Automatic Rendering Tests

Available sections:

1. What is an automatic rendering test?
2. How to create a rendering test

What is an automatic rendering test?

An automatic rendering test is a powerful tool to test rendering results automatically via dashboard. Regarding rendering lots of different sources influence the output on the screen (e.g. different mappers, renderers, camera settings or the algorithm creating the data). Thus, during the rendering process of an image many different classes are involved and can have impact on the output. A minor change in an important class (e.g. mitkVtkPropRenderer) can have major impact on the actual rendering. An automatic rendering test takes an arbitrary object as input (e.g. image, surface, point set), renders this into an mitkRenderWindow, makes a screen shot of that renderwindow and finally compares that screen shot to a given reference. Of course, the reference has to be defined by the user. Internally, a VTK test method is used to compare both screen shots and measure differences. In case of failure, a difference can be generated to show exactly which pixels are rendered incorrectly. Implementing automatic rendering tests for algorithms ensures that algorithms deliver the same output as they used to do in previous version of MITK.

How to create your own automatic rendering test

To create an automatic rendering test you should use an existing test as example (e.g. mitkImageVtkMapper2DTest).

Adding the test to CMake

Like adding any test with parameters to CMake, you have to add a custom test to the files.cmake and the corresponding CMakeLists.txt:

For instance a test for the mitkImageVtkMapper2D has to be added like this:

files.cmake

set(MODULE_CUSTOM_TESTS
    ...
    mitkImageVtkMapper2D.cpp
)

CMakeLists.txt

mitkAddCustomModuleTest(mitkImageVtkMapper2D_rgbaImage640x480 mitkImageVtkMapper2D #custom name of the test and executable
                        ${MITK_DATA_DIR}/RenderingTestData/rgbaImage.png #input image to load in data storage
                        -V ${MITK_DATA_DIR}/RenderingTestData/ReferenceScreenshots/rgbaImage640x480REF.png #corresponding reference screenshot
)

The first parameter defines a custom name for the test. This is a feature to distinguish between tests with different inputs. In this example the test is named mitkImageVtkMapper2D_rgbaImage640x480 to show that this test is using the test image rbgaImage640x480 as input.

The next parameters sets test executable name (i.e. the name of the test class). Here: mitkImageVtkMapper2D.

The next parameter(s) are used to pass the input to the test. For instance, it is possible to set multiple objects as input for a test (e.g. /path/to/img1.jpg /path/to/img2.nrrd /path/to/pointset.mps). All test data for core tests should be placed into the MITK-DATA repository inside the folder: ${MITK_DATA_DIR}/RenderingTestData/. It is possible to create other folders for different modules or bundles.

The option -V defines the path to the reference screen shot and is internally used by VTK. The reference screen shot is highly important and has to be proven if is correct. The mitkRenderingTestHelper offers means to capture a screen shot of a renderwindow. Capturing a reference screen shot should happen just once and NOT be a permanent part of the test.

It is also possible to set the option -T /path/to/directory/. This option is internally used by VTK to save a difference image. This is meant for debugging and should not be used for the final implementation of a test.

Coding the test

Writing the test code is pretty straight forward. In the example of the mitkImageVtkMapper2DTest the input parameters are added to a datastorage and rendered into a render window via the mitkRenderingTestHelper. Last, the vtkTesting macro is called to compare the given reference to the data rendered in the renderwindow:

int retVal = vtkRegressionTestImage( renderingHelper.GetVtkRenderWindow() );
 
//retVal meanings: (see VTK/Rendering/vtkTesting.h)
//0 = test failed
//1 = test passed
//2 = test not run
//3 = something with vtkInteraction
MITK_TEST_CONDITION( retVal == 1, "VTK test result positive" );

If the content of the previously filled renderwindow does not equal the reference, the test will fail.

Feel free to modify the data before rendering. E.g. create a surface from the loaded image and render that surface afterwards or add compute QBalls for an image and render those. Happy testing!