ArcObjects - Create and Draw a Transparent Polygon

Question

I'm trying to create a transparent Polygon in ArcMap; however, it's never transparent. Here's the code I have.

// Create polygon
IPolygon polygon = new PolygonClass();
// create & add points,......

// Create lineSymbol
ISimpleLineSymbol lineSymbol = new SimpleLineSymbolClass();
// set color & style,....

// Create fillSymbol
ISimpleFillSymbol fillSymbol = new SimpleFillSymbolClass();
fillSymbol.Outline = lineSymbol;
fillSymbol.Color = //set to color red//;
fillSymbol.Color.Transparent = 0;
fillSymbol.Style = esriSimpleFillStyle.esriSFSSolid;

// Draw Shape
myMap.DrawShape(polygon, fillSymbol);

RESULT ==> A solid red polygon is drawn. The Transparent setting is ignored.

Thoughts?

Answer 1

As per ArcObjects docs

IColor.Transparency Property: Transparency is the degree to which a color is opaque. True transparency is only supported at the layer level. For graphic elements, 0 for transparent and 255 for opaque are the only supported values. Use either the ILayerEffects or ITransparencyRenderer interfaces to set layer transparency. For layer transparency to work properly the display device must be set to "True Color (32 bit)".

Answer 2

When you need your polygon symbol to be fully transparent, assign a null color to the fill symbol as others have suggested. Another way would be to set the Transparency of the fill color to 255, which has the same effect. You can also set your simple fill style to hollow. All this can be done for graphic elements so your drawings persist in the map.

If you want to draw with transparency values other than 0 or 255, you can draw using display filters. See the code below:

        // as an example, we'll draw a rectangle in the middle of the current extent
        var rectangle = activeView.Extent;
        rectangle.Expand(0.5, 0.5, true); // create a rectangle in the middle

        var lineSymbol = new SimpleLineSymbolClass();
        lineSymbol.Style = esriSimpleLineStyle.esriSLSSolid;
        lineSymbol.Color = new RgbColorClass {Red = 0, Green = 0, Blue = 0};

        var fillSymbol = new SimpleFillSymbolClass
                         {
                             Style = esriSimpleFillStyle.esriSFSSolid,
                             Outline = lineSymbol,
                             Color = new RgbColorClass { Red = 255, Green = 0, Blue = 0}
                         };

        var screenDisplay = activeView.ScreenDisplay;
        var transparencyDisplayFilter = new TransparencyDisplayFilterClass
                            {
                                Transparency = 127
                            };

        try
        {
            screenDisplay.StartDrawing(0, (short)esriScreenCache.esriNoScreenCache);
            screenDisplay.Filter = transparencyDisplayFilter;
            screenDisplay.SetSymbol(fillSymbol);
            screenDisplay.DrawRectangle(rectangle);
        }
        finally
        {
            screenDisplay.FinishDrawing();
        }

This draws a red rectangle semi-transparent (semi-opaque) in the map's display. It can be easily modified to draw any geometry, like polygons, the rectangle was just easy to construct for the sake of the example.

However, as you can see, the code does not (and cannot) use any graphic elements, so if you draw this way, the drawing does not persist in the display when it gets refreshed. One way to solve is to run any drawing code like this in the IActiveViewEvents.AfterDraw event so it gets executed after every refresh.

Answer 3

fillSymbol.Color.Transparent = 0;

should be multiple operations.

RgbColor selectedColor = getColor(0, 0, 0);
selectedColor.Transparency = (byte)0;
fillSymbol.Color = selectedColor;  


public static RgbColor getColor(int r, int g, int b)
{
RgbColor color = new RgbColor();
color.Red = r;
color.Green = g;
color.Blue = b;
return color;
}

This code works for me, I am creating an transparent point, so I assume it will work for polygons.

See this for more context: Setting color of a feature "street" with a FeatureRenderer

you actually get a copy of the renderer's symbol object. If you apply any changes to the symbol:... you need to assign the symbol back to the renderer:

thanks to petr k.

Answer 4

'Code example - ILayerEffects

Dim pGeoFeatureLayer as IGeoFeatureLayer

Dim pLayerEffects As ILayerEffects

Set pLayerEffects = pGeoFeatureLayer

pLayerEffects.Transparency = 70

Answer 5

You're looking to have the 'No Fill' effect, correct? If so, then you really want a "null color".

fillSymbol.Color.NullColor = true;

Answer 6

Use esriSimpleFillStyle.esriSFSHollow. Or even esriSimpleLineStyle.esriSLSNull can be used.