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---
title: Data
---
# Data: Ins and Outs
Data flows through Caffe as [Blobs](net_layer_blob.html#blob-storage-and-communication).
Data layers load input and save output by converting to and from Blob to other formats.
Common transformations like mean-subtraction and feature-scaling are done by data layer configuration.
New input types are supported by developing a new data layer -- the rest of the Net follows by the modularity of the Caffe layer catalogue.
This data layer definition
layer {
name: "mnist"
# Data layer loads leveldb or lmdb storage DBs for high-throughput.
type: "Data"
# the 1st top is the data itself: the name is only convention
top: "data"
# the 2nd top is the ground truth: the name is only convention
top: "label"
# the Data layer configuration
data_param {
# path to the DB
source: "examples/mnist/mnist_train_lmdb"
# type of DB: LEVELDB or LMDB (LMDB supports concurrent reads)
backend: LMDB
# batch processing improves efficiency.
batch_size: 64
}
# common data transformations
transform_param {
# feature scaling coefficient: this maps the [0, 255] MNIST data to [0, 1]
scale: 0.00390625
}
}
loads the MNIST digits.
**Tops and Bottoms**: A data layer makes **top** blobs to output data to the model.
It does not have **bottom** blobs since it takes no input.
**Data and Label**: a data layer has at least one top canonically named **data**.
For ground truth a second top can be defined that is canonically named **label**.
Both tops simply produce blobs and there is nothing inherently special about these names.
The (data, label) pairing is a convenience for classification models.
**Transformations**: data preprocessing is parametrized by transformation messages within the data layer definition.
layer {
name: "data"
type: "Data"
[...]
transform_param {
scale: 0.1
mean_file_size: mean.binaryproto
# for images in particular horizontal mirroring and random cropping
# can be done as simple data augmentations.
mirror: 1 # 1 = on, 0 = off
# crop a `crop_size` x `crop_size` patch:
# - at random during training
# - from the center during testing
crop_size: 227
}
}
**Prefetching**: for throughput data layers fetch the next batch of data and prepare it in the background while the Net computes the current batch.
**Multiple Inputs**: a Net can have multiple inputs of any number and type. Define as many data layers as needed giving each a unique name and top. Multiple inputs are useful for non-trivial ground truth: one data layer loads the actual data and the other data layer loads the ground truth in lock-step. In this arrangement both data and label can be any 4D array. Further applications of multiple inputs are found in multi-modal and sequence models. In these cases you may need to implement your own data preparation routines or a special data layer.
*Improvements to data processing to add formats, generality, or helper utilities are welcome!*
## Formats
Refer to the layer catalogue of [data layers](layers.html#data-layers) for close-ups on each type of data Caffe understands.
## Deployment Input
For on-the-fly computation deployment Nets define their inputs by `input` fields: these Nets then accept direct assignment of data for online or interactive computation.
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