Drawing mirrored bitmaps in android

Question

I'm trying to learn how to make an animated sprite in android and couldn't figure out how to go about organising my bitmaps. I have a sprite sheet of my character walking to the Right: a bitmap of five copies of a character, equally spaced (every 45px), in a walk cycle.

I planned to draw each frame by drawing a tiny section of my sprite sheet bitmap at a time by going:

Rect sourceRect = new Rect(0, 0, 45, 75);
canvas.drawBitmap(spriteSheetBitmap, sourceRect, new Rect(0, 0, 45, 75), null);

Then for the next frames, increment "sourceRect.x" by 45, then redraw and so forth.

However, I'm now not sure how to go about making my sprite walk to the Left. I had initially thought I could just mirror my rectangle that I am drawing from to get a flipped picture. Something like:

sourceRect = new Rect(45, 0, 0, 75);

which doesn't seem to work (not sure what actually happens here, but nothing gets drawn to my surface).

Searching online, it seems I should make a copy of my original bitmap, mirror it with a transform matrix, then use that bitmap for drawing when walking to the left. However I've also found implementations where many smaller bitmap objects get created out of the original sprite sheet, stored (and transformed for the mirrored motion), then used as needed.

So I'm wondering what would be the best in this case or if there is really any difference (performance/memory):

Method 1: Load in my original sprite sheet, make a new bitmap instance, mirror it,, then calculate all the rectangles and use those + two entire sheets to draw from (admittedly there is some extra bitmap space where the sprite sheet is unused).

Method 2: Load in my original sprite sheet, for every frame create a new two bitmap objects (1 mirrored, 1 normal) and store those to draw from.

Method 3: Other better ways?

Answer 1

Method 2 would be way too expensive, and you don't need a canvas to flip a bitmap. Simply create another bitmap with a Matrix applied, like so:

BitmapDrawable flip(BitmapDrawable d)
{
    Matrix m = new Matrix();
    m.preScale(-1, 1);
    Bitmap src = d.getBitmap();
    Bitmap dst = Bitmap.createBitmap(src, 0, 0, src.getWidth(), src.getHeight(), m, false);
    dst.setDensity(DisplayMetrics.DENSITY_DEFAULT);
    return new BitmapDrawable(dst);
}

Answer 2

To mirror your sprite simply apply the following transform on the Canvas: scale(-1, 1). You will have to offset the sprite by its width too.

Answer 3

To draw a vertical mirrored bitmap bmp on a canvas :

Matrix m = new Matrix();
// Mirror is basically a rotation
m.setScale( -1 , 1 );
// so you got to move your bitmap back to it's place. otherwise you will not see it
m.postTranslate(canvas.getWidth(), 0);
canvas.drawBitmap(bmp, m, p);