public void setPhysicalDimension(int[] physicalDimension) {
        this.physicalDimension = (int[])(physicalDimension.clone());
        physicalDimensionSet = true;
    }
Sets the physical dimension information to be stored with this
image.  The physicalDimension parameter should be a 3-entry
array containing the number of pixels per unit in the X
direction, the number of pixels per unit in the Y direction,
and the unit specifier (0 = unknown, 1 = meters).

<p> The 'pHYS' chunk will encode this information.
plugins/svg-plugin/batik-codec-fix/src/main/java/org/apache/batik/ext/awt/image/codec/png/PNGEncodeParam.java