Opacity in Image.getPixelReader.getColor()

[Tom Schindl]

Hi,

Yes it looks like i mixed this up with transparency!

Tom

On 17.03.14 10:03, Kevin Rushforth wrote:
> 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
>>>