diff --git a/data_source.lua b/data_source.lua
index ff4b520..71b9b6c 100644
--- a/data_source.lua
+++ b/data_source.lua
@@ -1,12 +1,9 @@
 local classic = require 'classic'
 local hdf5 = require 'hdf5'
 local image = require 'image'
-local lmdb = require 'lmdb'
 local torch = require 'torch'
 local __ = require 'moses'
-require 'classic.torch'
-
-local video_frame_proto = require 'video_util.video_frames_pb'
+require 'classic.torch'  -- Necessary for serializing classic classes.
 
 local DataSource = classic.class('DataSource')
 DataSource:mustHave('num_samples')
@@ -168,403 +165,8 @@ function DiskFramesHdf5LabelsDataSource.static.parse_frame_key(frame_key)
     return filename, frame_number
 end
 
-
-local LabeledVideoFramesLmdbSource = classic.class(
-    'LabeledVideoFramesLmdbSource', 'VideoDataSource')
-function LabeledVideoFramesLmdbSource:_init(
-        lmdb_path, lmdb_without_images_path, num_labels, _ --[[options]])
-    --[[
-    Data source for LabeledVideoFrame protobufs in LMDBs.
-
-    Args:
-       lmdb_path (str): Path to LMDB containing LabeledVideoFrames as values,
-           and keys of the form "<video>-<frame_number>".
-       lmdb_without_images_path (str): Path to LMDB of the same form as
-           lmdb_path, but where the images have been stripped from the
-           protobufs.
-        num_labels (int)
-    ]]--
-    self.lmdb_path = lmdb_path
-    self.lmdb_without_images_path = lmdb_without_images_path
-    self.num_labels_ = num_labels
-
-    self.num_keys = 0
-    self.video_keys_ = {}
-    local key_labels = self:key_label_map()
-    for key, _ in pairs(key_labels) do
-        local video, frame = self:frame_video_offset(key)
-        if self.video_keys_[video] == nil then
-            self.video_keys_[video] = {}
-        end
-        self.video_keys_[video][frame] = key
-        self.num_keys = self.num_keys + 1
-    end
-end
-
-function LabeledVideoFramesLmdbSource:num_samples()
-    return self.num_keys
-end
-
-function LabeledVideoFramesLmdbSource:video_keys()
-    return self.video_keys_
-end
-
-function LabeledVideoFramesLmdbSource:num_labels() return self.num_labels_ end
-
-function LabeledVideoFramesLmdbSource:key_label_map(return_label_map)
-    --[[
-    Load mapping from frame keys to labels array.
-
-    Note: This is a giant array, and should be destroyed as soon as it is no
-    longer needed. If this array is stored permanently (e.g. globally or as an
-    object attribute), it will slow down *all* future calls to collectgarbage().
-
-    Args:
-        return_label_map (bool): If true, return a map from label names to label
-            id.
-
-    Returns:
-        key_labels: Table mapping frame keys to array of label indices.
-        (optional) label_map: See doc for return_label_map arg.
-
-    ]]--
-    return_label_map = return_label_map == nil and false or return_label_map
-
-    -- Get LMDB cursor.
-    local db = lmdb.env { Path = self.lmdb_without_images_path }
-    db:open()
-    local transaction = db:txn(true --[[readonly]])
-    local cursor = transaction:cursor()
-
-    -- Create mapping from keys to labels.
-    local key_label_map = {}
-
-    local label_map = {}
-    local num_keys = db:stat().entries
-    for i = 1, num_keys do
-        local key, value = cursor:get('MDB_GET_CURRENT')
-        local video_frame = video_frame_proto.LabeledVideoFrame()
-        video_frame:ParseFromString(value:storage():string())
-        local num_frame_labels = #video_frame.label
-        if num_frame_labels == 0 then
-            -- Add frame to list of background frames.
-            key_label_map[key] = {self.num_labels_ + 1}
-        else
-            local labels = {}
-            for j, label in ipairs(video_frame.label) do
-                -- Label ids start at 0.
-                labels[j] = label.id + 1
-                if label_map[label.name] == nil then
-                    label_map[label.name] = label.id + 1
-                end
-            end
-            key_label_map[key] = labels
-        end
-        if i ~= db:stat().entries then cursor:next() end
-    end
-
-    -- Cleanup.
-    cursor:close()
-    transaction:abort()
-    db:close()
-
-    if return_label_map then
-        return key_label_map, label_map
-    else
-        return key_label_map
-    end
-end
-
--- luacheck: push no unused args
-function LabeledVideoFramesLmdbSource:frame_video_offset(key)
-    return DiskFramesHdf5LabelsDataSource.static.parse_frame_key(key)
-end
--- luacheck: pop
-
-function LabeledVideoFramesLmdbSource:load_data(keys, load_images)
-    --[[
-    Load images and labels for a set of keys from the LMDB.
-
-    Args:
-        keys (array): Array of array of string keys. Each element must be
-            an array of the same length as every element, and contains keys for
-            one step of the image sequence.
-        load_images (bool): Defaults to true. If false, load only labels,
-            not images. The ByteTensors in batch_images will simply be empty.
-
-    Returns:
-        batch_images: Array of array of ByteTensors
-        batch_labels: ByteTensor of shape (num_steps, batch_size, num_labels)
-    ]]--
-    load_images = load_images == nil and true or load_images
-    local db = lmdb.env {
-        Path = load_images and self.lmdb_path or self.lmdb_without_images_path}
-    db:open()
-    local transaction = db:txn(true --[[readonly]])
-
-    local num_steps = #keys
-    local batch_size = #keys[1]
-    local batch_labels = torch.ByteTensor(
-        num_steps, batch_size, self.num_labels_)
-    local batch_images = {}
-    for step = 1, num_steps do
-        batch_images[step] = {}
-    end
-    for i = 1, batch_size do
-        for step = 1, num_steps do
-            if keys[step][i] == VideoDataSource.END_OF_SEQUENCE then
-                table.insert(batch_images[step],
-                             VideoDataSource.END_OF_SEQUENCE)
-                batch_labels[{step, i}]:zero()
-            else
-                -- Load LabeledVideoFrame.
-                local video_frame = video_frame_proto.LabeledVideoFrame()
-                video_frame:ParseFromString(
-                    transaction:get(keys[step][i]):storage():string())
-
-                -- Load image and labels.
-                local img, labels =
-                    LabeledVideoFramesLmdbSource._load_image_labels_from_proto(
-                        video_frame)
-                labels = LabeledVideoFramesLmdbSource._labels_to_tensor(
-                    labels, self.num_labels_)
-                table.insert(batch_images[step], img)
-                batch_labels[{step, i}] = labels
-            end
-        end
-    end
-
-    transaction:abort()
-    db:close()
-
-    return batch_images, batch_labels
-end
-
-function LabeledVideoFramesLmdbSource.static._labels_to_tensor(
-    labels, num_labels)
-    --[[
-    Convert an array of label ids into a 1-hot encoding in a binary Tensor.
-    ]]--
-    local labels_tensor = torch.ByteTensor(num_labels):zero()
-    for _, label in ipairs(labels) do
-        labels_tensor[label + 1] = 1 -- Label ids start at 0.
-    end
-    return labels_tensor
-end
-
-function LabeledVideoFramesLmdbSource.static._image_proto_to_tensor(image_proto)
-    local image_storage = torch.ByteStorage()
-    if image_proto.data:len() == 0 then
-        return torch.ByteTensor()
-    end
-    image_storage:string(image_proto.data)
-    return torch.ByteTensor(image_storage):reshape(
-        image_proto.channels, image_proto.height, image_proto.width)
-end
-
-function LabeledVideoFramesLmdbSource.static._load_image_labels_from_proto(
-        frame_proto)
-    --[[
-    Loads an image tensor and labels for a given key.
-
-    Returns:
-        img (ByteTensor): Image of size (num_channels, height, width).
-        labels (Array): Contains numeric id for each label.
-    ]]
-
-    local img = LabeledVideoFramesLmdbSource._image_proto_to_tensor(
-        frame_proto.frame.image)
-
-    -- Load labels in an array.
-    local labels = __.pluck(frame_proto.label, 'id')
-    return img, labels
-end
-
-local PositiveVideosLmdbSource, PositiveVideosLmdbSourceSuper = classic.class(
-    'PositiveVideosLmdbSource', 'LabeledVideoFramesLmdbSource')
-function PositiveVideosLmdbSource:_init(
-        lmdb_path, lmdb_without_images_path, num_labels, options)
-    --[[
-    Like LabeledVideoFramesLmdbSource, but only use 'positive' videos.
-
-    A 'positive' video is a video containing at least one frame that has one of
-    the options.labels labels assigned to it.
-
-    Args:
-       lmdb_path (str): Path to LMDB containing LabeledVideoFrames as values,
-           and keys of the form "<video>-<frame_number>".
-       lmdb_without_images_path (str): Path to LMDB of the same form as
-           lmdb_path, but where the images have been stripped from the
-           protobufs.
-        num_labels (int): Total number of labels in the dataset.
-        options:
-            labels (table of strings): List of labels to consider as positive.
-            output_all_labels (bool): If true, still output the groundtruth
-                for all labels in the dataset - just limit the frames to be
-                from positive videos. By default, this is false, and we only
-                output the groundtruth for labels in options.labels.
-    ]]--
-    self.lmdb_path = lmdb_path
-    self.lmdb_without_images_path = lmdb_without_images_path
-    self.positive_label_names = options.labels
-    self.output_all_labels =
-        options.output_all_labels == nil and false or options.output_all_labels
-    self.num_labels_ = num_labels
-
-    self.num_keys = 0
-    self.video_keys_ = {}
-    local key_labels, label_map = self:_unfiltered_key_label_map()
-    self.label_map = label_map -- Maps label names to ids
-
-    for key, _ in pairs(key_labels) do
-        local video, frame = DiskFramesHdf5LabelsDataSource.parse_frame_key(key)
-        if self.video_keys_[video] == nil then
-            self.video_keys_[video] = {}
-        end
-        self.video_keys_[video][frame] = key
-        self.num_keys = self.num_keys + 1
-    end
-
-    self.positive_label_ids  = {}
-    local positive_label_ids_set = {}
-    for i, label in ipairs(self.positive_label_names) do
-        positive_label_ids_set[self.label_map[label]] = true
-        self.positive_label_ids[i] = self.label_map[label]
-    end
-    table.sort(self.positive_label_ids)
-
-    for video, keys in pairs(self.video_keys_) do
-        local positive_video = false
-        for _, key in ipairs(keys) do
-            for _, label_id in ipairs(key_labels[key]) do
-                if positive_label_ids_set[label_id] then
-                    positive_video = true
-                    break
-                end
-            end
-            if positive_video then break end
-        end
-        if not positive_video then self.video_keys_[video] = nil end
-    end
-end
-
-function PositiveVideosLmdbSource:num_labels()
-    if self.output_all_labels then
-        return self.num_labels_
-    else
-        return #self.positive_label_ids
-    end
-end
-
-function PositiveVideosLmdbSource:_unfiltered_key_label_map()
-    return PositiveVideosLmdbSourceSuper.key_label_map(
-        self, true --[[return_label_map]])
-end
-
-function PositiveVideosLmdbSource:key_label_map(return_label_map)
-    -- Remove keys that are not in positive videos.
-    local key_label_map = PositiveVideosLmdbSourceSuper.key_label_map(
-        self, return_label_map)
-    for key, _ in pairs(key_label_map) do
-        local video, _ = self:frame_video_offset(key)
-        if self.video_keys_[video] == nil then
-            key_label_map[key] = nil
-        end
-    end
-end
-
-function PositiveVideosLmdbSource:load_data(keys, load_images)
-    local batch_images, batch_labels = PositiveVideosLmdbSourceSuper.load_data(
-        self, keys, load_images)
-    if not self.output_all_labels then
-        batch_labels = batch_labels[{{}, {}, self.positive_label_ids}]
-    end
-    return batch_images, batch_labels
-end
-
-local SubsampledLmdbSource, SubsampledLmdbSourceSuper = classic.class(
-    'SubsampledLmdbSource', 'LabeledVideoFramesLmdbSource')
-function SubsampledLmdbSource:_init(
-        lmdb_path, lmdb_without_images_path, num_labels, options)
-    --[[
-    Subsample video to reduce the frame rate.
-
-    Args:
-       lmdb_path (str): Path to LMDB containing LabeledVideoFrames as values,
-           and keys of the form "<video>-<frame_number>".
-       lmdb_without_images_path (str): Path to LMDB of the same form as
-           lmdb_path, but where the images have been stripped from the
-           protobufs.
-        num_labels (int): Total number of labels in the dataset.
-        options:
-            subsample_rate (int): How much to subsample the frames by.
-    ]]--
-    self.lmdb_path = lmdb_path
-    self.lmdb_without_images_path = lmdb_without_images_path
-    self.num_labels_ = num_labels
-    -- The whole point is to set a subsample rate, we should mandate that it is
-    -- specified.
-    assert(options ~= nil and options.subsample_rate ~= nil)
-    self.subsample_rate = options.subsample_rate
-
-    self.num_keys = 0
-    self.video_keys_ = {}
-    local key_labels = self:key_label_map()
-
-    for key, _ in pairs(key_labels) do
-        local video, frame = self:frame_video_offset(key)
-        if self.video_keys_[video] == nil then
-            self.video_keys_[video] = {}
-        end
-        self.video_keys_[video][frame] = key
-        self.num_keys = self.num_keys + 1
-    end
-end
-
-function SubsampledLmdbSource:use_frame_q(frame_key)
-    local _, frame_index = DiskFramesHdf5LabelsDataSource.parse_frame_key(
-        frame_key)
-    return ((frame_index - 1) % self.subsample_rate) == 0
-end
-
-function SubsampledLmdbSource:frame_video_offset(frame_key)
-    assert(self:use_frame_q(frame_key))
-    local video, frame_index = DiskFramesHdf5LabelsDataSource.parse_frame_key(
-        frame_key)
-    return video, (frame_index - 1) / self.subsample_rate + 1
-end
-
-function SubsampledLmdbSource:key_label_map(return_label_map)
-    --[[
-    Load mapping from frame keys to labels array.
-
-    Args:
-        return_label_map (bool): See LabeledVideoFramesLmdbSource:key_label_map.
-
-    Returns:
-        key_labels: See LabeledVideoFramesLmdbSource:key_label_map.
-    ]]--
-    local key_label_map, label_map = SubsampledLmdbSourceSuper.key_label_map(
-        self, return_label_map)
-
-    for key, _ in pairs(key_label_map) do
-        if not self:use_frame_q(key) then
-            key_label_map[key] = nil
-        end
-    end
-
-    if return_label_map then
-        return key_label_map, label_map
-    else
-        return key_label_map
-    end
-end
-
 return {
     DataSource = DataSource,
     VideoDataSource = VideoDataSource,
-    DiskFramesHdf5LabelsDataSource = DiskFramesHdf5LabelsDataSource,
-    LabeledVideoFramesLmdbSource = LabeledVideoFramesLmdbSource,
-    PositiveVideosLmdbSource = PositiveVideosLmdbSource,
-    SubsampledLmdbSource = SubsampledLmdbSource
+    DiskFramesHdf5LabelsDataSource = DiskFramesHdf5LabelsDataSource
 }
diff --git a/debug/debug_compare_data_sources.lua b/debug/debug_compare_data_sources.lua
index 3443a5f..fca1db1 100644
--- a/debug/debug_compare_data_sources.lua
+++ b/debug/debug_compare_data_sources.lua
@@ -6,6 +6,7 @@ local data_source = require 'data_source'
 -- local data_loader = require 'data_loader'
 local log = require 'util/log'
 local samplers = require 'samplers'
+require 'lmdb_data_source'
 
 local parser = argparse() {
     description = 'Compare lmdb loader to disk loader'
diff --git a/debug/debug_sampler.lua b/debug/debug_sampler.lua
index c786135..6547f43 100644
--- a/debug/debug_sampler.lua
+++ b/debug/debug_sampler.lua
@@ -7,6 +7,7 @@ local data_source = require 'data_source'
 local data_loader = require 'data_loader'
 local log = require 'util/log'
 local samplers = require 'samplers'
+require 'lmdb_data_source'
 
 local parser = argparse() {
     description = 'Compute distribution of labels from samples.'
diff --git a/lmdb_data_source.lua b/lmdb_data_source.lua
new file mode 100644
index 0000000..f705e4e
--- /dev/null
+++ b/lmdb_data_source.lua
@@ -0,0 +1,406 @@
+local classic = require 'classic'
+local lmdb = require 'lmdb'
+local torch = require 'torch'
+local __ = require 'moses'
+require 'classic.torch'
+
+local data_source = require 'data_source'
+local DiskFramesHdf5LabelsDataSource =
+    data_source.DiskFramesHdf5LabelsDataSource
+
+local video_frame_proto = require 'video_util.video_frames_pb'
+
+local LabeledVideoFramesLmdbSource = classic.class(
+    'LabeledVideoFramesLmdbSource', 'VideoDataSource')
+function LabeledVideoFramesLmdbSource:_init(
+        lmdb_path, lmdb_without_images_path, num_labels, _ --[[options]])
+    --[[
+    Data source for LabeledVideoFrame protobufs in LMDBs.
+
+    Args:
+       lmdb_path (str): Path to LMDB containing LabeledVideoFrames as values,
+           and keys of the form "<video>-<frame_number>".
+       lmdb_without_images_path (str): Path to LMDB of the same form as
+           lmdb_path, but where the images have been stripped from the
+           protobufs.
+        num_labels (int)
+    ]]--
+    self.lmdb_path = lmdb_path
+    self.lmdb_without_images_path = lmdb_without_images_path
+    self.num_labels_ = num_labels
+
+    self.num_keys = 0
+    self.video_keys_ = {}
+    local key_labels = self:key_label_map()
+    for key, _ in pairs(key_labels) do
+        local video, frame = self:frame_video_offset(key)
+        if self.video_keys_[video] == nil then
+            self.video_keys_[video] = {}
+        end
+        self.video_keys_[video][frame] = key
+        self.num_keys = self.num_keys + 1
+    end
+end
+
+function LabeledVideoFramesLmdbSource:num_samples()
+    return self.num_keys
+end
+
+function LabeledVideoFramesLmdbSource:video_keys()
+    return self.video_keys_
+end
+
+function LabeledVideoFramesLmdbSource:num_labels() return self.num_labels_ end
+
+function LabeledVideoFramesLmdbSource:key_label_map(return_label_map)
+    --[[
+    Load mapping from frame keys to labels array.
+
+    Note: This is a giant array, and should be destroyed as soon as it is no
+    longer needed. If this array is stored permanently (e.g. globally or as an
+    object attribute), it will slow down *all* future calls to collectgarbage().
+
+    Args:
+        return_label_map (bool): If true, return a map from label names to label
+            id.
+
+    Returns:
+        key_labels: Table mapping frame keys to array of label indices.
+        (optional) label_map: See doc for return_label_map arg.
+
+    ]]--
+    return_label_map = return_label_map == nil and false or return_label_map
+
+    -- Get LMDB cursor.
+    local db = lmdb.env { Path = self.lmdb_without_images_path }
+    db:open()
+    local transaction = db:txn(true --[[readonly]])
+    local cursor = transaction:cursor()
+
+    -- Create mapping from keys to labels.
+    local key_label_map = {}
+
+    local label_map = {}
+    local num_keys = db:stat().entries
+    for i = 1, num_keys do
+        local key, value = cursor:get('MDB_GET_CURRENT')
+        local video_frame = video_frame_proto.LabeledVideoFrame()
+        video_frame:ParseFromString(value:storage():string())
+        local num_frame_labels = #video_frame.label
+        if num_frame_labels == 0 then
+            -- Add frame to list of background frames.
+            key_label_map[key] = {self.num_labels_ + 1}
+        else
+            local labels = {}
+            for j, label in ipairs(video_frame.label) do
+                -- Label ids start at 0.
+                labels[j] = label.id + 1
+                if label_map[label.name] == nil then
+                    label_map[label.name] = label.id + 1
+                end
+            end
+            key_label_map[key] = labels
+        end
+        if i ~= db:stat().entries then cursor:next() end
+    end
+
+    -- Cleanup.
+    cursor:close()
+    transaction:abort()
+    db:close()
+
+    if return_label_map then
+        return key_label_map, label_map
+    else
+        return key_label_map
+    end
+end
+
+-- luacheck: push no unused args
+function LabeledVideoFramesLmdbSource:frame_video_offset(key)
+    return DiskFramesHdf5LabelsDataSource.static.parse_frame_key(key)
+end
+-- luacheck: pop
+
+function LabeledVideoFramesLmdbSource:load_data(keys, load_images)
+    --[[
+    Load images and labels for a set of keys from the LMDB.
+
+    Args:
+        keys (array): Array of array of string keys. Each element must be
+            an array of the same length as every element, and contains keys for
+            one step of the image sequence.
+        load_images (bool): Defaults to true. If false, load only labels,
+            not images. The ByteTensors in batch_images will simply be empty.
+
+    Returns:
+        batch_images: Array of array of ByteTensors
+        batch_labels: ByteTensor of shape (num_steps, batch_size, num_labels)
+    ]]--
+    load_images = load_images == nil and true or load_images
+    local db = lmdb.env {
+        Path = load_images and self.lmdb_path or self.lmdb_without_images_path}
+    db:open()
+    local transaction = db:txn(true --[[readonly]])
+
+    local num_steps = #keys
+    local batch_size = #keys[1]
+    local batch_labels = torch.ByteTensor(
+        num_steps, batch_size, self.num_labels_)
+    local batch_images = {}
+    for step = 1, num_steps do
+        batch_images[step] = {}
+    end
+    for i = 1, batch_size do
+        for step = 1, num_steps do
+            if keys[step][i] == VideoDataSource.END_OF_SEQUENCE then
+                table.insert(batch_images[step],
+                             VideoDataSource.END_OF_SEQUENCE)
+                batch_labels[{step, i}]:zero()
+            else
+                -- Load LabeledVideoFrame.
+                local video_frame = video_frame_proto.LabeledVideoFrame()
+                video_frame:ParseFromString(
+                    transaction:get(keys[step][i]):storage():string())
+
+                -- Load image and labels.
+                local img, labels =
+                    LabeledVideoFramesLmdbSource._load_image_labels_from_proto(
+                        video_frame)
+                labels = LabeledVideoFramesLmdbSource._labels_to_tensor(
+                    labels, self.num_labels_)
+                table.insert(batch_images[step], img)
+                batch_labels[{step, i}] = labels
+            end
+        end
+    end
+
+    transaction:abort()
+    db:close()
+
+    return batch_images, batch_labels
+end
+
+function LabeledVideoFramesLmdbSource.static._labels_to_tensor(
+    labels, num_labels)
+    --[[
+    Convert an array of label ids into a 1-hot encoding in a binary Tensor.
+    ]]--
+    local labels_tensor = torch.ByteTensor(num_labels):zero()
+    for _, label in ipairs(labels) do
+        labels_tensor[label + 1] = 1 -- Label ids start at 0.
+    end
+    return labels_tensor
+end
+
+function LabeledVideoFramesLmdbSource.static._image_proto_to_tensor(image_proto)
+    local image_storage = torch.ByteStorage()
+    if image_proto.data:len() == 0 then
+        return torch.ByteTensor()
+    end
+    image_storage:string(image_proto.data)
+    return torch.ByteTensor(image_storage):reshape(
+        image_proto.channels, image_proto.height, image_proto.width)
+end
+
+function LabeledVideoFramesLmdbSource.static._load_image_labels_from_proto(
+        frame_proto)
+    --[[
+    Loads an image tensor and labels for a given key.
+
+    Returns:
+        img (ByteTensor): Image of size (num_channels, height, width).
+        labels (Array): Contains numeric id for each label.
+    ]]
+
+    local img = LabeledVideoFramesLmdbSource._image_proto_to_tensor(
+        frame_proto.frame.image)
+
+    -- Load labels in an array.
+    local labels = __.pluck(frame_proto.label, 'id')
+    return img, labels
+end
+
+local PositiveVideosLmdbSource, PositiveVideosLmdbSourceSuper = classic.class(
+    'PositiveVideosLmdbSource', 'LabeledVideoFramesLmdbSource')
+function PositiveVideosLmdbSource:_init(
+        lmdb_path, lmdb_without_images_path, num_labels, options)
+    --[[
+    Like LabeledVideoFramesLmdbSource, but only use 'positive' videos.
+
+    A 'positive' video is a video containing at least one frame that has one of
+    the options.labels labels assigned to it.
+
+    Args:
+       lmdb_path (str): Path to LMDB containing LabeledVideoFrames as values,
+           and keys of the form "<video>-<frame_number>".
+       lmdb_without_images_path (str): Path to LMDB of the same form as
+           lmdb_path, but where the images have been stripped from the
+           protobufs.
+        num_labels (int): Total number of labels in the dataset.
+        options:
+            labels (table of strings): List of labels to consider as positive.
+            output_all_labels (bool): If true, still output the groundtruth
+                for all labels in the dataset - just limit the frames to be
+                from positive videos. By default, this is false, and we only
+                output the groundtruth for labels in options.labels.
+    ]]--
+    self.lmdb_path = lmdb_path
+    self.lmdb_without_images_path = lmdb_without_images_path
+    self.positive_label_names = options.labels
+    self.output_all_labels =
+        options.output_all_labels == nil and false or options.output_all_labels
+    self.num_labels_ = num_labels
+
+    self.num_keys = 0
+    self.video_keys_ = {}
+    local key_labels, label_map = self:_unfiltered_key_label_map()
+    self.label_map = label_map -- Maps label names to ids
+
+    for key, _ in pairs(key_labels) do
+        local video, frame = DiskFramesHdf5LabelsDataSource.parse_frame_key(key)
+        if self.video_keys_[video] == nil then
+            self.video_keys_[video] = {}
+        end
+        self.video_keys_[video][frame] = key
+        self.num_keys = self.num_keys + 1
+    end
+
+    self.positive_label_ids  = {}
+    local positive_label_ids_set = {}
+    for i, label in ipairs(self.positive_label_names) do
+        positive_label_ids_set[self.label_map[label]] = true
+        self.positive_label_ids[i] = self.label_map[label]
+    end
+    table.sort(self.positive_label_ids)
+
+    for video, keys in pairs(self.video_keys_) do
+        local positive_video = false
+        for _, key in ipairs(keys) do
+            for _, label_id in ipairs(key_labels[key]) do
+                if positive_label_ids_set[label_id] then
+                    positive_video = true
+                    break
+                end
+            end
+            if positive_video then break end
+        end
+        if not positive_video then self.video_keys_[video] = nil end
+    end
+end
+
+function PositiveVideosLmdbSource:num_labels()
+    if self.output_all_labels then
+        return self.num_labels_
+    else
+        return #self.positive_label_ids
+    end
+end
+
+function PositiveVideosLmdbSource:_unfiltered_key_label_map()
+    return PositiveVideosLmdbSourceSuper.key_label_map(
+        self, true --[[return_label_map]])
+end
+
+function PositiveVideosLmdbSource:key_label_map(return_label_map)
+    -- Remove keys that are not in positive videos.
+    local key_label_map = PositiveVideosLmdbSourceSuper.key_label_map(
+        self, return_label_map)
+    for key, _ in pairs(key_label_map) do
+        local video, _ = self:frame_video_offset(key)
+        if self.video_keys_[video] == nil then
+            key_label_map[key] = nil
+        end
+    end
+end
+
+function PositiveVideosLmdbSource:load_data(keys, load_images)
+    local batch_images, batch_labels = PositiveVideosLmdbSourceSuper.load_data(
+        self, keys, load_images)
+    if not self.output_all_labels then
+        batch_labels = batch_labels[{{}, {}, self.positive_label_ids}]
+    end
+    return batch_images, batch_labels
+end
+
+local SubsampledLmdbSource, SubsampledLmdbSourceSuper = classic.class(
+    'SubsampledLmdbSource', 'LabeledVideoFramesLmdbSource')
+function SubsampledLmdbSource:_init(
+        lmdb_path, lmdb_without_images_path, num_labels, options)
+    --[[
+    Subsample video to reduce the frame rate.
+
+    Args:
+       lmdb_path (str): Path to LMDB containing LabeledVideoFrames as values,
+           and keys of the form "<video>-<frame_number>".
+       lmdb_without_images_path (str): Path to LMDB of the same form as
+           lmdb_path, but where the images have been stripped from the
+           protobufs.
+        num_labels (int): Total number of labels in the dataset.
+        options:
+            subsample_rate (int): How much to subsample the frames by.
+    ]]--
+    self.lmdb_path = lmdb_path
+    self.lmdb_without_images_path = lmdb_without_images_path
+    self.num_labels_ = num_labels
+    -- The whole point is to set a subsample rate, we should mandate that it is
+    -- specified.
+    assert(options ~= nil and options.subsample_rate ~= nil)
+    self.subsample_rate = options.subsample_rate
+
+    self.num_keys = 0
+    self.video_keys_ = {}
+    local key_labels = self:key_label_map()
+
+    for key, _ in pairs(key_labels) do
+        local video, frame = self:frame_video_offset(key)
+        if self.video_keys_[video] == nil then
+            self.video_keys_[video] = {}
+        end
+        self.video_keys_[video][frame] = key
+        self.num_keys = self.num_keys + 1
+    end
+end
+
+function SubsampledLmdbSource:use_frame_q(frame_key)
+    local _, frame_index = DiskFramesHdf5LabelsDataSource.parse_frame_key(
+        frame_key)
+    return ((frame_index - 1) % self.subsample_rate) == 0
+end
+
+function SubsampledLmdbSource:frame_video_offset(frame_key)
+    assert(self:use_frame_q(frame_key))
+    local video, frame_index = DiskFramesHdf5LabelsDataSource.parse_frame_key(
+        frame_key)
+    return video, (frame_index - 1) / self.subsample_rate + 1
+end
+
+function SubsampledLmdbSource:key_label_map(return_label_map)
+    --[[
+    Load mapping from frame keys to labels array.
+
+    Args:
+        return_label_map (bool): See LabeledVideoFramesLmdbSource:key_label_map.
+
+    Returns:
+        key_labels: See LabeledVideoFramesLmdbSource:key_label_map.
+    ]]--
+    local key_label_map, label_map = SubsampledLmdbSourceSuper.key_label_map(
+        self, return_label_map)
+
+    for key, _ in pairs(key_label_map) do
+        if not self:use_frame_q(key) then
+            key_label_map[key] = nil
+        end
+    end
+
+    if return_label_map then
+        return key_label_map, label_map
+    else
+        return key_label_map
+    end
+end
+
+data_source.LabeledVideoFramesLmdbSource = LabeledVideoFramesLmdbSource
+data_source.PositiveVideosLmdbSource = PositiveVideosLmdbSource
+data_source.SubsampledLmdbSource = SubsampledLmdbSource
diff --git a/main.lua b/main.lua
index d7657a5..8e618aa 100644
--- a/main.lua
+++ b/main.lua
@@ -116,6 +116,7 @@ local experiment_saver = require 'util/experiment_saver'
 local log = require 'util/log'
 local samplers = require 'samplers'
 local trainer = require 'trainer'
+require 'lmdb_data_source'
 require 'last_step_criterion'
 require 'layers/init'
 require 'util/strict'
diff --git a/scripts/batch_label_distribution.lua b/scripts/batch_label_distribution.lua
index 47365f1..d003db2 100644
--- a/scripts/batch_label_distribution.lua
+++ b/scripts/batch_label_distribution.lua
@@ -8,6 +8,7 @@ local data_source = require 'data_source'
 local data_loader = require 'data_loader'
 local log = require 'util/log'
 local samplers = require 'samplers'
+require 'lmdb_data_source'
 
 local parser = argparse() {
     description = 'Compute distribution of labels from samples.'
diff --git a/scripts/compute_video_activations.lua b/scripts/compute_video_activations.lua
index c78ee45..54c417a 100644
--- a/scripts/compute_video_activations.lua
+++ b/scripts/compute_video_activations.lua
@@ -20,6 +20,7 @@ local data_source = require 'data_source'
 local data_loader = require 'data_loader'
 local log = require 'util/log'
 local samplers = require 'samplers'
+require 'lmdb_data_source'
 
 local parser = argparse() {
     description = 'Computes activations for a given video for a VGG network.'
diff --git a/tests/PositiveVideosLmdbSource_test.lua b/tests/PositiveVideosLmdbSource_test.lua
index 0ba6fcd..f41bb09 100644
--- a/tests/PositiveVideosLmdbSource_test.lua
+++ b/tests/PositiveVideosLmdbSource_test.lua
@@ -4,6 +4,7 @@ local data_loader = require 'data_loader'
 local data_source = require 'data_source'
 local samplers = require 'samplers'
 local lyaml = require 'lyaml'
+require 'lmdb_data_source
 
 local data_paths = lyaml.load(io.open('config/data_paths.yaml', 'r'):read('*a'))