Question

A seemingly simple issue, I have an off-screen bitmap that I perform some transformations to (rotation, scaling, etc) and I’d like to store a copy of the bitmap prior to the transformations, such that in my View’s onDraw(), I can display the transformed off-screen bitmap AND a smaller scaled version of the un-transformed bitmap as a thumbnail.

No problem writing the off-screen bitmap in onDraw(), but the copied ‘preserved’ bitmap is also being transformed. Here is the code where I am making the copy of the bitmap, where mCanvas was created via mCanvas = new Canvas(mBitmap);:

mPreservedBitmap = Bitmap.createBitmap(mBitmap);

// save the canvas
mCanvas.save();

// do some rotations, scaling
mCanvas.rotate(rotation, px, py);
mCanvas.scale(scaleFactor, scaleFactor, scaleFocusX, scaleFocusY);

// draw the bitmaps to the screen
invalidate();

// restore the bitmap
mCanvas.restore();

In onDraw(), I have:

// draw the off-screen bitmap to the on-screen bitmap
canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

// draw the preserved image, scaling it to a thumbnail first
canvas.drawBitmap(
    Bitmap.createScaledBitmap(mPreservedBitmap, (int) thumbWidth, (int) thumbHeight, true), 
null, 
thumbnailRectF, 
thumbCanvasPaint);

The thumbnail gets scaled to the appropriate size, but the bitmap that is being scaled down to thumbnail size is also rotated and scaled the exact same as mBitmap, which I don’t want. I’ve also tried the Bitmap.copy() method, but with the same results. Any pointers/assitance/advice?

Thanks,

Paul

Answer 1

My final solution to this was to generate a copy of the canvas Bitmap PRIOR to it being scaled via:

mPreservedBitmap = Bitmap.createBitmap(bitmap, 0, 0, bitmap.getWidth(), bitmap.getHeight(), null, true);

Then, when the Canvas and the primary Bitmap is scaled, I can draw the non-scaled ‘preserved’ Bitmap to the Canvas in onDraw() via:

canvas.drawBitmap(mPreservedBitmap, null, thumbnailRectF, thumbCanvasPaint);

Per Romain’s comments above, I scale the preserved Bitmap off-screen to improve performance in onDraw().