I'm trying to use MobileNets to localize a rectangular object in an image. I want to construct a model that inputs an image, and outputs the keypoints/coordinates (8 total points) of each corner of the rectangular object.

The rectangular object in each image is pretty unique and I would guess should be easy to generalize. For example (my problem is very similar to this): 900 images of the Chicago Bull's court along with the 8 given coordinates for each. The image is always with the center logo front-facing, but from any angle/rotation, & can be taken from close range or at a distance (so the court dimensions/rotations vary).

The model would be able to give me the 8 points:

[upper_left_x, upper_left_y, upper_right_x, upper_right_y, bottom_left_x, bottom_left_y, bottom_right_x, bottom_right_y]

We could then manually (I don't want the network to do this) apply the points:

I initialize with a base model:

model = MobileNet(weights=None,              # can also be Imagenet
                  include_top=False,         # the top is for classification
                  input_shape=(224, 224, 3))

and for the output layers I apply something very similar to face-keypoints model

x = model.output
x = Flatten()(x)
x = Dense(500, activation="relu")(x)
x = Dense(90, activation="relu")(x)
predictions = Dense(8)(x)

Other Setup and Problems

I've used Adam optimizers, SGD, initialized the model with imagenet weights (tried frozen/unfrozen layers), changed learning rates , restructured & sized the output layers in many ways, opted for different/custom model implementations, validated my dataset (X,y) processing, trained on 100s-1000s of epochs....

I do not get anywhere as near accurate results as the facial-keypoints example. In fact, the predictions seem completely random OTHER THAN the model accurately outputting a rectangle with the points in the correct order. Any model seems to only recognize the shape, without worry to any correct dimensions or rotation -- I've played with the facial keypoints and even that variable accuracy is low, but seems to perform well due to the faces having very similar dimension & virtually no rotation (if facial dimensions don't change, the outputted shape typically fits)

No matter the structure, the lowest validation loss I can achieve seems to be around 0.0xxx and accuracy is completely off (in terms of correct dimension/rotation). I've accounted for over-fitting, but think the structure of my model is the real issue.

Is mobilenet the wrong structure? Should the output layers be structured differently?

I would appreciate any help, thanks.