Strange behaviour when using BlendMode “erase” in flash AS3

Question

Can any one expain how to avoid seeing lines when using BlendMode.ERASE in AS3?

Here is an example. I draw a black background to the stage and then draw 2 overlaping circles to a sprite and try to erase them from the background.

            var solidBitmapData = new BitmapData(550,400,true,0x000000);
            var mySpriteLayer = new Sprite();

            // Create black background.
            mySpriteLayer.graphics.beginFill(0x000000);
            mySpriteLayer.graphics.drawRect(0,0,550,400);
            mySpriteLayer.graphics.endFill();

            // Draw it to bitmap data.
            solidBitmapData.draw(mySpriteLayer);

            // Clear sprite.
            mySpriteLayer.graphics.clear();

            // Draw two circles
            mySpriteLayer.graphics.beginFill(0xFF0000);
            mySpriteLayer.graphics.drawCircle(200,200,50);
            mySpriteLayer.graphics.endFill();
            mySpriteLayer.graphics.beginFill(0xFF0000);
            mySpriteLayer.graphics.drawCircle(250,200,50);
            mySpriteLayer.graphics.endFill();

            // Draw circles to bitmap with blend mode erase.
            solidBitmapData.draw(mySpriteLayer,null,null,BlendMode.ERASE);

            // Create bitmap and add to stage.
            var solidBitmap = new Bitmap(solidBitmapData);
            addChild(solidBitmap);

I'm talking about the lines in the middle of the circles. It seems to be something to do with linestyle but I've tried setting it to zero and the alpha to 0 but I can't get rid of the lines.

Any ideas?

Answer 1

You need to set cacheAsBitmap property of ' mySpriteLayer ' to ' true ' :

     mySpriteLayer.cacheAsBitmap = true;

As blend mode performs computations on pixels, it works more precisely with raster data, than with vector data.

Answer 2

this.blendMode = BlendMode.LAYER;

Adobe's ActionScript 3.0 Reference says about class BlendMode and field ERASE:

Erases the background based on the alpha value of the display object. This process requires that the blendMode property of the parent display object be set to flash.display.BlendMode.LAYER.

Answer 3

Perhaps this is the way the Sprite blend is reacting to the background. Have you tried this with two black circles on a white background? If you get the same lines (only white) you are able to conclude this is the way the blend works.