public interface PostProcessor

Helper interface for adding custom processing to an image.

The image being processed may be a Drawable, a Bitmap, or a frame of an AnimatedImageDrawable produced by ImageDecoder. This is called before the requested object is returned.

This custom processing can even be applied to images that will be returned as immutable objects, such as a Bitmap with Config HARDWARE returned by ImageDecoder.

On an AnimatedImageDrawable, the callback will only be called once, but the drawing commands will be applied to each frame, as if the Canvas had been returned by Picture.beginRecording.

Supplied to ImageDecoder via setPostProcessor.


Public methods

abstract int onPostProcess(Canvas canvas)

Do any processing after (for example) decoding.

Public methods


int onPostProcess (Canvas canvas)

Do any processing after (for example) decoding.

Drawing to the Canvas will behave as if the initial processing (e.g. decoding) already exists in the Canvas. An implementation can draw effects on top of this, or it can even draw behind it using PorterDuff.Mode.DST_OVER. A common effect is to add transparency to the corners to achieve rounded corners. That can be done with the following code:

  Path path = new Path();
  int width = canvas.getWidth();
  int height = canvas.getHeight();
  path.addRoundRect(0, 0, width, height, 20, 20, Path.Direction.CW);
  Paint paint = new Paint();
  paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
  canvas.drawPath(path, paint);
  return PixelFormat.TRANSLUCENT;

canvas Canvas: The Canvas to draw to.

This value must never be null.

int Opacity of the result after drawing. PixelFormat.UNKNOWN means that the implementation did not change whether the image has alpha. Return this unless you added transparency (e.g. with the code above, in which case you should return PixelFormat.TRANSLUCENT) or you forced the image to be opaque (e.g. by drawing everywhere with an opaque color and PorterDuff.Mode.DST_OVER, in which case you should return PixelFormat.OPAQUE). PixelFormat.TRANSLUCENT means that the implementation added transparency. This is safe to return even if the image already had transparency. This is also safe to return if the result is opaque, though it may draw more slowly. PixelFormat.OPAQUE means that the implementation forced the image to be opaque. This is safe to return even if the image was already opaque. PixelFormat.TRANSPARENT (or any other integer) is not allowed, and will result in throwing an IllegalArgumentException.