title: Ground truth pixel labels in PASCAL VOC for semantic segmentation
body: <p>I'm experimenting with FCN(Fully Convolutional Network), and trying to reproduce the results reported in the original paper (Long et al. CVPR'15).</p>
  
  <p>In that paper the authors reported results on PASCAL VOC dataset. After downloading and untarring the train-val dataset for 2012 (<a href="http://host.robots.ox.ac.uk/pascal/VOC/voc2012/VOCtrainval_11-May-2012.tar" rel="noreferrer">http://host.robots.ox.ac.uk/pascal/VOC/voc2012/VOCtrainval_11-May-2012.tar</a>
  ), I noticed there are 2913 png files in the <strong>SegmentationClass</strong> and same number of files in <strong>SegmentationObject</strong> subdirectory. </p>
  
  <p>The pixel values in these png files seem to be multiples of 32 (e.g. 0, 128, 192, 224...), which don't fall in the range between 0 and 20. I'm just wondering what's the correspondence between the pixel values and ground truth labels for pixels. Or am I looking at the wrong files? </p>