PmgCanvas - Deatiled object description

See: the PmgCanvas object.

The PmgCanvas object permanently contains the drawing area (raster image in the memory - bitmap) the size of the Pmg object. The drawing area (the CanvasCtx object) is available in the onDraw event (JavaScript) by the pEvent.GetCtx(state) method. When drawing the object physicaly into the PROMOTIC panel window the current remembered content of the drawing area is used, the onDraw event is not used for redrawing the area.

The concept of foreground/background:
If the animated 2D content is complex, large and some portion of it is changing rapidly, then the CPU load could be quite high. The possible solution of such situation is to divide the complex PmgCanvas object up into two overlaping PmgCanvas objects. One portion being static complex background and the second part being dynamic simple foreground. The Pmg object representing the dynamic foreground is embedded in the static background, constituting one compound Pmg object. This is the standard solution used for creating most of the preconfigured PmgCanvas objects.
The embedded Pmg object (foreground) can access Vars variables of the parent object (background). If the Vars variables of the object item (background) are modified, the refresh of embedded object (foreground) is provided. See the "In onDraw the pEvent.GetParentVar() method will be used" configurator.

The PmgCanvas object allows automated triggering of the onDraw event for redrawing the draw area CanvasCtx if needed:
1) If the drawing area is not initialised (empty) after opening the panel or after change the size of the PmgCanvas object, or after calling the CtxReset method. In this case the drawing area is always empty (transparent).
2) If the input variables (Variables) of the PmgCanvas object were changed. The drawing method may request an existing drawing area, see pEvent.GetCtx(1), or more commonly non-initialized (empty) drawing area, see pEvent.GetCtx(0).
3) If the variables (Variables) of the parent Pmg object (background) PmgCanvas were changed (valid for embedded object - foreground).

The drawing area is redrawn in the onDraw event only if changed. If no change occurs, then it is not necessary to use the script for redrawing the area and the remembered appearance is used. The onDraw event can access only the values of local variables (Variables) of the PmgCanvas object, that are currently used as data interface of the drawing function. It can also use the Pm object and the drawing area object CanvasCtx. The scripting interface of the CanvasCtx object enables most of the methods and properties for 2D drawing used in HTML5 in the <canvas> tag.

- Drawing in the script allows conditioned drawing, repeated drawing, recounting, obtaining and formatting values, (statements if...else, for, Pm object, local variables Variables, etc.).
- It is functional also in the Web panel, drawing directly in client computer browsers, most browsers are supported. The properties and methods for drawing are present in the CanvasCtx object. A standard HTML5 <canvas> tag that is commonly supported; there are many exemples, editors and testers online.
- Faster than SVG in PmgRasterImage (localy and on the Web) and supports text displaying.
- when editing the PmgCanvas object including the onDraw drawing event the object is displayed immediatelly in the development environment (in the graphics editor).
- The designers can use and modify the preconfigured objects based on PmgCanvas.
- On the Web, the graphics (drawing scripts) are integral part of the panel, there is no need to download any aditional graphic files.
See also:
- CanvasCtx (object)
- CanvasCtxExt (object)
- The onDraw event

Pm8.02.05: Created
PROMOTIC 9.0.27 SCADA system documentation MICROSYS, spol. s r.o.

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