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);

Lines shown here

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?

Engineer · Accepted Answer · 2012-03-24T07:43:33.370

6

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.

edited Mar 24 '12 at 07:43

answered Mar 23 '12 at 15:00

Engineer

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Perfect. I was sure I had tried that, but must have missed it because it worked perfectly. – Ben Mar 23 '12 at 15:30
any insight on why that actually makes it work? i think that's one of the most inobvious fixes i've ever seen :D – divillysausages Mar 23 '12 at 22:39
1

@divillysausages Check again my answer. – Engineer Mar 24 '12 at 07:45
@user1113426 yeah, i wasn't understanding that. `draw()`ing turns it into raster, and using a `BlendMode` does the same, so I didn't see how rastering it *again* made it work :) Makes sense now I suppose – divillysausages Mar 24 '12 at 11:31
Thanks - this fixed a similar issue for me – tarling Sep 28 '12 at 09:20
Thanks , my same problem get fixed. – Katax Emperore Apr 11 '13 at 04:05

score 1 · Answer 2 · answered Mar 23 '12 at 15:04

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.

score 0 · Answer 3 · answered Mar 23 '12 at 14:41

0

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.

answered Mar 23 '12 at 14:41

Brooks Hanes

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Strange behaviour when using BlendMode "erase" in flash AS3

3 Answers3