Re: [JAVA2D] alphacomposite for combining drop shadows

[Brian Nahas]

Title: Message

Pulled the trigger on the send button too soon.

 

I thought it was relatively complex to implement a composite but I should have explored it more prior to sending my email.  My problem was fixed by a relatively simple implementation where the alpha values of the src and dest pixels are compared and the greater is kept.  However, this didn't produce the desired visual effect, so I ended up going with an implementation that adds the alpha values of the src and dest, but applies a ceiling value to them.  Here's the final implementation of the compose method for those interested:

 

public void compose(Raster src,
                    Raster dstIn,
                    WritableRaster dstOut) {
    int[] srcPixels = new int[4];
    int[] dstPixels = new int[4];
    for (int x=0; x < dstOut.getWidth(); x++){

 

        for (int y=0; y < dstOut.getHeight(); y++){
            src.getPixel(x, y, srcPixels);
            dstIn.getPixel(x, y, dstPixels);

 

            dstPixels[3] = dstPixels[3] + srcPixels[3];
            if (dstPixels[3] > maxDarkness) {
                dstPixels[3] = maxDarkness;
            }

 

            dstOut.setPixel(x, y, dstPixels);
        }

    }

}

Note that you'd have to do much more than that to take the rgb values into account.  In this case I was only working with black so I just ignored them.

-Brian

-----Original Message-----
From: Discussion list for Java 2D API [mailto:[EMAIL PROTECTED] On Behalf Of Brian Nahas
Sent: Wednesday, March 31, 2004 12:34 PM
To: [EMAIL PROTECTED]
Subject: [JAVA2D] alphacomposite for combining drop shadows

For those that don't want to read my motivation for this email, here's essentially what I'm looking for: Is there some sort of Composite that does a comparison of the SRC and DEST pixels and retains the darker of the two?  If not, how would I go about implementing one?  Or is there a better way?

 

Here's why I'm asking:  I'm trying to implement feathered drop shadows for a number of rectangles and I'm having some difficulty combining the resulting shadows into a smooth backdrop.  For example, let's say I have a rectangle A.  The shadow that A casts is essentially a rounded rectangle, but instead of using a RoundRectangle2D, I composed the shadow out of a rectangle of the same size as A in the middle, 4 smaller rectangles on the sides, top and bottom, and then 4 arcs for the 4 corners:

 

Arc  Rect Arc

Rect Rect Rect

Arc  Rect Arc

 

The rectangle in the middle is then painted black with an alpha value, and then the 4 sides are painted the same black with a gradient paint that is reduced heading outwards from the middle rectangle.  The 4 arcs on the corners are painted black with a RadialGradient paint the lightens as you head outwards from the corner.  So what I get for the shadow is a nice rounded rectangle that is larger than the original with a gradient paint on the edges that gradually lightens.  This looks great.

 

The problem arises when I have the rectangle A near or next to another Rectangle, say B.  If I use SRC_OVER as an alpha composite for the shadow buffer, then the shadows (because they are larger than the rectangles that cast them) combine as if they are from two light sources and you get a dark section where they come together.  If I use SRC, then whichever shadow get's drawn second destroys the feathering around the first shadow because the gradient paint on the edges of the second shadow overwrites the edges on the first shadow.

 

So what I'm looking for is a Composite that looks at two pixels and retains the darker of the two, so that when the shadows overlap, it will look like their pixles have been merged together instead of combined. 

 

Thanks for any help.

-Brian

 

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