Fastbook Chp. 6 - Multiple PointBlocks

I’m working through fastbook chapter 6, using the Facial Keypoints Dataset on Kaggle.

I’ve manipulated the data a little, converting the tensors to images and saving them down so I can use transforms later. When I use the following DataBlock, everything works correctly and trains well.

data = DataBlock(
blocks=(ImageBlock, PointBlock),

However, when I try to add additional PointBlocks I get some errors. The DataBlock runs fine, so does the subsequent set up of Dataloaders. dls.show_batch() also runs, and shows the points in the correct positions.

xb,yb = dls.one_batch() gives the following error: ValueError: too many values to unpack (expected 2)

When I try to train I get the error TypeError: 'L' object is not callable

Here is the troublesome DataBlock set up…

data = DataBlock(
    blocks=(ImageBlock, PointBlock, PointBlock, PointBlock),

I feel like i’m making a simple error in the way i’m structuring get_y, but after much googling and trial and error, i’ve been unable to make progress. Any help much appreciated.


One single PointBlock works on any number of points. For example see the cats dataset (from Kaggle). I made an example notebook here. We identify 9 points in each image (and only one PointBlock is used)


Thanks very much (as always!) @muellerzr, i’ll take a look at the notebook and try to work back from that. :+1:

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No problem @PDiTO :slight_smile: The key is inside the get_y function, you should have it return a list of x,y. IIRC in v1 it wanted y,x unless you enabled something but this isn’t a thing in v2 (you can check the notebook)

But for instance this should be the output of your get_y:

tensor([[563., 411.],
        [736., 404.],
        [669., 545.],
        [404., 340.],
        [380., 148.],
        [528., 261.],
        [739., 254.],
        [869., 123.],
        [811., 325.]])

Now the order will matter here, and if there are missing values I’d recommend putting them in a place you know already, such as maybe -1,-1 (this is done with object detection to define a point is missing)

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Thanks again @muellerzr. It works :slight_smile: and I now understand what get_y expects better!

For anyone interested, my solution that now trains looks like:-

def get_y(r):
    return [

data = DataBlock(
    blocks=(ImageBlock, PointBlock),

Just in time for my work. Thanks.

Hi everyone,

I am having difficulties in interpreting the predictions from the learner object, with the original problem described in Chapter 6, when I run predict on one of the images from the dataset, I get

db = DataBlock(blocks=(ImageBlock, PointBlock), 
               get_y= get_ctr, 
               splitter=FuncSplitter(lambda fn: == "01"),
               batch_tfms=aug_transforms() # this is different to that described in the book.
ds = db.datasets(path)
dls = db.dataloaders(path)
learn = cnn_learner(dls, resnet18, y_range=(-1,1))
# steps to train
# ....
# ...

# get one image from the dataset
img = ds[0][0]

# make a prediction
# output:
#   (TensorPoint([[123.3277, 121.6380]]),
#    tensor([0.1011, 0.0861]),
#    tensor([0.1011, 0.0861]))

How can I reconstruct the point on the image from this output?
While using the first TensorPoint (123.3277, 121.6380), it is going way off the face, while learn.show_results() works basically fine!

plt.scatter(123.3277, 121.6380)



I believe the result is based on the resized image (224x224) so the prediction of (123, 121) seems reasonable. Therefore, to use the original image, you would need to scale the prediction.

This link may help with a solution:

To rescale the prediction output to the original image, the point in encoded coordinates can be decoded with a PointScaler:

img, ref_point = ds[0]
_,_,p = learn.predict(img) # p = tensor([0.1011, 0.0861])
sclr = PointScaler()
sclr(img) # Transforming the image stores the image size in
dp = sclr.decode(TensorPoint.create(p)) # need a TensorPoint
print(dp, ref_point)