something about voc groudtruth label

[chenpengf0223]

Hi sthalles, Thanks for your great work!

I get a little confusion about CreateTfRecord.ipynb.
In CreateTfRecord.ipynb, I found the two types of annotation are different.
1,
If "annotation_np = imread(os.path.join(annotations_dir_voc, image_name.strip() + ".png"))",
then the result label is :
annotation_np.dtype : uint8, annotation_np.shape: (w,h,4), np.max(annotation_np): 255, np.min(annotation_np)): 0.
2,
If "annotation_np = read_annotation_from_mat_file(annotations_dir_aug_voc, image_name)",
then the result label is :
annotation_np.dtype : uint8, annotation_np.shape: (w,h), np.max(annotation_np): 15, np.min(annotation_np)): 0.

What is the meaning of the annotation_np when its shape is (w,h,4)? If it will cause something wrong when train the model?