private void drawPreviousShadow(Canvas canvas) {
        final float degreesFlipped = getDegreesFlipped();
        if (degreesFlipped > 90) {
            final int alpha = (int) (((degreesFlipped - 90) / 90f) * MAX_SHADOW_ALPHA);
            mShadowPaint.setAlpha(alpha);
            canvas.drawPaint(mShadowPaint);
        }
    }
draw top/left half shadow

@param canvas
app/src/main/java/home/smart/fly/animations/customview/flipview/FlipView.java