Question

I have a custom View that always draws a Bitmap at a certain rotation. I overwrite the onDraw method, rotate the Canvas and draw the bitmap with an anti-aliased Paint.

public RotatedImageView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    someBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.placeholder);
}

@Override
protected void onDraw(Canvas canvas) {

    // Save and rotate canvas
    canvas.save();
    canvas.rotate(3F, getWidth() / 2, getHeight());

    // Draw the icon
    Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);
    canvas.drawBitmap(someBitmap, 0, 0, p);
    canvas.drawRoundRect(new RectF(75, 50, 225, 200), 10, 10, p);

    // All done; restore canvas
    canvas.restore();
}

However, I always get jagged edges on the Bitmap. Note that the roudned rectangle gets nicely anti-aliased edges. Also, when I apply p.setFilterBitmap(true); this works (the bitmap surface gets filtered/smoothed) correctly. Am I missing something?

Here’s a minimal Android project with isolated example of one screen that shows the View that draws the non-anti-aliased Bitmap, loaded from a resource: https://bitbucket.org/erickok/antialiastest

UPDATE: I have also tried the following:

    Paint p = new Paint();
    p.setAntiAlias(true);
    p.setFilterBitmap(true);
    p.setDither(true);
    canvas.drawBitmap(someBitmap, 0, 0, p);

But this doesn’t help as setFilterBitmap filters the surface; it does not anti-alias the edges. Also, setAntiAlias does the same as directly setting the flag in the Paint constructor. If in doubt, please try my minimal test project. Thanks so much for any help!

Answer 1

I have found why I did not get any anti-aliasing: hardware acceleration. After I (by pure accident) tested on my Nexus One it suddenly worked, while my Nexus 7 didn’t. Turns out that the ANTI_ALIAS_FLAG does nothing when the Canvas is drawn using the hardware acceleration. With that insight I found Romain Guy’s answer on StackOverflow which answers my question.

Solutions seem to be: adding a transparent border to every bitmap (manually or on the fly) or ‘do something with shaders’. Neither are very appealing to me. Instead I disabled hardware acceleration on my custom view using the setLayerType method. Note that this is available since API level 11, but (not by accident) you won’t have to deal with hardware acceleration before any way. So I have added, in the view constructor:

    if (android.os.Build.VERSION.SDK_INT >= 11) {
        setLayerType(View.LAYER_TYPE_SOFTWARE, null);
    }

I’ve updated the above referenced open-source project on BitBucket for those interested.