olordata.black</a>
      and colordata.cblack) will be adjusted too.</p>
    <p>This call should be used if you postprocess RAW data by your own code.
      LibRaw <a href="#dcrawemu">postprocessing functions</a> will call <strong>subtract_black()</strong>
      by oneself.</p>
    <p>The function returns an integer number in accordance with the <a href="API-notes.html#errors">return
        code convention</a>: positive if any system call has returned an error,
      negative (from the <a href="API-datastr