public void setBitShift(int bitShift) {
            if (bitShift < 0) {
                throw new RuntimeException();
            }
            this.bitShift = bitShift;
            bitShiftSet = true;
        }
Sets the desired bit shift for a grayscale image.
Pixels in the source image will be shifted right by
the given amount prior to being clamped to the maximum
value given by the encoded image's bit depth.
plugins/svg-plugin/batik-codec-fix/src/main/java/org/apache/batik/ext/awt/image/codec/png/PNGEncodeParam.java