Btukfyl

I am just a Keras beginner and I try to implement a OCR project by Keras.So I try to learn from Keras OCR example.Here's a link!

I do not understand why "get_output_size" in class TextImageGenerator is len(alphabet) + 1 but not len(alphabet)?
I will appreciate it if someone can tell me why ..

It's related to the CTC layer used as cost function. Maybe reading the scientific papers will give you more perspective, but it's related to a "extra" class used by the model to say ("there is no letter").
Paper by Graves explaining the algorithm behind

There is one extra-character needed in neural networks trained with CTC loss. This extra-character essentially means "no character seen at this position" and is called CTC blank.

It is used to allow different alignments of a text or to allow some white-space between characters (think of an image containing " hello" or "hello " with whitespace around them, for both you want to recognize "hello").
When recognizing the text, these blanks are removed: e.g. when using best path decoding, the best-scoring character at each position is taken, but the blanks will be removed.

To get a better idea of this special CTC blank character, let's look at the illustration below. The output of the neural network contains the characters a, b and the CTC blank (denoted as "-").
Let's pick the best-scoring characters for each position t0...t4, this gives us "aaa-b". Best path decoding removes repeated characters, this gives us "a-b", and finally removes all blanks, which gives us "ab".

If you want some more information, you can look at my CTC article, or this article, or the original paper.