Add ScaledBrightnessARGBConverter

[tinevez]

This is to enable in BDV a better display control for label images

This converter allows for better controling the visibility of ARGB segmentation. The scale value is used to alter the brightness of colors, which is better suited than altering R G and B in bulk.

This could be used in the BDV, to finely control the visibility of a segmentation overlay in the shape of an ARGB image.

[tpietzsch]

If possible, I would like to avoid the dependency on java.awt.Color.

We could just inline the Color.HSBtoRGB and Color.RGBtoHSB implementations.
This would also avoid to have the (non-threadsafe) float[] hsb array.
But I strongly suspect that full conversion to/from HSB is not necessary to change the brightness.
I didn't look into the math (https://en.wikipedia.org/wiki/HSL_and_HSV#HSV_to_RGB etc) in detail, but from looking over it, I think there should be shortcuts. Do you want to dig into that? (Otherwise, I can also dig a bit deeper when I find time)

[tpietzsch]

I couldn't resist... I think it should be more or less:

v := max(r,g,b)
m := min(1, v/scale) - v

and then

r' := min(255, r+m)
g' := min(255, g+m)
b' := min(255, b+m)

Could you try whether that works?

[tinevez]

Done!
Tested with the BDV overlaying a segmentation results as a label image. The scale properly alters the visibility of the segmentation overlay without touching the teint.

[tinevez]

Here is what I have tested, with a label image overlay in the BDV, in 'Sum' mode, and adjusting the scale to 255, 122, and 0.

The current implementation

https://github.com/imglib/imglib2/blob/master/src/main/java/net/imglib2/display/ScaledARGBConverter.java#L102

Scaling brightness via HSB conversion

imglib2/src/main/java/net/imglib2/display/ScaledBrightnessARGBConverter.java

Line 47 in 183efc2

private static final int getScaledColor( final int color, final double scale )

Simple RGB scaling

https://github.com/imglib/imglib2/blob/argb-converter/src/main/java/net/imglib2/display/ScaledBrightnessARGBConverter.java#L47

Conclusion

The simple RGB scaling gives perceptually similar results and is considerably simpler. I suggest we stick to it.

[tpietzsch]

Could you try what I suggested above:
(input color: r,g,b)
v := max(r,g,b)
m := min(1, v/scale) - v
r' := min(255, r+m)
g' := min(255, g+m)
b' := min(255, b+m)
(output color: r,g,b)

I think this should be exactly the same as via HSB conversion.
(I just took the "convert-to-HSB-then-modify-S-then-convert-to-RGB" sequence and simplify the equations).

[tinevez]

I had tried and it does not seem to work. The display is unsensitive to scale and the colors are wrong. Here is what I implemented

	private static final int getScaledColor( final int color, final double scale )
	{
		final int r = ARGBType.red( color );
		final int g = ARGBType.green( color );
		final int b = ARGBType.blue( color );
		final int v = Math.max( r, Math.max( g, b ) );
		final double m = Math.min( 1., v / scale ) - v;

		final int nr = ( int ) Math.min( 255, r + m );
		final int ng = ( int ) Math.min( 255, g + m );
		final int nb = ( int ) Math.min( 255, b + m );

		return 0xff000000 | ( nr << 16 ) | ( ng << 8 ) | nb;
	}