Assignment 1 Spatial Pyramid Matching for Scene Classification1


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COMP5421 Computer Vision
Homework Assignment 1
Spatial Pyramid Matching for Scene Classification1
Figure 1: Scene Classification: Given an image, can a computer program determine where it was
taken? In this homework, you will build a representation based on bags of visual words and use
spatial pyramid matching for classifying the scene categories.
1. Please pack your code into a single file named <ustlogin-id>.zip, see the complete submission checklist at the end.
2. All questions marked with a Q require a submission.
3. For the implementation part, please stick to the headers, variable names, and file
conventions provided. You will lose marks if you don’t.
4. Start early! This homework will take a long time to complete.
5. Attempt to verify your implementation as you proceed: If you don’t verify that your
implementation is correct on toy examples, you will risk having a huge mess when you put
everything together.
6. Use relative paths with respect to the working directory.
1Credit to CMU Simon Lucey, Nate Chodosh, Allie Chang, Akshita Mittel, Gaurav Mittal, Chengqian Che, Purna
The bag-of-words (BoW) approach, which you learned about in class, has been applied to a myriad
of recognition problems in computer vision. For example, two classic ones are object recognition
[5, 7] and scene classification [6, 8]2
Beyond that, the BoW representation has also been the subject of a great deal of study aimed
at improving it, and you will see a large number of approaches that remain in the spirit of bagof-words but improve upon the traditional approach which you will implement here. For example,
two important extensions are pyramid matching [2, 4] and feature encoding [1].
An illustrative overview of the homework is shown in Figure 2. In Section 1, we will build the
visual words from the training set images. With the visual words, i.e. the dictionary, in Section
2 we will represent an image as a visual-word vector. Then the comparison between images is
realized in the visual-word vector space. Finally, we will build a scene recognition system based on
the visual bag-of-words approach to classify a given image into 8 types of scenes.
Figure 2: An overview of the bags-of-words approach to be implemented in the homework. Given
the training set of images, the visual features of the images are extracted. In our case, we will
use the filter responses of the pre-defined filter bank as the visual features. The visual words,
i.e. dictionary, are built as the centers of clusterings of the visual features. During recognition,
the image is first represented as a vector of visual words. Then the comparison between images
is realized in the visual-word vector space. Finally, we will build a scene recognition system that
classifies the given image into 8 types of scenes.
What you will be doing: You will implement a scene classification system that uses the
bag-of-words approach with its spatial pyramid extension. The paper that introduced the pyramid
matching kernel [2] is:
K. Grauman and T. Darrell. The Pyramid Match Kernel: Discriminative Classification
2This homework aims at being largely self-contained; however, reading the listed papers (even without trying to
truly understand them) is likely to be helpful.
Figure 3: The provided multi-scale filter bank
with Sets of Image Features. ICCV 2005.
Spatial pyramid matching [4] is presented at:
S. Lazebnik, C. Schmid, and J. Ponce, Beyond Bags of Features: Spatial Pyramid
Matching for Recognizing Natural Scene Categories, CVPR 2006.
You will be working with a subset of the SUN database3
. The data set contains 1600 images
from various scene categories like “auditorium”, “desert” and “kitchen”. And to build a recognition
system, you will:
• first, take responses of a filter bank on images and build a dictionary of visual words;
• then, learn a model for the visual world based on the bag of visual words (with spatial pyramid
matching [4]), and use nearest-neighbor to predict scene classes in a test set.
In terms of number of lines of code, this assignment is fairly small. However, it may take a few
hours to finish running the baseline system, so make sure you start early so that you have time to
debug things.
Also, try each component on a subset of the data set first before putting everything
We provide you with a number of functions and scripts in the hopes of alleviating some tedious
or error-prone sections of the implementation. You can find a list of files provided in Section 4.
Notice that, we include num workers as input for some functions you need to implement. Those
are not necessary, but can be used with multi-threading python libraries to significantly speed up
your code. This homework was tested using python3.5 and pytorch 0.4.1.
Figure 4: An input image and filter responses for all of the filters in the filter bank. (a) The input
image (b) The filter responses of Lab image corresponding to the filters in Figure 3.
1 Representing the World with Visual Words
1.1 Extracting Filter Responses
We want to run a filter bank on an image by convolving each filter in the bank with the image and
concatenating all the responses into a vector for each pixel. In our case, we will be using 20 filters
consisting of 4 types of filters in 5 scales. The filters are: (1) Gaussian, (2) Laplacian of Gaussian,
(3) derivative of Gaussian in the x direction, and (4) derivative of Gaussian in the y direction. The
convolution function scipy.ndimage.convolve() can be used with user-defined filters, but the
functions scipy.ndimage.gaussian filter() and scipy.ndimage.gaussian laplace() may be
useful here for improved efficiency. The 5 scales we will be using are 1, 2, 4, 8, and 8√
2, in pixel
Q1.1.1 (5 points): What properties do each of the filter functions (See Figure 3) pick up?
You should group the filters into broad categories (e.g. all the Gaussians). Also, why do we need
multiple scales of filter responses? Answer in your write-up.
Q1.1.2 (10 points): For the code, loop through the filters and the scales to extract responses.
Since color images have 3 channels, you are going to have a total of 3F filter responses per pixel if
the filter bank is of size F. Note that in the given dataset, there are some gray-scale images. For
those gray-scale images, you can simply duplicated them into three channels using the command
repmat. Then output the result as a 3F channel image. Complete the function
visual words.extract filter responses(image)
and return the responses as filter responses. We have provided you with a template code
with detailed instructions in it. You would be required to input a 3-channel RGB or gray-scale
image and filter bank to get the responses of the filters on the image.
Remember to check the input argument image to make sure it is a floating point type with range
[0, 1], and convert it if necessary. Be sure to check the number of input image channels and convert
it to 3-channel if it is not. Before applying the filters, use the function skimage.color.rgb2lab()
to convert your image into the Lab color space, which was designed to more effectively quantify
color differences with respect to human perception. (See Wikipedia for more information.) If image
is an M × N × 3F matrix, then filter responses should be a matrix of size M × N × 3F. Make sure
your convolution function call handles image padding along the edges sensibly.
Apply all 20 filters on a sample image, and visualize as an image collage (as shown in Figure 4).
You can use the included helper function util.display filter responses() (which expects a
list of filter responses with those of the Lab channels grouped together with shape M × N × 3) to
create the collage. Submit the collage of 20 images in the write-up.
1.2 Creating Visual Words
You will now create a dictionary of visual words from the filter responses using k-means. After
applying k-means, similar filter responses will be represented by the same visual word. You will
use a dictionary with fixed-size. Instead of using all of the filter responses (that can exceed the
memory capacity of your computer), you will use responses at α random pixels4
. If there
are T training images, then you should collect a matrix filter responses over all the images that
is αT × 3F, where F is the filter bank size. Then, to generate a visual words dictionary with K
words, you will cluster the responses with k-means using the function sklearn.cluster.KMeans
as follows:
kmeans = sklearn.cluster.KMeans(n clusters=K).fit(filter responses)
dictionary = kmeans.cluster centers
You may alternatively pass the n jobs argument into the KMeans() object to utilize parallel
Q1.2 (10 points): You should write the functions
visual words.compute dictionary one image(args) visual words.compute dictionary()
to generate a dictionary given a list of images. The overall goal of compute dictionary() is to
load the training data, iterate through the paths to the image files to read the images, and extract
αT filter responses over the training files, and call k-means. This can be slow to run; however, the
images can be processed independently and in parallel. Inside compute dictionary one image(),
you should read an image, extract the responses, and save to a temporary file. Here, args is a
collection of arguments passed into the function. Inside compute dictionary(), you should load
all the training data and create subprocesses to call compute dictionary one image(). After all
the subprocesses are finished, load the temporary files back, collect the filter responses, and run
k-means. A sensible initial value to try for K is between 100 and 300, and for α is between 50 and
500, but they depend on your system configuration and you might want to play with these values.
Finally, execute compute dictionary() and go get a coffee. If all goes well, you will have a
file named dictionary.npy that contains the dictionary of visual words. If the clustering takes
too long, reduce the number of clusters and samples. If you have debugging issues, try passing in
a small number of training files manually.
1.3 Computing Visual Words
Q1.3 (10 points): We want to map each pixel in the image to its closest word in the dictionary.
Complete the following function to do this:
and return wordmap, a matrix with the same width and height as image, where each pixel in
wordmap is assigned the closest visual word of the filter response at the respective pixel in image.
We will use the standard Euclidean distance to do this; to do this efficiently, use the function
scipy.spatial.distance.cdist(). Some sample results are shown in Figure 5.
4Try using numpy.random.permutation().
Figure 5: Visual words over images. You will use the spatially un-ordered distribution of visual
words in a region (a bag of visual words) as a feature for scene classification, with some coarse
information provided by spatial pyramid matching [4].
Visualize three wordmaps of three images from any one of the category and submit in the writeup along with their original RGB image. Also, provide your comments about the visualization.
They should look similar to the ones in Figure 5 5
2 Building a Recognition System
We have formed a convenient representation for recognition. We will now produce a basic recognition system with spatial pyramid matching. The goal of the system is presented in Figure 1: given
an image, classify (colloquially, “name”) the scene where the image was taken.
Traditional classification problems follow two phases: training and testing. During training
time, the computer is given a pile of formatted data (i.e., a collection of feature vectors) with
corresponding labels (e.g., “desert”, “kitchen”) and then builds a model of how the data relates to
the labels: “if green, then kitchen”. At test time, the computer takes features and uses these rules
to infer the label: e.g., “this is green, so therefore it is kitchen”.
In this assignment, we will use the simplest classification model: nearest neighbor. At test
time, we will simply look at the query’s nearest neighbor in the training set and transfer that label.
In this example, you will be looking at the query image and looking up its nearest neighbor in a
collection of training images whose labels are already known. This approach works surprisingly
well given a huge amount of data, e.g., a very cool graphics applications from [3].
The components of any nearest-neighbor system are: features (how do you represent your instances?) and similarity (how do you compare instances in the feature space?). You will implement
2.1 Extracting Features
We will first represent an image with a bag of words approach. In each image, we simply look at
how often each word appears.
Q2.1 (10 points): Write the function
visual_recog.get_feature_from_wordmap(wordmap,dict size)
5Try using numpy.random.permutation().
that extracts the histogram6 of visual words within the given image (i.e., the bag of visual words).
As inputs, the function will take:
• wordmap is a H × W image containing the IDs of the visual words
• dict size is the maximum visual word ID (i.e., the number of visual words, the dictionary
size). Notice that your histogram should have dict size bins, corresponding to how often
that each word occurs.
As output, the function will return hist, a dict size histogram that is L1 normalized, (i.e.,
the sum equals 1). You may wish to load a single visual word map, visualize it, and verify that
your function is working correctly before proceeding.
2.2 Multi-resolution: Spatial Pyramid Matching
Bag of words is simple and efficient, but it discards information about the spatial structure of
the image and this information is often valuable. One way to alleviate this issue is to use spatial
pyramid matching [4]. The general idea is to divide the image into a small number of cells, and
concatenate the histogram of each of these cells to the histogram of the original image, with a
suitable weight.
Here we will implement a popular scheme that chops the image into 2l × 2
cells where l is the
layer number. We treat each cell as a small image and count how often each visual word appears.
This results in a histogram for every single cell in every layer. Finally to represent the entire image,
we concatenate all the histograms together after normalization by the total number of features in
the image.If there are are L + 1 layers and K visual words, the resulting vector has dimensionality
l=0 4
l = K(4(L+1) − 1)/3.
Now comes the weighting scheme. Note that when concatenating all the histograms, histograms
from different levels are assigned different weights. Typically (in [4]), a histogram from layer l gets
half the weight of a histogram from layer l + 1, with the exception of layer 0, which is assigned a
weight equal to layer 1. A popular choice is for layer 0 and layer 1 the weight is set to 2−L, and
for the rest it is set to 2l−L−1
(e.g., in a three layer spatial pyramid, L = 2 and weights are set to
1/4, 1/4 and 1/2 for layer 0, 1 and 2 respectively, see Figure 6). Note that the L1 norm (absolute
values of all dimensions summed up together) for the final vector is 1.
Q2.2 (15 points): Create a function getImageFeaturesSPM that form a multi-resolution representation of the given image.
visual_recog.get_feature_from_wordmap_SPM(wordmap,layer num,dict size)
As inputs, the function will take:
• layer num the number of layers in the spatial pyramid, i.e., L + 1
• wordmap is a H × W image containing the IDs of the visual words
• dict size is the maximum visual word ID (i.e., the number of visual words, the dictionary
As output, the function will return hist all, a vector that is L1 normalized. Please use a
3-layer spatial pyramid (L = 2) for all the following recognition tasks.
One small hint for efficiency: a lot of computation can be saved if you first compute the
histograms of the finest layer, because the histograms of coarser layers can then be aggregated from
finer ones. Make sure you normalize the histogram after aggregation.
6Look into numpy.histogram().
Figure 6: Spatial Pyramid Matching: From [4]. Toy example of a pyramid for L = 2. The image
has three visual words, indicated by circles, diamonds, and crosses. We subdivide the image at
three different levels of resolution. For each level of resolution and each channel, we count the
features that fall in each spatial bin. Finally, weight each spatial histogram.
2.3 Comparing images
We will also need a way of comparing images to find the “nearest” instance in the training data.
In this assignment, we’ll use the histogram intersection similarity. The histogram intersection
similarity between two histograms is the sum of the minimum value of each corresponding bins.
Note that since this is a similarity, you want the largest value to find the “nearest” instance.
Q2.3 (10 points): Create the function
visual recog.distance to set(word hist,histograms)
where word hist is a K(4(L+1) − 1)/3 vector and histograms is a T × K(4(L+1) − 1)/3 matrix
containing T features from T training samples concatenated along the rows. This function returns
the histogram intersection similarity between word hist and each training sample as a vector of
length T. Since this is called every time you want to look up a classification, you want this to be
fast, so doing a for-loop over tens of thousands of histograms is a very bad idea.
2.4 Building A Model of the Visual World
Now that we’ve obtained a representation for each image, and defined a similarity measure to
compare two spatial pyramids, we want to put everything up to now together.
You will need to load the training file names from data/train data.npz and the filter bank
and visual word dictionary from dictionary.npy. You will save everything to a .npz file named
trained system.npz. Included will be:
1. dictionary: your visual word dictionary.
2. features: a N × K(4(L+1) − 1)/3 matrix containing all of the histograms of the N training
images in the data set. A dictionary with 150 words will make a train features matrix of
size 1440 × 3150.
3. labels: an N vector containing the labels of each of the images. (features[i] will correspond to label labels[i]).
4. SPM layer num: the number of spatial pyramid layers you used to extract the features for the
training images.
We have provided you with the names of the training images in data/train data.npz. You
want to use the dictionary entry image names for training. You are also provided the names of
the test images in data/test data.npz, which is structured in the same way as the training data;
however, you cannot use the testing images for training.
If it’s any helpful, the below table lists the class names that correspond to the label indices:
0 1 2 3 4 5 6 7
auditorium baseball field desert highway kitchen laundromat waterfall windmill
Q2.4 (15 points): Implement the function
that produces trained system.npz. You may include any helper functions you write in visual
visual recog.get image feature(file path,dictionary,layer num,K)
that load image, extract word map from the image, compute SPM feature and return the computed
feature. Use this function in your visual recognition system().
2.5 Quantitative Evaluation
Qualitative evaluation is all well and good (and very important for diagnosing performance gains
and losses), but we want some hard numbers.
Load the corresponding test images and their labels, and compute the predicted labels of each,
i.e., compute its distance to every image in training set and return the label with least distance
difference as the predicted label. To quantify the accuracy, you will compute a confusion matrix
C: given a classification problem, the entry C(i,j) of a confusion matrix counts the number of
instances of class i that were predicted as class j. When things are going well, the elements on the
diagonal of C are large, and the off-diagonal elements are small. Since there are 8 classes, C will be
8 × 8. The accuracy, or percent of correctly classified images, is given by the trace of C divided by
the sum of C.
Q2.5 (10 points): Implement the function
that tests the system and outputs the confusion matrix. Report the confusion matrix and accuracy
for your results in your write-up. This does not have to be formatted prettily: if you are using
LATEX, you can simply copy/paste it into a verbatim environment. Additionally, do not worry
if your accuracy is low: with 8 classes, chance is 12.5%. To give you a more sensible number, a
reference implementation with spatial pyramid matching gives an overall accuracy of around 50%.
2.6 Find the failed cases
There are some classes/samples that are more difficult to classify than the rest using the bags-ofwords approach. As a result, they are classified incorrectly into other categories.
Q2.6 (5 points): List some of these classes/samples and discuss why they are more difficult in
your write-up.
3 Deep Learning Features – An Alternative to “Bag of Words”
As we have discussed in class, another powerful method for scene classification in computer vision
is the employment of convolutional neural networks (CNNs) – sometimes referred to generically
as “deep learning” It is important to understand how previously trained (pretrained) networks
can be used as another form of feature extraction, and how they relate to classical Bag of Words
(BoW) features. We will be covering details on how one chooses the network architecture and
training procedures later in the course. For this question, however, we will be asking you to deal
with the VGG-16 pretrained network. VGG-16 is a pretrained Convolutional Neural Network
(CNN) that has been trained on approximately 1.2 million images from the ImageNet Dataset
( by the Visual Geometry Group (VGG) at University of Oxford.
The model can classify images into a 1000 object categories (e.g. keyboard, mouse, coffee mug,
pencil) or later installed.
One lesson we want you to take away from this exercise is to understand the effectiveness of “deep
features” for general classification tasks within computer vision – even when those features have
been previously trained on a different dataset (i.e. ImageNet) and task (i.e. object recognition).
3.1 Extracting Deep Features
To complete this question, you need to install the torchvision library from Pytorch, a popular
Pythonbased deep learning library. If you are using the Anaconda package manager (https:
//, this can be done with the following command:
conda install pytorch torchvision -c pytorch
To check that you have installed it correctly, make sure that you can import torch in a Python
interpreter without errors. Please refer to for more detailed installation
Q3.1 (25 points): We want to extract out deep features corresponding to the convolutional layers
of the VGG-16 network.
To load the network, add the line
vgg16 = torchvision.models.vgg16(pretrained=True).double()
followed by
The latter line ensures that the VGG-16 network is in evaluation mode, not training mode.
We want you to complete a function that is able to output VGG-16 network outputs at the fc7
layer in network
network_layers.extract_deep_feature(x,vgg16 weights)
where x refers to the input image and vgg16 weights is a structure containing the CNN’s network
parameters. In this function you will need to write sub-functions multichannel conv2d, relu,
max pool2d, and linear corresponding to the fundamental elements of the CNN: multi-channel
convolution, rectified linear units (ReLU), max pooling, and fully-connected weighting.
We have provided a helper function util.get VGG16 weights() that extracts the weight parameters of VGG-16 and its meta information. The returned variable is a numpy array of shape
L × 3, where L is the number of layers in VGG-16. The first column of each row is a string indicating the layer type. The second/third columns may contain the learned weights and biases, or
other meta-information (e.g. kernel size of max-pooling). Please refer to the function docstring for
VGG-16 assumes that all input imagery to the network is resized to 224×224 with the three color
channels preserved (use skimage.transform.resize() to do this before passing any imagery to
the network). And be sure to normalize the image using suggested mean and std before extracting
the feature:
In order to build the extract deep feature function, you should run a for-loop through each
layer index until layer “fc7”, which corresponds to the second linear layer (Refer to VGG
structure to see where “fc7” is). Remember: the output of the preceding layer should be passed
through as an input to the next. Details on the sub-functions needed for the extract deep feature
function can be found below. Please use scipy.ndimage.convolve and numpy functions to implement these functions instead of using pytorch. Please keep speed in mind when implementing your
function, for example, using double for loop over pixels is not a good idea.
1. multichannel conv2d(x,weight,bias): a function that will perform multi-channel 2D convolution which can be defined as follows,
y(j) = X
∗ h
] + b[j] (1)
where ∗ denotes 2D convolution, x = {x
k=1 is our vectorized K-channel input signal,
h = {h
k=1,j=1 is our J ×K set of vectorized convolutional filters and r = {y
j=1 is our
J channel vectorized output response. Further, unlike traditional single-channel convolution
CNNs often append a bias vector b whose J elements are added to the J channels of the
output response.
To implement multichannel conv2d, use the Scipy convolution function
scipy.ndimage.convolve() with for loops to cycle through the filters and channels. All the
necessary details concerning the number of filters (J), number of channels (K), filter weights
(h) and biases (b) can be inferred from the shapes/dimensions of the weights and biases.
2. relu(x): a function that shall perform the Rectified Linear Unit (ReLU) which can be defined
mathematically as,
ReLU(x) = max(x, 0)
and is applied independently to each element of the matrix/vector x passed to it.
3. max pool2d(x,size): a function that will perform max pooling over x using a receptive
field of size × size (we assume a square receptive field here for simplicity). If the function
receives a multi-channel input, then it should apply the max pooling operation across each
input channel independently. (Hint: making use of smart array indexing can drastically speed
up the code.)
4. linear(x,W,b): a function that will compute a node vector where each element is a linear
combination of the input nodes, written as
y[j] = X
W[j, k]x[k] + b[j] (2)
or more succinctly in vector form as y = Wx + b, where x is the (K × 1) input node vector,
W is the (J × K) weight matrix, b is the (J × 1) bias vector and y is the (J × 1) output
node vector. You should not need for-loops to implement this function.
For efficiency you should check that each sub-function is working properly before putting them
all together – otherwise it will be hard to track any errors. You can check the performance of each
layer by creating your own single-layer network.
3.2 Building a Visual Recognition System: Revisited
We want to compare how useful deep features are in a visual recognition system.
Q3.2 (5 points): Implement the functions
both of which takes the pretrained VGG-16 network as the input arguments. These functions
should follow a very similar pipeline to those in the previous questions, so you are free to reuse
much of the code. The former function should produce trained system deep.npz as the output.
Included will be:
1. features: a N × K matrix containing all the deep features of the N training images in the
data set.
2. labels: an N vector containing the labels of each of the images. (features[i] will correspond
to label labels[i]).
The latter function should produce the confusion matrix, as with the previous question. Instead
of using the histogram intersection similarity, write a function to just use the negative Euclidean
distance (as larger values are more similar). Report the confusion matrix and accuracy for your
results in your write-up. Can you comment in your writeup on whether the results are better or
worse than classical BoW – why do you think that is?
4 HW1 Distribution Checklist
After unpacking, you should have a folder hw1 containing one folder for the data (data)
and one for your code (code). In the code folder, where you will primarily work, you will find:
• visual function definitions for extracting visual words.
• visual function definitions for building a visual recognition system.
• network function definitions for implementing deep network layers.
• deep function definitions for building a visual recognition system using deep features.
• utility functions
• main function for running the system
The data folder contains:
• data/: a directory containing .jpg images from SUN database.
• data/train data.npz: a .npz file containing the training set.
• data/test data.npz: a .npz file containing the test set.
• data/vgg16 list.npy: a .npy file with the weights of VGG-16.
5 HW1 Submission Checklist
The assignment should be submitted to CASS. The writeup should be submitted as a pdf file named
<ustlogin-id> hw1.pdf. The code should be submitted as a zip file named <ustlogin-id>.zip.
By extracting the zip file, it should have the following files in the structure defined below. (Note:
Missing to follow the structure will incur huge penalty in scores).
When you submit, remove the folder data/, as well as any large temporary files
that we did not ask you to create.
• <ustlogin-id>/ (a directory inside .zip file)
– code/
∗ dictionary.npy
∗ trained system.npz
∗ all .py files inside code directory
– <ustlogin-id> hw1.pdf
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on, volume 2, pages 1458 – 1465 Vol. 2, 2005.
3. James Hays and Alexei A Efros. Scene completion using millions of photographs. ACM
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recognizing natural scene categories. In Computer Vision and Pattern Recognition (CVPR),
2006 IEEE Conference on, volume 2, pages 2169–2178, 2006.
5. D.G. Lowe. Object recognition from local scale-invariant features. In Computer Vision
(ICCV), 1999 IEEE International Conference on, volume 2, pages 1150–1157 vol.2, 1999.
6. Laura Walker Renninger and Jitendra Malik. When is scene identification just texture recognition? Vision research, 44(19):2301–2311, 2004.
7. J. Winn, A. Criminisi, and T. Minka. Object categorization by learned universal visual
dictionary. In Computer Vision (ICCV), 2005 IEEE International Conference on, volume 2,
pages 1800–1807 Vol. 2, 2005.
8. Jianxiong Xiao, J. Hays, K.A. Ehinger, A. Oliva, and A. Torralba. Sun database: Large-scale
scene recognition from abbey to zoo. In Computer Vision and Pattern Recognition (CVPR),
2010 IEEE Conference on, pages 3485–3492, 2010.

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