Displaying Colormap over Image

[davidweb]

Hey everyone,

I tried to display a heatmap over an image both using matplotlib - which I was able to get done now. This is the result:

More particularly, I have:

• a tensor of shape (7, 7) with floats between 0 and 1 - let’s call it the activation_map
• the image tensor of shape (3, 224, 224)

I now upsampled the activation_map to (224, 224) and overlayed it with a custom colormap that is based on the gray colormap, just making high values transparent instead of white.

However, when I plot multiple of these pictures with activation_map, I always want them to show the same amount of pixels of the picture. Basically, I always want the highest 10.000 values of the activation_map to be ‘1’ and the other ones to be ‘0’. How can I achieve that?

I know I need to squeeze the activation_map to an array, sort it, then make the biggest values to ‘1’, the smallest ones to ‘0’. But how do I get the values back in the same order as they were in the original activation_map? I need to work with the indices but I am struggling to do so…

I would be happy to receive any help, overall it is for research as this will be used in an experiment.
BR, David

[davidweb]

Well, sometimes it helps to just approach the problem again after you explained it to others… Here is the solution I got the problem solved with:

# make a copy of the original activation_map
nact = act.copy()

#Reshape the activation map to a 1D Matrix / Array
nact = nact.reshape(-1)

#Get the indices that would sort the array from low to high
sort_ind = np.argsort(nact)

# indices of the values I want to set to 0
sort_zero = sort_ind[:47175]

#indices of the values I want to set to 1
sort_ones = sort_ind[47176:]

#set values to zero
nact[sort_zero] = 0.0
# set values to one
nact[sort_ones] = 1.0

#Reshape activation_map back to original size
d_act = nact.reshape(224, 224)

#Visualize it over the image
plt.imshow(img)
plt.imshow(d_act, alpha=1.0, cmap=my_cmap)
plt.axis('off'); plt.tight_layout(); plt.show()

Result: