I would like if someone could explain to me something. The architecture in YOLO from the Figure 3 in your YOLO paper https://pjreddie.com/media/files/papers/yolo.pdf is like this:

(448,448,3), (112,112,192), (56,56,256), (28,28,512), (14,14,1024),(7,7,1024),(7,7,1024), Dense(4096), (7,7,30)

I don't understand how to implement the last three parts, bolded ones If it is not the problem, I would appreciate if you help me understand that part. I use Keras and everything is OK for me to implement except those parts. I really don't know how to pass from (7,7,1024) to (7,7,1024) and also from Dense to (7,7,30).


1 Answer 1


You can use the Flatten and Reshape layers to go to Dense and back to HWC format. The last layers in keras would look like this:

7_7_1024_1 = ...  # The first (7,7,1024)
x = keras.layers.Conv2D(1024, 3, padding='same')(7_7_1024_1)
x = keras.layers.Flatten()(x)
x = keras.layers.Dense(4096)(x)
x = keras.layers.Dense(7 * 7 * 30)(x)
x = keras.layers.Reshape((7, 7, 30))(x)
  • $\begingroup$ Thank you very much for answer. There is only one thing I really don't understand. Input shape is (448,448,3) and then they got (112,112,192). I don't understand how they got that with 64 kernels of size (7,7) and stride = 2. $\endgroup$
    – Alem
    Jun 26, 2018 at 18:41
  • $\begingroup$ The first layer contains a conv stride 2 and max pool (2, 2) stride 2 which results in a 4x spatial reduction. $\endgroup$
    – kenny
    Jun 27, 2018 at 5:35
  • $\begingroup$ When we use conv stride = 2, shouldn't we get (448-7+2*0)/2 + 1 = 221.5 output dimension, instead of 224? This is what I didn't understand. I uderstand that from 224 with maxpooling we get 112, but I don't understand how we get 224. $\endgroup$
    – Alem
    Jun 27, 2018 at 10:53
  • $\begingroup$ I think I understand now. Padding is used here different from zero. Somewhere 1, somewhere 3. Fix me if I'm wrong. I used to code in Keras something like: model.add(something). Is there a way you could write your code in that way? $\endgroup$
    – Alem
    Jun 27, 2018 at 12:58

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