Posts Tagged alpha

BlendMode.LAYER; A must when changing the alpha property of a DisplayObject containing other DisplayObjects

Posted by zedia.net in ActionScript 3 on April 28th, 2008

I don’t know if you ever tweened a MovieClip’s alpha while this MovieClip contained multiple assets, but I kinda do this all the time (fade in ,fade out being the easiest transition for almost everything). The default way Flash handles this is by changing the alpha of every assets in the containing MovieClip (or Sprite). Sometimes it doesn’t matter but most of the time that’s not the behavior you would like it to have. When you have shapes or bitmaps that overlap or hide other graphical assets you will see what is under it when changing the alpha instead of actually changing the alpha of the container as if it was a whole.

Well I don’t know if it’s clear but I have an example below that should clarify that. Anyway, I have stumbled upon the solution to this while browsing through the help files for BlendMode. If you set the blendMode of your containing DisplayObject to BlendMode.LAYER Flash will consider this DisplayObject as only one layer and will change the alpha the way you normally think it would.

Here is the example that will illustrate my point better. The left Sprite is the reference object, the middle Sprite has no blendMode on and the right Sprite has a blendMode of BlendMode.LAYER. Mostly look at the colors while tweening and the overlapping regions of the green and blue ellipses.

I really wish I had found this earlier and I hope this will help others that were in my situation.

Cet article en Français

, , , ,

14 Comments


Converting a RGB color and Alpha to ARGB in ActionScript 3

Posted by zedia.net in ActionScript 3 on March 25th, 2008

So I was using the color picker component that comes with the Flash IDE. When the user selects a color, the color picker fire a ColorPickerEvent.CHANGE event and from that event you can access the new color that was chosen. The format of this color is a uint; when working with colors I am more used to the hexadecimal format.  From the event you can also access to the hexadecimal value, but as a String which is not that useful if you want to do operation on the color.

The color you get from the color picker component is in the RGB format, so no transparency. If you want to add an alpha channel to that color (converting it to ARGB), let’s say to use the setPixel32 method of the BitmapData class  and produce Bitmap with transparency, you’re going to have to modify the uint you can do so using this simple function (AS3):

1
2
3
4
5
6
7

private function returnARGB(rgb:uint, newAlpha:uint):uint{
  //newAlpha has to be in the 0 to 255 range
  var argb:uint = 0;
  argb += (newAlpha<<24);
  argb += (rgb);
  return argb;
}

What this function does is that it bit shift the value of the Alpha you input and adds it to the original RGB color making it ARGB. You see, RGB format uses 8 bits for each color for each pixel (so 24 bits for each pixel), when you add transparency, the alpha channel takes up another 8 bits for each pixels (making it 32 bits for each pixel thus the 32 in setPixel32). That also explains why newAlpha has to be from 0 to 255, because 255 is the highest number you can store in 8 bits. The Blue color takes the bits from 1 to 8, the Green color takes the bits from 9 to 16, Red from 17 to 24 and finally Alpha from 25 to 32. This is why we bit shift alpha from 24 bits (newAlpha<<24);

I don’t use the color picker component often, but there seem to be missing the end of the sentence here.

, , , , , , ,

5 Comments