[fpc-devel] TFPCustomCanvas Expanded Clipping

[michael.vancanneyt at wisa.be]

On Wed, 30 Nov 2011, Felipe Monteiro de Carvalho wrote:

> Hello,
>
> I am facing a problem with clipping in TFPCustomCanvas because it has
> these properties:
>
>    property ClipRect : TRect read GetClipRect write SetClipRect;
>    property Clipping : boolean read GetClipping write SetClipping;
>
> Which obviously is not extensible for non-rectangular formats. I
> already have written my own extensible and object oriented clipper,
> but I'm unsure how to properly integrate it into TFPCustomCanvas. To
> diminish code duplication and to allow fpvectorial to use
> TFPCustomCanvas it would be good if TFPCustomCanvas and it's
> descendents are at least aware that someone might use a different kind
> of Clipping.
>
> So I am thinking of adding these properties in TFPCustomCanvas:
>  property ClipRegion: TObject read GetClipRegion write SetClipRegion;
>  property RegionClipping: Boolean read GetRegionClipping write
> SetRegionClipping;
>
> So TFPCustomCanvas would be aware that we are can have an extended
> clipping, although it by itself does not know how to process it. Here
> I see 2 options of how to interprete the 2 clipping systems:
> 1> Make them independent, so if you set both Clipping and
> RegionClipping you might get 2 clippings done o.O
> 2> Make them dependent, so RegionClipping only sets if ClipRect or
> ClipRegion should be used. Instead of checking for Clipping it would
> check for (Clipping and not RegionClipping) to apply the ClipRect
>
> The generic object TObject is utilized here for ClipRegion because
> TCanvas and TLazCanvas will use different regions, which match only in
> LCL-CustomDrawn.
>
> what do you think?

Good basics.

What about the following:

Create TBaseClipRegion with abstract method PointIsInRegion(P : TPoint)
(and maybe some others, like line intersections or so).

Create a descendent TRectClipRegion with property ClipRect: TRect.
It can implement the abstract method(s).

Then implement the ClipRegion property, as you propose.

Setting the ClipRect property just sets ClipRegion to a TRectClipRegion 
instance with the Rect filled in. (TFPCustomCanvas should own this instance, i.e.
destroy when canvas is destroyed)

Property clipping just determines whether the ClipRegion object 
is used, yes or no.

Seems pretty straightforward to me, backwards compatible, and only a single
extra property is introduced (ClipRegion).

Michael.