How to create a Giotto Object

In this tutorial, the methodology and syntax to create a giottoObject is shown and osmFISH data is used throughout the tutorial.

Import Giotto and Download the Data

To download this data, please ensure that wget is installed locally.


genv_exists = checkGiottoEnvironment()
  # The following command need only be run once to install the Giotto environment.

# Specify path from which data may be retrieved/stored
data_directory = paste0(getwd(),'/gobject_data/')
# alternatively, "/path/to/where/the/data/lives/"

# Specify path to which results may be saved
results_directory = paste0(getwd(),'/gobject_results/')
# alternatively, "/path/to/store/the/results/"

# Download osmFISH dataset to data_directory
getSpatialDataset(dataset = 'osmfish_SS_cortex', directory = data_directory, method = 'wget')

Minimum requirements for a Giotto Object

  • Expression matrix

  • Spatial locations (unnecessary for scRNAseq analysis)

Here, creating a giottoObject with the minimum requirements is shown in two examples. Data formatting guidelines are shown below this code block.

# Example 1.
# Create a Giotto object using data directly from file paths
osm_exprs = paste0(data_directory, "osmFISH_prep_expression.txt")
osm_locs = paste0(data_directory, "osmFISH_prep_cell_coordinates.txt")

minimum_gobject1 = createGiottoObject(expression = osm_exprs,
                                      spatial_locs = osm_locs)

# Example 2.
# Create a Giotto object using objects already loaded into workspace
expression_matrix = readExprMatrix(path = osm_exprs) # fast method to read expression matrix
cell_locations = data.table::fread(file = osm_locs)

minimum_gobject2 = createGiottoObject(expression = expression_matrix,
                                      spatial_locs = cell_locations)

Expression file formatting

Expression count matrices accepted by Giotto should have columns correlating to individual cells (cell IDs) and rows to individual features (feature IDs).
data.table objects and text-based files ie: (.csv, .tsv, .txt) to be read in should have column 1 be feature IDs.











matrix and DelayedMatrix objects should have feature IDs already incorporated as the rownames.







Locations file formatting

Numerical columns will be interpreted in order of x, y, and (optionally) z coordinate. The first non-numerical column will be taken as cell IDs








*Note: multiple expression files during can be given at once during Giotto object creation by using a named list.

# Arbitrary modifications
scaled_matrix = expression_matrix * 1.2
custom_matrix = expression_matrix * 0.5

# Provide multiple expression matrices at once to the Giotto Object
# If these matrices are stored in files rather than in the workspace,
# file paths may be provided instead of variables
multi_expr_gobject = createGiottoObject(expression = list(raw = expression_matrix,
                                                          scaled = scaled_matrix,
                                                          custom = custom_matrix),
                                        spatial_locs = cell_locations)

Customizing the Giotto Object

By providing values to other createGiottoObject parameters, it is possible to add:

Providing giottoInstructions allows the specification of:

  • An alternative python path if using the Giotto Environment (default) is not desired

  • A directory to which resulting plots will save

  • Plot formatting

There are four functions to utilize when setting instructions:

  • createGiottoInstructions: creates instructions that are provided to createGiottoObject

  • showGiottoInstructions: displays the instructions of a Giotto object

  • changeGiottoInstructions: replaces 1 or more of the instruction parameters (e.g. plotting format)

  • replaceGiottoInstructions: replaces all instructions with new instructions (e.g after subsetting)

Here is an example of a more customized Giotto object.

# Specify data with file paths
osm_exprs = paste0(data_directory, "osmFISH_prep_expression.txt")
osm_locs = paste0(data_directory, "osmFISH_prep_cell_coordinates.txt")
meta_path = paste0(data_directory, "osmFISH_prep_cell_metadata.txt")

# Create instructions

# Optional: Specify a path to a Python executable within a conda or miniconda
# environment. If set to NULL (default), the Python executable within the previously
# installed Giotto environment will be used.
my_python_path = NULL # alternatively, "/local/python/path/python" if desired.

instrs = createGiottoInstructions(python_path = my_python_path,
                                  save_dir = results_directory,
                                  plot_format = 'png',
                                  dpi = 200,
                                  height = 9,
                                  width = 9)

# Create Giotto object
custom_gobject = createGiottoObject(expression = osm_exprs,
                                    spatial_locs = osm_locs,
                                    instructions = instrs)

# Add field annotations as cell metadata
metadata = data.table::fread(file = meta_path)
custom_gobject = addCellMetadata(custom_gobject, new_metadata = metadata,
                                 by_column = T, column_cell_ID = 'CellID')

# Check which Giotto instructions are associated with the Giotto object

Note that although parameters show_plot, return_plot, and save_plot were not specified within the call to createGiottoInstructions, default values were provided to these instruction parameters. All instruction parameters have default values, such that createGiottoInstructions may be called with some or no arguments yet all instruction parameters will have a value after its execution.

Alternatively, a named list may also be provided to the instructions argument of createGiottoObject. However, ensure that all arguments to createGiottoInstructions are defined when providing instructions as a named list, since default values are only applied to instructions when made with createGiottoInstructions. Note that python_path must be specified when providing instructions as a named list, and may not be provided as NULL.

The giottoInstructions may be changed, or completely replaced:

# Change a previously set parameter, e.g. change dpi = 200 to dpi = 300
custom_gobject = changeGiottoInstructions(custom_gobject,
                                          param = 'dpi',
                                          new_value = 300)

# Observe that the instructions have changed

# Create new instructions using a named list
sub_results_directory = paste0(results_directory, 'specific_results/')
my_python_path = custom_gobject@instructions$python_path
new_instrs = list(python_path = my_python_path,
                  show_plot = TRUE,
                  return_plot = FALSE,
                  save_plot = TRUE,
                  save_dir = sub_results_directory,
                  plot_format = 'jpg',
                  dpi = 250,
                  units = 'in',
                  height = 12,
                  width = 12,
                  is_docker = FALSE)

# Change all instructions
custom_gobject = replaceGiottoInstructions(custom_gobject,
                                           instructions = new_instrs)

# Observe that the instructions have changed

Plotting Data from a Giotto Object

Each plotting function in Giotto has three important binary parameters:

  • show_plot: print the plot to the console, default is TRUE

  • return_plot: return the plot as an object, default is TRUE

  • save_plot: automatically save the plot, default is FALSE

These parameters are stored within a giottoObject that was provided instructions from createGiottoInstructions and are provided to plotting functions accordingly. To change these parameters from the default values, the instructions may be changed or replaced, or these parameters may be manually overwritten within plotting functions.

See showSaveParameters and the Saving Options tutorial for alternative methods to save plots.

# Plot according to Giotto Instructions (default)

# Plot clusters, create, and save to a new subdirectory, all while overwriting formatting
         cell_color = 'ClusterName',
         save_plot = TRUE,
         return_plot = TRUE,
         show_plot = TRUE,
         save_param = list(save_folder = 'plots/', # Create subdirectory
                           save_name = 'cell_clusters',
                           save_format = 'png',
                           units = 'in',
                           base_height = 9,
                           base_width = 9))

Giotto Object Structure

Usage of the Giotto package revolves around the giottoObject. This is an S4 object class that holds spatial expression data and facilitates its manipulation and visualization with the Giotto package’s functions. Additional metadata and other outputs generated from certain Giotto functions, which may used in downstream analyses, are also be stored within the giottoObject. Its self-contained nature renders a convenient representation of the entire spatial experiment and is why most Giotto functions take a given giottoObject as input and return a giottoObject as output.

Data is organized within the giottoObject in defined slots as described in the diagram below.

Nested Organization of the Giotto Object

Spatial data has tiered organization and thus so too does Giotto. Tissue regions can be divided down into individual cells and then further into organelles. The data structure of Giotto mirrors biological structure by nesting data within slots related to spatial data by spat_unit (spatial unit). Additonally, to facilitate the integration of multiple -omic data, slots related to feature data will be nested with feat_type (feature type).
Included below is a description of this nesting and also, for advanced users and contributors/developers, accessor functions for the slots are also provided.

Slots and Subnesting

Slot Nested Example Accessors ———————— ————————————————————————————————————————————————————————————————————————————————————————- ————————————————————————————————————————————————————————————————————————————————————————————- ———————————————————————————– ———————— ————————————————————————————————————————————————————————————————————————————————————————- ————————————————————————————————————————————————————————————————————————————————————————————– ———————————————————————————–