# Image¶

Theia provides a basic FloatImage class to use for SfM and SLAM applications. The class is fairly lightweight, and is largely a wrapper for the OpenImageIO but allows for more general use. It is the standard image class that used for feature detection within Theia.

class FloatImage

Images can be stored either as RGB or grayscale, and the number of channels used to describe an image can be determined with :func:Channels. Images can be easily read in from a file upon contruction or with the Read() command.

FloatImage my_rgb_image(img_filename);

// Convert to grayscale.
FloatImage my_grayscale_image = my_rgb_image.AsGrayscaleImage();

// ... do some stuff ... //

// Write the image back out.
my_rgb_image.Write(out_filename);
my_grayscale_image.Write(out_filename);


Once an image is loaded, pixel values can be accessed through several accessor functions. To avoid ambiguity between matrix and image notation, we provide explicit methods to get image pixel values based on either the xy coordinate or the row and column.

// Load RGB and scale images.
FloatImage my_img("test_img.jpg");

// Get the middle pixel location.
int middle_x = my_img.Width()/2;
int middle_y = my_img.Height()/2;

// Grab the middle pixel.
const Eigen::Vector3f middle_xy_pixel = my_img.GetXY(middle_x, middle_y);

// Get the middle pixel location.
int middle_row = my_img.Rows()/2;
int middle_col = my_img.Cols()/2;

// Grab the middle pixel.
const Eigen::Vector3f middle_row_pixel = my_img.GetRowCol(middle_row, middle_col);

// The rgb values should be the same for both pixels.
CHECK_EQ(middle_xy_pixel(0), middle_row_pixel(0));
CHECK_EQ(middle_xy_pixel(1), middle_row_pixel(1));
CHECK_EQ(middle_xy_pixel(2), middle_row_pixel(2));


Similar operations apply for grayscale images and for obtaining the value of specific color channels at a pixel. The BilinearInterpolate() function allows for interpolation at non-discrete pixel locations. See the image.h file for more details.

We have also implemented some useful member functions of the FloatImage class. For a full list of functions, theia/image/image.h

int FloatImage::Rows() const
int FloatImage::Cols() const
int FloatImage::Channels() const
float *FloatImage::Data()
const float *FloatImage::Data() const
void FloatImage::Read(const std::string &filename)
void FloatImage::Write(const std::string &filename)
void FloatImage::ConvertToGrayscaleImage()
void FloatImage::ConvertToRGBImage()
FloatImage FloatImage::AsGrayscaleImage() const
FloatImage FloatImage::AsRGBImage() const
FloatImage FloatImage::Integrate() const
FloatImage FloatImage::ComputeGradient() const
void FloatImage::Resize(int new_width, int new_height)
void FloatImage::ResizeRowsCols(int new_rows, int new_cols)
void FloatImage::Resize(double scale)