I am using Keras to build a CNN model that takes two types of images as inputs (input1, input2) and produce one output. The model classifies the two inputs into a class, and the number of images in each type are different. (The number of images in the input1 is 1500 and in input2 is 4000).

This is my code:

in1 = Input(...)  
 x = Conv2D(...)(in1)
# rest of the model
 out1 = Dense(...)(x)  

 in2 = Input(...)  
x = Conv2D(...)(in2)
# rest of the model
out2 = Dense(...)(x)  

concatenated_layer = concatenate([out1, out2])  # merge the outputs of the two models
output_layer = Dense(no_classes, activation='softmax', name='prediction')(concatenated_layer)
modal= Model(inputs=[in1, in2], outputs=[output_layer])

Image generator code:

input_imgen = ImageDataGenerator(rescale = 1./255, 
                                   shear_range = 0.2, 
                                   zoom_range = 0.2,
                                   horizontal_flip = True)

test_imgen = ImageDataGenerator()

def generate_generator_multiple(generator,dir1, dir2, batch_size, img_height,img_width):
    genX1 = generator.flow_from_directory(dir1,
                                          target_size = (img_height,img_width),
                                          class_mode = 'categorical',
                                          batch_size = batch_size,

    genX2 = generator.flow_from_directory(dir2,
                                          target_size = (img_height,img_width),
                                          class_mode = 'categorical',
                                          batch_size = batch_size,
    while True:
            X1i = genX1.next()
            X2i = genX2.next()
            yield [X1i[0], X2i[0]], X2i[1]  #Yield both images and their mutual label



My questions are:

How can I determine the value of steps_per_epoch and validation_steps if the number of the images in each type are different?

In general, How to deal with the multi input Convolutional Neural Network that takes different number of images?


1 Answer 1


You need to define what your training set is in this case. If a single input to your network is a pair of images $(a, b)$, and you only have 1500 examples of $b$, then you only have 1500 training examples.

If each image in $A$ can map to any arbitrary image in $B$, you can potentially re-use images in $B$ to create these input pairs. If that's not the case, which images in $A$ correspond to the image of interest in $B$?

Alternatively, if you don't want to (or it's not appropriate to) simply re-use the images, you could consider augmenting the smaller set by, for example, rotating, cropping, or otherwise manipulating the images in that dataset.

To re-initialize the generator, you would just execute this code again :

genX2 = generator.flow_from_directory(dir2, target_size = (img_height,img_width), class_mode = 'categorical', batch_size = batch_size, shuffle=False, seed=7)

When genX2.next() doesn't return anything. Alternatively, and more pythonically, you could use itertools.chain() to form a generator capable of returning the right number of observations:

n_times_through = math.ceil(4000/1500) 
genX2_new = itertools.chain(*([genX2] * n_times_through)
  • $\begingroup$ My case is different images from the first input can map to different images in B. I mean different images in A and different images in B have the same class. Also, I am using data augmentation for the two inputs. I think I have to re-use images in B, right ?? How can I do that? $\endgroup$
    – Noran
    Commented Nov 2, 2018 at 15:52
  • $\begingroup$ you could, in generate_generator_multiple, simply re-initialize genX2 when genX2.next() stops generating an image from the iterator. $\endgroup$ Commented Nov 2, 2018 at 16:20
  • 1
    $\begingroup$ @Thomas Cleberg How to re-initialize genX2? Could you explain? or give an example? please. $\endgroup$
    – N.IT
    Commented Nov 4, 2018 at 12:23
  • $\begingroup$ Can you explain how can I re-initialize genX2? Also, during training I want to paired images (a,b) from the same class @ThomasCleberg $\endgroup$
    – Noran
    Commented Nov 7, 2018 at 18:28
  • $\begingroup$ Added content on methods to re-use. $\endgroup$ Commented Nov 8, 2018 at 17:13

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