Re-updating OCR model for reading Captchas Keras 3 example (TF-Only)

[sitamgithub-MSIT]

Re-updates the PR here #1788

This PR updates the OCR model for reading Captchas Keras 3.0 example [TF Only Backend]. Recently released Keras ops ctc_decode is added in place of the custom function.

For example, here is the notebook link provided:
https://colab.research.google.com/drive/1vCDb45wLmSI3iBI2_BfDDYDSztgxZ4Qp?usp=sharing

cc: @fchollet

The following describes the Git difference for the changed files:

Changes:

diff --git a/examples/vision/captcha_ocr.py b/examples/vision/captcha_ocr.py
index a6bac599..6ebf1a7a 100644
--- a/examples/vision/captcha_ocr.py
+++ b/examples/vision/captcha_ocr.py
@@ -359,30 +359,6 @@ and try the demo on [Hugging Face Spaces](https://huggingface.co/spaces/keras-io
 """
 
 
-def ctc_decode(y_pred, input_length, greedy=True, beam_width=100, top_paths=1):
-    input_shape = ops.shape(y_pred)
-    num_samples, num_steps = input_shape[0], input_shape[1]
-    y_pred = ops.log(ops.transpose(y_pred, axes=[1, 0, 2]) + keras.backend.epsilon())
-    input_length = ops.cast(input_length, dtype="int32")
-
-    if greedy:
-        (decoded, log_prob) = tf.nn.ctc_greedy_decoder(
-            inputs=y_pred, sequence_length=input_length
-        )
-    else:
-        (decoded, log_prob) = tf.compat.v1.nn.ctc_beam_search_decoder(
-            inputs=y_pred,
-            sequence_length=input_length,
-            beam_width=beam_width,
-            top_paths=top_paths,
-        )
-    decoded_dense = []
-    for st in decoded:
-        st = tf.SparseTensor(st.indices, st.values, (num_samples, num_steps))
-        decoded_dense.append(tf.sparse.to_dense(sp_input=st, default_value=-1))
-    return (decoded_dense, log_prob)
-
-
 # Get the prediction model by extracting layers till the output layer
 prediction_model = keras.models.Model(
     model.input[0], model.get_layer(name="dense2").output
@@ -394,9 +370,11 @@ prediction_model.summary()
 def decode_batch_predictions(pred):
     input_len = np.ones(pred.shape[0]) * pred.shape[1]
     # Use greedy search. For complex tasks, you can use beam search
-    results = ctc_decode(pred, input_length=input_len, greedy=True)[0][0][
-        :, :max_length
-    ]
+    results = ops.ctc_decode(pred, sequence_lengths=input_len, strategy="greedy")[0][0]
+    # Convert the SparseTensor to a dense tensor
+    dense_results = tf.sparse.to_dense(results, default_value=-1)
+    # Slice the dense tensor to keep only up to max_length
+    dense_results = dense_results[:, :max_length]
     # Iterate over the results and get back the text
     output_text = []
     for res in results:
(END)

[fchollet]

Thanks for the PR! Are you also able to replace ctc_batch_cost with keras.losses.CTC?

[sitamgithub-MSIT]

Thanks for the PR! Are you also able to replace ctc_batch_cost with keras.losses.CTC?

I had thought about it and was about to discuss it with you. So clearly, we have two OCR examples: this and a handwritten one. Previously, you could see the original captcha PR and handwriting PR. Both architectures are identical and came from the same authors. They mentioned why they added CTC as a layer rather than a direct loss in model.compile().

Now, following the updated Keras 3 PR, we provided the TF backend while using TF compat.v1 and all. Recently, this draft PR demonstrated the implementation of CTC as a loss with no (tf compat.v1 and all) using keras.losses.CTC and ctc_decode.

In response to your query, implementing it with Keras CTC loss will not be a problem because both examples use the same architecture. So, instead of utilizing the CTC layer, would you like to modify both the OCR example with the CTC loss implementation along with ctc_decode ops?

[sitamgithub-MSIT]

I tried with keras.loss.CTC() implementation similar to what is done here and model.fit() giving this error:

ValueError: None values not supported.

cc: @fchollet