Drawable implementation which draws a chess board to express transparency

ChessBoardDrawable.java

package com.thinkfree.touchspan;

import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.ColorFilter;
import android.graphics.Paint;
import android.graphics.PixelFormat;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;

/**
 * @author Alan Goo(behumble@hanjava.net)
 */
public class ChessBoardDrawable extends Drawable {
    public static final int LIGHTGRAY = 0xFFDDDDDD;
    public static final int DARKGRAY = 0xFF222222;

    private int thinknessPx;

    private Paint bgPaint = new Paint();

    private Paint fgPaint = new Paint();

    public ChessBoardDrawable(int thinknessPx) {
        this.thinknessPx = thinknessPx;
        bgPaint.setColor(Color.WHITE);
        fgPaint.setColor(LIGHTGRAY);
    }

    public void adjustFor(int textColor) {
        // perceived brightness
        // http://stackoverflow.com/a/12043228/111890
        int r = Color.red(textColor);
        int g = Color.green(textColor);
        int b = Color.blue(textColor);
        double luma = 0.2126 * r + 0.7152 * g + 0.0722 * b;
        boolean textDark = luma < 128;
        if (textDark) {
            bgPaint.setColor(Color.WHITE);
            fgPaint.setColor(LIGHTGRAY);
        } else {
            bgPaint.setColor(Color.BLACK);
            fgPaint.setColor(DARKGRAY);
        }
        invalidateSelf();
    }

    @Override
    public void draw(Canvas canvas) {
        canvas.drawPaint(bgPaint);
        Rect b = getBounds();
        int twiceThickness = thinknessPx << 1;
        boolean flipper = true;
        for (int x = b.left; x < b.right; x += thinknessPx) {
            int offset = flipper ? 0 : thinknessPx;
            for (int y = b.top; y < b.bottom; y += twiceThickness) {
                canvas.drawRect(x, y + offset, x + thinknessPx, y + offset + thinknessPx, fgPaint);
            }
            flipper = !flipper;
        }
    }

    @Override
    public void setAlpha(int i) {
        // just ignore
    }

    @Override
    public void setColorFilter(ColorFilter colorFilter) {
        // just ignore
    }

    @Override
    public int getOpacity() {
        return PixelFormat.OPAQUE;
    }
}