Drawing line less than one pixel thick requires anti-aliasing in Android 4.2

3
votes

I'm trying to draw a very thin line (less than one pixel thick) in android. I'm using 

Paint blackThin = new Paint();
blackThin.setStrokeWidth(0.1f);
blackThin.setColor(Color.BLACK);

to initialize the paint object.

This works fine in Android 2.2, but when I try it in 4.2 (also 4.1, I think - I tested that one briefly - I haven't tested any other versions other that 2.2, 4.1.2 and 4.2) the lines won't show up when I draw them. To make them show up in 4.2, it seems like I have to set the anti-aliasing flag to be true. (I tried that, and the lines showed up.) But I really don't want that, because that blurs the lines and such.

Is this a bug in Android? Is there something I can do to make it work other than turning on anti-aliasing?

EDIT: I also tested this with a pixel size of 0.9, and the problem is the same.

Thanks for the help!

2
androidcanvaspaintantialiasingstroke
What do you expect a 0.1 pixel thick line to look like without anti-aliasing? – Matti Virkkunen
In version 2.2, it works fine. It's just a very thin line. And as an aside, 0.9 has the same problem as 0.1. – Shdus
More likely: in 2.2 it's working accidentally. The smallest thing that can be drawn is a single pixel, why are you expecting things to be drawn that are smaller than that? – robertc

2 Answers

3
votes

If you want a very thin line, use setStrokeWidth(0), as the documentation says Pass 0 to stroke in hairline mode.

Sub-pixel drawing obviously requires Anitaliasing or other workarounds. It is pretty possible that e.g. with software rendering, lines below one pixel are always drawn as hairlines, but with hardware rendering (which may be automatically used in Android 4.2 but not in 2.2), the behavior changes and many of the convenient features disappear. (This is often the reason for drawing differences and weird glitches, btw.)

3
votes

You can't draw lines less than a pixel in width without anti-aliasing.

The whole point of anti-aliasing is to calculate what a pixel's colour would be, if a element doesn't completely fill the pixel (Like a line that's only a tenth of a pixel's width).

You could somewhat simulate it by drawing a lighter line, but that still wouldn't come close to a actually anti-aliased line.

Take a look at this image, for example:

enter image description here

It's really not possible to replicate the anti-aliased result by changing the line's colour.

I realise the lines in the example are > 1 pixel in width, but the same principle applies. The "darkness" of a pixel is calculated as a result of how much of the pixel is filled with the line. For that reason, a solid grey line won't work.