mlpack

Image Utilities Tutorial

Image datasets are becoming increasingly popular in deep learning.

mlpack’s image saving/loading functionality is based on stb.

🔗 Model API

Image utilities supports loading and saving of images.

It supports filetypes jpg, png, tga, bmp, psd, gif, hdr, pic, pnm for loading and jpg, png, tga, bmp, hdr for saving.

The datatype associated is unsigned char to support RGB values in the range 1-255. To feed data into the network typecast of arma::Mat may be required. Images are stored in matrix as (width * height * channels, numberOfImages). Therefore imageMatrix.col(0) would be the first image if images are loaded in imageMatrix.

🔗 ImageInfo

The ImageInfo class contains the metadata of the images.

/**
 * Instantiate the ImageInfo object with the image width, height, channels.
 *
 * @param width Image width.
 * @param height Image height.
 * @param channels number of channels in the image.
 */
ImageInfo(const size_t width,
          const size_t height,
          const size_t channels);

Other public members include the quality compression of the image if saved as jpg (0-100).

🔗 Loading

Standalone loading of images can be done with the function below.

/**
 * Load the image file into the given matrix.
 *
 * @param filename Name of the image file.
 * @param matrix Matrix to load the image into.
 * @param info An object of ImageInfo class.
 * @param fatal If an error should be reported as fatal (default false).
 * @param transpose If true, flips the image, same as transposing the
 *    matrix after loading.
 * @return Boolean value indicating success or failure of load.
 */
template<typename eT>
bool Load(const std::string& filename,
          arma::Mat<eT>& matrix,
          ImageInfo& info,
          const bool fatal,
          const bool transpose);

Loading a test image is shown below. It also fills up the ImageInfo class object.

data::ImageInfo info;
data::Load("test_image.png", matrix, info, false, true);

ImageInfo requires height, width, number of channels of the image.

size_t height = 64, width = 64, channels = 1;
data::ImageInfo info(width, height, channels);

More than one image can be loaded into the same matrix.

Loading multiple images can be done using the function below.

/**
 * Load the image file into the given matrix.
 *
 * @param files A vector consisting of filenames.
 * @param matrix Matrix to save the image from.
 * @param info An object of ImageInfo class.
 * @param fatal If an error should be reported as fatal (default false).
 * @param transpose If true, flips the image, same as transposing the
 *    matrix after loading.
 * @return Boolean value indicating success or failure of load.
 */
template<typename eT>
bool Load(const std::vector<std::string>& files,
          arma::Mat<eT>& matrix,
          ImageInfo& info,
          const bool fatal,
          const bool transpose);

data::ImageInfo info;
std::vector<std::string>> files{"test_image1.bmp","test_image2.bmp"};
data::Load(files, matrix, info, false, true);

🔗 Saving

Saving images expects a matrix of type unsigned char in the form (width * height * channels, NumberOfImages). Just like loading, it can be used to save one image or multiple images. Besides image data it also expects the shape of the image as input (width, height, channels).

Saving one image can be done with the function below:

/**
 * Save the image file from the given matrix.
 *
 * @param filename Name of the image file.
 * @param matrix Matrix to save the image from.
 * @param info An object of ImageInfo class.
 * @param fatal If an error should be reported as fatal (default false).
 * @param transpose If true, flips the image, same as transposing the
 *    matrix after loading.
 * @return Boolean value indicating success or failure of load.
 */
template<typename eT>
bool Save(const std::string& filename,
          arma::Mat<eT>& matrix,
          ImageInfo& info,
          const bool fatal,
          const bool transpose);

data::ImageInfo info;
info.width = info.height = 25;
info.channels = 3;
info.quality = 90;
data::Save("test_image.bmp", matrix, info, false, true);

If the matrix contains more than one image, only the first one is saved.

Saving multiple images can be done with the function below.

/**
 * Save the image file from the given matrix.
 *
 * @param files A vector consisting of filenames.
 * @param matrix Matrix to save the image from.
 * @param info An object of ImageInfo class.
 * @param fatal If an error should be reported as fatal (default false).
 * @param transpose If true, Flips the image, same as transposing the
 *    matrix after loading.
 * @return Boolean value indicating success or failure of load.
 */
template<typename eT>
bool Save(const std::vector<std::string>& files,
          arma::Mat<eT>& matrix,
          ImageInfo& info,
          const bool fatal,
          const bool transpose);

data::ImageInfo info;
info.width = info.height = 25;
info.channels = 3;
info.quality = 90;
std::vector<std::string>> files{"test_image1.bmp", "test_image2.bmp"};
data::Save(files, matrix, info, false, true);

Multiple images are saved according to the vector of filenames specified.