Is there a way to achieve a stroke-like effect on bitmaps?

Question

I would like to achieve on a bitmap an effect equivalent to what

paint.setStyle(Paint.Style.STROKE) 
canvas.drawText(string, x, y, paint);

does for text.

Something akin to BlurMaskFilter, but for stroke, not glow, nor shadow.

Or if there isn't a built-in way, perhaps someone can suggest an algorithm to achieve this?

Answer 1

private Bitmap multiplyAlpha(final Bitmap bitmap, Paint paint,
        final boolean color, final float x) {
    paint = new Paint(paint.getFlags());
    Bitmap result = Bitmap.createBitmap(bitmap.getWidth(),
            bitmap.getHeight(), color ? Config.ARGB_8888 : Config.ALPHA_8);
    // ColorMatrixColorFilter requires ARGB.
    Bitmap auxiliary = Bitmap.createBitmap(bitmap.getWidth(),
            bitmap.getHeight(), Config.ARGB_8888);
    new Canvas(auxiliary).drawBitmap(bitmap, 0, 0, paint);
    // @formatter:off
    paint.setColorFilter(new ColorMatrixColorFilter(new float[] { 
               1,    0,    0,    0,    0,
               0,    1,    0,    0,    0,
               0,    0,    1,    0,    0,
               0,    0,    0,    x,    0
    }));
    // @formatter:on
    new Canvas(result).drawBitmap(auxiliary, 0, 0, paint);
    return result;
}

private Bitmap opaque(final Bitmap bitmap, Paint paint, final boolean color) {
    return multiplyAlpha(bitmap, paint, color, 255);
}

private RectF inset(final RectF rectF, final float dx, final float dy) {
    RectF result = new RectF(rectF);
    result.inset(dx, dy);
    return result;
}

private RectF offset(final RectF rectF, final float dx, final float dy) {
    RectF result = new RectF(rectF);
    result.offset(dx, dy);
    return result;
}

private Bitmap antialias(final Bitmap bitmap, Paint paint,
        final float radius) {
    Bitmap result = Bitmap.createBitmap(bitmap.getWidth(),
            bitmap.getHeight(), Config.ALPHA_8);
    if (radius > 0) {
        paint = new Paint(paint.getFlags());
        Canvas canvas = new Canvas(result);
        Bitmap opaque = opaque(bitmap, paint, false);
        canvas.drawBitmap(opaque, 0, 0, paint);
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        paint.setMaskFilter(new BlurMaskFilter(radius, Blur.INNER));
        canvas.drawBitmap(opaque, 0, 0, paint);
    }
    return result;
}

private Bitmap stroke(final Bitmap bitmap, Paint paint, final float radius,
        final RectF rectF, final float dx, final float dy) {
    paint = new Paint(paint.getFlags());
    Bitmap result = Bitmap.createBitmap(
            (int) Math.ceil(rectF.width() + 2 * radius),
            (int) Math.ceil(rectF.height() + 2 * radius), Config.ALPHA_8);
    if (radius > 0) {
        paint.setMaskFilter(new BlurMaskFilter(radius, Blur.NORMAL));
        new Canvas(result).drawBitmap(opaque(bitmap, paint, false), null,
                offset(rectF, -dx, -dy), paint);
    }
    return result;
}

private Bitmap stroke(final Bitmap bitmap, Paint paint, final float radius,
        final RectF rectF, final int color, final float antialias,
        final float factor, final boolean fill, final float dx,
        final float dy) {
    paint = new Paint(paint.getFlags());
    Canvas canvas = new Canvas();
    paint.setColor(color);
    Bitmap stroke = stroke(bitmap, paint, radius, rectF, dx, dy);

    Bitmap auxiliary = Bitmap.createBitmap(stroke.getWidth(),
            stroke.getHeight(), Config.ALPHA_8);
    canvas.setBitmap(auxiliary);
    // Paint [opaque] stroke.
    canvas.drawBitmap(opaque(stroke, paint, false), 0, 0, paint);
    // Antialias stroke with outside.
    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
    Bitmap outer = multiplyAlpha(antialias(stroke, paint, antialias),
            paint, false, factor);

    canvas.drawBitmap(outer, 0, 0, paint);
    paint.setXfermode(null);

    // If FILL, leave the inside filled with color (this is the way e.g.
    // Photoshop strokes); otherwise, the stroke will be only on the outside
    // of the bitmap; the more transparent the bitmap, the more noticeable
    // the effect.
    if (!fill) {
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        canvas.drawBitmap(opaque(bitmap, paint, false), null,
                offset(rectF, -dx, -dy), paint);
        paint.setXfermode(null);
    }

    Bitmap result = Bitmap.createBitmap(auxiliary.getWidth(),
            auxiliary.getHeight(), bitmap.getConfig());
    canvas.setBitmap(result);
    RectF output = offset(rectF, -dx, -dy);
    canvas.drawBitmap(auxiliary, null, inset(output, -radius, -radius),
            paint);
    // Paint bitmap.
    canvas.drawBitmap(bitmap, null, output, paint);
    // Antialias stroke with bitmap.
    Bitmap inner = multiplyAlpha(antialias(bitmap, paint, antialias),
            paint, false, factor);
    canvas.drawBitmap(inner, null, output, paint);

    return result;
}

private void stroke(final Bitmap bitmap, Paint paint, final float radius,
        final RectF rectF, final int color, final float antialias,
        final float factor, final boolean fill, Canvas canvas) {
    float dx = rectF.left - radius;
    float dy = rectF.top - radius;
    Bitmap stroke = stroke(bitmap, paint, radius, rectF, color, antialias,
            factor, fill, dx, dy);
    canvas.drawBitmap(stroke, dx, dy, paint);
}

where:

• antialias is the width of the antialias effect
• factor modifies the antialias strength
• fill indicates whether to false paint only the stroke outside, or true also fill the inside (this is the way e.g. Photoshop strokes); the more transparent the bitmap, the more noticeable the effect
• rectF indicates the RectF to draw in

usage (e.g.):

stroke(bmp, paint, radius, rectF, 0xffff0000, antialias, factor, fill, canvas);