vdiffr v0.0.0.9000


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by Lionel Henry

Visual regression testing and graphical diffing

vdiffr is an extension to testthat that makes it easy to add tests for graphics. It provides a Shiny application to manage the test cases.



vdiffr is an extension to the package testthat that makes it easy to test for visual regressions. It provides a Shiny app to manage failed tests and visually compare a graphic to its expected output.


Get the development version from github with:

# install.packages("devtools")

How to use vdiffr

Adding expectations

vdiffr integrates with testthat through the expect_doppelganger() expectation. It takes as argument:

  • A figure. This can be a ggplot object, a recordedplot, a function to be called, or more generally any object with a print method.
  • A name identifying the test case.
  • Optionally, a path where to store the figures. By default, they are stored in tests/figs/.
disp_hist_base <- function() hist(mtcars$disp)
disp_hist_ggplot <- ggplot(mtcars, aes(disp)) + geom_histogram()

vdiffr::expect_doppelganger(disp_hist_base, "disp-histogram-base")
vdiffr::expect_doppelganger(disp_hist_ggplot, "disp-histogram-ggplot")

Running tests

You can run the tests the usual way, for example with devtools::test(). New cases for which you just wrote an expectation will be skipped. Failed tests will show as an error.

Managing the tests

When you have added new test cases or detected regressions, you can manage those from the R command line with the functions collect_cases() and validate_cases(). However it's often more comfortable to run the shiny application manage_cases(). With this app you can:

  • Check how a failed case differs from its expected output using three widgets: Toggle (click to swap the images), Slide and Diff. If you use Github, you may be familiar with the last two.

  • Validate cases. You can do so groupwise (all new cases or all failed cases) or on a case by case basis. When you validate a failed case, the old expected output is replaced by the new one.

Both manage_cases() and collect_cases() take package as first argument, the path to your package sources. This argument has exactly the same semantics as in devtools. You can use vdiffr tools the same way as you would use devtools::check(), for example. The default is ".", meaning that the package is expected to be found in the current folder.

All validated cases are stored in tests/figs/ or in the path specified as an option to expect_doppelganger(). This folder may be handy to showcase the different graphs offered by your package. You can also keep track of how your plots change as you tweak their layout by checking the history on Github.

RStudio integration

An addin to launch manage_cases() is provided with vdiffr. Use the addin menu to launch the Shiny app in an RStudio dialog.

RStudio addin

ESS integration

The next version of ESS will feature devtools integration. Include this in your config file to add vdiffr's Shiny app to the package development keymap. Call it with C-c C-w C-v.

(defun ess-r-vdiffr-manage-cases ()
  (ess-r-package-send-process "vdiffr::manage_cases(%s)\n"
                              "Manage vdiffr cases for %s"))

(define-key ess-r-package-dev-map "\C-v" 'ess-r-vdiffr-manage-cases)

Dependency notes

vdiffr currently uses svglite to save the plots in a text format that makes it easy to perform comparisons. This makes the test cases dependent on that package as the SVG translation of the plot may change across different versions of svglite (though that should not happen often). For this reason, whenever you validate a graphical test case, your DESCRIPTION file is updated with a note containing the svglite version. This works the same way as the roxygen version note.

Your graphics might be dependent on other packages besides svglite. If your package is an extension to ggplot2 for instance, the appearance of your plot may change as ggplot2 evolves (as with the 2.0 version which tweaked the grayness of the background color among other changes). For this reason, expect_doppelganger() adds a dependence on ggplot2 when you supply a ggplot2 object. Next time you validate a case, the DESCRIPTION file will be updated with a note describing the ggplot2 version with which the tested plots were rendered. You can also manually add a dependency on any other package by calling vdiffr::add_dependency() anywhere in a test file.


Changing default figures folder

By default, figures will be stored in tests/figs/. You can change this path by providing the path argument to expect_doppelganger(). To set it globally, just write a small wrapper in a testthat helper file. Helper files have names starting with helper- and are executed before the unit tests.

default_path <- "path/default/"
this_path <- "path/this/"
that_path <- "path/that/"

expect_doppelganger <- function(fig, fig_name) {
  vdiffr::expect_doppelganger(fig, fig_name, default_path)

expect_doppelganger_this <- function(fig, fig_name) {
  vdiffr::expect_doppelganger(fig, fig_name, this_path)

expect_doppelganger_that <- function(fig, fig_name) {
  vdiffr::expect_doppelganger(fig, fig_name, that_path)


testthat Reporter

vdiffr extends testthat through a custom Reporter. Reporters are classes (R6 classes in recent versions of testthat) whose instances collect cases and output a summary of the tests. While reporters are usually meant to provide output for the end user, you can also use them in functions to interact with testthat.

vdiffr has a special reporter that does nothing but activate a collecter for the visual test cases. collect_cases() calls devtools::test() with this reporter. When expect_doppelganger() is called, it first checks whether the case is new or failed. If that's the case, and if it finds that vdiffr's collecter is active, it calls the collecter, which in turns records the current test case.

This enables the user to run the tests with the usual development tools and get feedback in the form of skipped or failed cases. On the other hand, when vdiffr's tools are called, we collect information about the tests of interest and wrap them in a data structure.

SVG comparison

Comparing SVG files is convenient and should work correctly in most situations. However, SVG is not suitable for tracking really subtle changes and regressions. See vdiffr's issue #1 for a discussion on this. vdiffr may gain additional comparison backends in the future to make the tests more stringent.

Functions in vdiffr

Name Description
manage_cases Manage visual test cases with a Shiny app
add_dependency Add a vdiffr dependency
expect_doppelganger Does a figure look like its expected output?
htmlwidgets HTML Widgets for graphical comparison
collect_cases Collect and validate cases
shinybindings Shiny bindings for graphical comparison widgets
vdiffrAddin RStudio Addin for managing visual cases
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