Opacity in Image.getPixelReader.getColor()

[Kevin Rushforth]

I'm sure Jim could word this better, but here goes...

Opacity and alpha are mostly used interchangeably. Whether the range is 
expressed as a normalized value [0.0,1.0] or an 8-bit pixel value 
[0,255], the meaning is the same. A value of 0 means that the color is 
completely transparent. When using the default SRC_OVER blend equation, 
a pixel with this color will have no effect. A value of 1.0 (if using a 
normalized value) or 255 (if using an 8-bit integer value) means that 
the color is fully opaque. When using the default SRC_OVER blend 
equation, a pixel with this color will overwrite whatever is underneath it.

Perhaps Tom is thinking of "transparency" which, if we were to have such 
an attribute, would be defined as (1.0-opacity).

-- Kevin


Jasper Potts wrote:
> 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
>>