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<p>A callback function that can be used to modify the DCT coefficients after they are losslessly transformed but before they are transcoded to a new JPEG image. </p>
<p>This allows for custom filters or other transformations to be applied in the frequency domain.</p>
<dl class="params"><dt>Parameters</dt><dd>
  <table class="params">
    <tr><td class="paramname">coeffs</td><td>pointer to an array of transformed DCT coefficients. (NOTE: this pointer is not guaranteed to be valid 