns

You can use the following operators : +,-,*,/,^  to combine catalogue parameters. These parameters are specified via the corresponding UCDs or column names. They can be used to define constraints but also in some action parameters.
${Bjmag}-$[PHOT_JHN_V]>1 {
# Do some action
draw ellipse( $[EXTENSION_RAD], 
              $[POS_POS-ANG] )

Moreover, you can use the following functions:

log(abs(${Fi})/${Fx})>44 {draw}

2.4  Syntax of Conditions

A Condition is an arithmetic combination of Parameters followed by a Comparison Operator followed by a Value.

The table below summarizes the allowed comparison operators for a numeric value.

Operator |Meaning |Example| |
= |Equality |=1| |!= |Inequality |!=1| |>= |Greater or equals |>=12.0| |> |Strictly greater |>12.0| |<= |Less or equals |<=12.0| |< |Strictly less |<12.0| |

The table below summarizes the allowed comparison operators for a string value.

Operator |Meaning |Example| |
= |String equality (case sensitive) |="Galaxy"| |!= |String inequality (case sensitive) |!="UV"| |

When performing a comparison on a string, you can only use one parameter, ie combination of parameters is not allowed in this case.
It is allowed to use the star * and the question mark ? as wildcards when performing string comparison.

# Draw in green sources whose name start with "IRAS"
$[ID_MAIN]="IRAS*" {draw green}
# Draw in red sources with object type "GNe" or "BNe" or "DNe" ...
${otyp}="?Ne" {draw red}
Remark: in order to escape the wildcard star * character, prefix it with a backslash.

Note : there is a special condition called undefined(ucd or column name) which is true if the entered ucd/column name is not present for the current object.

# Process first sources without the column Bjmag --> draw them black
undefined(${Bjmag}) {
    draw black
# Process sources with column Bjmag

2.5  Syntax of Constraints

The logical operators used to combine conditions are && – logical AND – and || – logical OR.
($[PHOT_PHG_B]<16 && $[PHOT_PHG_R]<15) || $[CLASS_OBJECT]="Star" {

A constraint can also be empty. In such a case, the corresponding actions will apply to all sources.

# No constraint, the action block is applied to all sources
{draw blue}

2.6  Syntax of Actions

Actions allow to change the appearance of symbols in a catalogue plane.
Available actions are :

2.6.1  How to define a color

You can specify the color you want to assign to either a shape or a string.
The color function is optional and can take place be either after the draw keyword, or after the optional shape function. E.g:
# Color function after the "draw" keyword
draw blue square
# Color function after the shape function
draw circle(-$[PHOT_PHG_B]) #00ff00

There are different ways to define a color :

2.6.2  How do I customize the shape

You can customize the shape of sources. It is an optional parameter, which takes place either just after the draw keyword, or after the color function.

There are shapes without parameter : they are the same as those in the properties of a catalog plane :


# Filter drawing different shapes according to the class of the object

# Draw a plus for Star objects
$[CLASS_OBJECT]="Star" {draw plus}
# Draw a rhomb for Radio objects
$[CLASS_OBJECT]="Radio" {draw rhomb}
# Draw a dot for other objects
{draw dot}
Furthermore, there are some shape functions requiring parameters :

3  Usage in Aladin

3.1  Creating a filter


Figure creationPic  describes how to create a filter. First, click on te Filter button in the tool bar. This will create in the stack a new plane dedicated to a filter. The Properties window will pop up, in order to let you enter your definition. Type in your filter definition, and press the Apply button.

In the properties window of a filter, predefined filters are intended to help you understand the syntax of a filter.
If you right-click in the definition text area, a pop-up help menu will appear and allows easy access to columns/UCDS names and list of actions.

3.2  Modify a filter

To modify a filter, open the Properties window of the filter. Then, modify the definition and press Apply to confirm the changes.
If the filter is active, the result will be updated. If it is not, the new definition will be taken into account as soon as the filter becomes activated.

3.3  Syntax errors

When creating or modifying a filter, you may enter a definition which is syntactically incorrect. If it happened, the filter is deactivated, a message pops up to let you know what the error is. At the same time, the status pastille next to the filter name in the stack becomes red.

If you try to activate a filter whose definition is incorrect, you will see a message asking you to correct the error.

3.4  Activate/Deactivate a filter

Filters are like other planes : you can easily activate or deactivate them, just by clicking on the logo.

3.5  Scope of a filter

A filter applies to all active catalogue planes located below it. If the filter is in a folder, it applies to all active catalogues located below it and in the same folder, including catalogues being in subfolders.


Figure scopePic  explains this principle. We have created 2 filters, TEST and CIRCLE. TEST just prints the string TEST, CIRCLE draws a circle according to the magnitude of the object.
As you can see, the filter TEST applies to planes GSC1.2 and USNO2 which are below it, but does not apply to plane Simbad. CIRCLE applies to GSC1,2, which is in the same folder, but does not apply to USNO2 even though it is located below CIRCLE.

Whenever a filter or a catalogue plane has moved, whenever a new catalogue plane is created, filters results are automatically reprocessed and updated.

3.6  Applying multiple filters

You can apply several filters simultaneously. Each filter has its own scope, and each filter performs its actions apart from each other.

3.7  Creating a filter in script mode

You can create filters in script mode, via in-line commands.
The syntax is almost identical to the one you use in graphical mode. The only difference is that you have to begin with filter filtername { and to finish with a closing bracket }. The filter definition can be on multiple lines : once you entered filter filtername {, the prompt becomes Aladin - Filter def. and waits until you enter the final }.
Aladin> filter circle {
Aladin - Filter def.> {draw circle(-$[PHOT*])}
Aladin - Filter def.> }

Filters can also be activated/deactivated with the command:

filter [filtername] [on|off]

3.8  Miscellaneous


Figure miscPic  shows 3 highlighted buttons contained in the Properties window of a filter.

This feature can be used regardless of the activated/deactivated status of the filter.


A  Backus Naur Form of filter syntax

This is an attempt to describe the syntax of a filter in the extended Backus-Naur-Form. It describes in a shorter way what has been explained in section sectionSyntax.

<filter> ::= <constraints block>+

<constraints block> ::= <constraint> "{" (<action><action separator>)* "}"

<action separator> ::= Carriage Return | ";"

<constraint> ::= ( <simple condition> [ <logical operator> <constraint> ] ) 
                         | "undefined(" (<UCD>|<column>) ")"

<condition> ::= <expression> <comparison operator> <value>

<expression> ::= ( ["+"|"-"] (<UCD>|<column>|<numeric>)
                 [ ( ("+"|"-"|"*"|"/") <expression>  | "^" <numeric> ) ] )
                 | <function>"("<expression>")"

<function> ::= abs | cos | deg2rad | exp | ln | log | rad2deg | sin | tan

<value> ::= <string> | ( <numeric> [<unit>] )

<logical operator> ::= "&&" | "||"

<comparison operator> ::=  "==" | "!=" | ">" | ">=" | "<" | "<=" 

<action> ::= 
"draw" [ <color function> ] [ (<shape function>|<UCD>|<column>|<string>) ] 
| "hide"

<UCD> ::= "$[" UCD name "]"

<column> ::= "${" column name "}"

<shape function> ::= (("circle"|"fillcircle"|"fixedcircle")"("
                       <expression> [","<numeric>","<numeric>]")")
                   | "ellipse(" <expression> "," <expression> "," <expression> ")"
                   | "pm(" <expression> "," <expression> ")"
                   | "rectangle(" <expression> "," <expression> "," <expression> ")"
                   | "line(" <expression> "," <expression> ","
                                <expression> "," <expression> ")"
                   | "square" | "rhomb" | "cross" | "plus" | "dot" | "microdot"

<color function> ::= "rgb(" <expression>, <expression>, <expression> ")" 
                   | "rainbow(" <expression> ")"
                   | "black" | "blue" | "cyan" | "darkGray" | "gray" | "green" 
                   | "lightGray" | "magenta" | "orange" | "pink" | "red" | "white"
                   | "yellow"