Opacity in Image.getPixelReader.getColor()

[Jasper Potts]

My understanding is alpha and opacity are same just different range. 

Opacity is 0.0 to 1.0
Alpha is 0 to 255

Jim or Kevin will be authority on this. 

Jasper

> On Mar 16, 2014, at 5:55 AM, Tom Schindl <tom.schindl at bestsolution.at> wrote:
> 
> Hi,
> 
> Maybe I'm completely wrong but to me it looks like the opacity I get
> from Image.getPixelReader.getColor() is wrong.
> 
> If not mistaken the relation between alpha and opacity is expressed with:
> 
> opacity = (255 - alpha) / 255.0
> 
> which means:
> 
> opacity 0 => alpha 255
> opacity 1 => alpha 0
> 
> Running the following programm on a gif
> 
>> public class OpacityBug extends Application {
>> 
>>    @Override
>>    public void start(Stage primaryStage) throws Exception {
>>        Image image = new Image(getClass().getResourceAsStream("methpri_obj.gif"));
>>        int width = (int) image.getWidth();
>>        int height = (int) image.getHeight();
>>        
>>        PixelReader reader = image.getPixelReader();
>>        for (int x = width - 1; x >= 0; x--) {
>>            for (int y = height - 1; y >= 0; y--) {
>>                int argb = reader.getArgb(x, y);
>>                int alphaArgb = (argb >> 24) & 0xFF;
>>                Color color = reader.getColor(x, y);
>>                double opacity = color.getOpacity();
>>                System.err.println(x+","+y + " => alpha: " +  alphaArgb + " op: " + opacity + " op-alpha: " + opacityToAlpha(opacity) + " color: " + color);
>>            }
>>        }
>>    }
>>    
>>    
>>    private static int opacityToAlpha(double opacity) {
>>        return (int)Math.round((((opacity * 255.0) - 255) * -1));
>>    }
>> 
>>    public static void main(String[] args) {
>>        Application.launch(args);
>>    }
>> }
> 
> produces and output like:
> 15,15 => alpha: 0 op: 0.0 op-alpha: 255 color: 0x00000000
> 
> which to my understanding is wrong. The argb value is correct (if
> compared to the image because the pixel at 15,15 is fully transparent)
> but then the the opacity should be 1.0 but is the opposite.
> 
> Looking at the code in Image I see:
>> @Override
>>                public Color getColor(int x, int y) {
>>                    int argb = getArgb(x, y);
>>                    int a = argb >>> 24;
>>                    int r = (argb >> 16) & 0xff;
>>                    int g = (argb >>  8) & 0xff;
>>                    int b = (argb      ) & 0xff;
>>                    return Color.rgb(r, g, b, a / 255.0);
>>                }
> 
> which means that:
> a) my formula from the above is wrong
> b) or there's the bug because it should be
>   return Color.rgb(r, g, b, (255 - a) / 255.0);
> 
> May I also suggest to add get a
> - Color.argb(int r,int g,int b,int alpha)
> maybe it is just me but I'm so much more used to alpha than opacity that
> I always have to look it up when i need it.
> 
> Tom