Editing OnFilterStart - FilterMeisterWiki

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= OnFilterStart =

== Syntax ==
:{{OnFilterStart: ((<compound-statement>))}}

== Return ==
:{{((false))}} if filter processing is to continue with the {{((ForEveryTile))}} handler, or {{((true))}} to abort further filter processing (though this latter option is currently ignored in FM 0.4.20).
== Description ==
:The {{OnFilterStart}} handler is invoked at the start of each filter run, just before the {{((ForEveryTile))}}, {{((ForEveryRow))}}, {{((ForEveryPixel))}}, and {{((R, G, B, A))}} handlers are called.  The {{OnFilterStart}} handler is a good place to perform one-time calculations at the start of a filter run, check for valid control inputs, valid image mode, etc.
== Default handler ==
:The default {{OnFilterStart}} handler simply returns {{((false))}}, passing control to the {{((ForEveryTile))}} handler for every tile in the image or preview.

== Example ==
:You can test the current image mode in the {{OnFilterStart}} handler, and abort the invocation if the image mode is inappropriate.  For example:
<code>
OnFilterStart:{
    if (imageMode != RGBMode) {
        ErrorOk("Not an RGB image.");
        doAction(CA_CANCEL); /* for now, until return true works correctly */
        return true; /* abort filter processing */
    }
    return false; /* continue with ForEveryTile */
}
</code>
:Note that we added a call to {{((doAction))}} to explicitly cancel the filter run, since return {{((true))}} does not yet correctly abort the filter run. Once this is fixed, the call to {{((doAction))}} can be removed.
== Also see ==
:((handlers)), ((ForEveryTile)), ((ForEveryRow)), ((ForEveryPixel)), ((OnFilterEnd))
== Comments ==
:Everyone can add his/her comments about this handler here. Tips for using it are welcome, too.

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