Re: Opacity in Image.getPixelReader.getColor()

[Jim Graham]

Kevin and Jasper hit the nail on the head (Jasper's comment that the 
words are usually associated with particular ranges is mostly true 
AFAIRemember, but I can't guarantee it).

If there is part of the documentation here that could be made clearer, 
let us know...

                        ...jim

On 3/17/14 1:07 PM, Tom Schindl wrote:
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.schi...@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