CODEWITHSUNDEEP
actionscript-3actionscriptcolorsbitmaprendering
Marty
Marty

Reputation: 39456

Multiply alpha channel in an AARRGGBB color

I have an AARRGGBB value which is used to fill cells in a grid that I am using in my attempt at a lighting engine:

var color:Number = 0xFF000000; // Full opaque black.

There are light sources which have a radius parameter. The distance is measured from the light source cell to nearby cells within that radius. A percentage value is then given to each nearby cell, which is:

distanceFromSource / sourceRadius

So a higher percentage represents a cell further away from the source.

I want to multiply the alpha channel in the color I have above by the percentage, and fill the cells with the resulting values. Basically I want between 0-100% AA in the AARRGGBB value. When I try to do straight multiplication I get odd results:

enter image description here

I believe I need to use special operators for this, alongside the values seen in BitmapDataChannel. Unfortunately this is where I've gotten stuck.

How can I multiply the alpha channel in an AARRGGBB color by a percentage?

Upvotes: 2

Views: 851

Answers (1)

mfa
mfa

Reputation: 5087

You need to preserve the rgb values of your pixel. Only multiply the alpha byte of the uint.

function multiplyAlpha(color:uint, percent:Number):uint
{
  //returns the pixel with it's aplha value multiplied by percent
  //percent is expected to be in the range 0..1
  var a:uint = (color >> 24) * percent;
  var rgb:uint = color & 0x00ffffff;
  return ((a<<24) | rgb);
}

function setAlphaByPercent(color:uint, percent:Number):uint
{
  //returns the pixel with it's a new alpha value based on percent
  //percent is expected to be in the range 0..1
  var a:uint = 0xff * percent;
  var rgb:uint = color & 0x00ffffff;
  return ((a<<24) | rgb);
}

Upvotes: 3

Related Questions