SSIM plugin

GstValidate plugin to run the ssim algorithm on the buffers flowing in the pipeline to find regressions and detect frame corruptions. It allows you to generate image files from the buffers flowing in the pipeline (either as raw in the many formats supported by GStreamer or as png) and then check them against pre generated, reference images.

The ssim algorithm will set a value of 1.0 when images are perfectly identical, and -1.0 if they have nothing in common. By default we consider images as similar if they have at least a ssim value of 0.95 but you can override it defining the value under which the test will be considered as failed.

Errors are reported on the GstValidate reporting system. You can also ask the plugin to generate grey scale output images. Those will be named in a way that should lets you precisely see where and how the test failed.


The configuration of the plugin is done through a validate configuration file, specified with the %GST_VALIDATE_CONFIG environment variable. Each line starting with 'ssim,' will configure the ssim plugin. In practice each configuration statement will lead to the creation of a #GstValidateOverride object which will then dump image files and if wanted compare those with a set of reference images.

The following parameters can be passed in the configuration file:

  • element-classification: The target element classification as define in gst_element_class_set_metadata
  • output-dir: The directory in which the image files will be saved
  • min-avg-priority: (default 0.95): The minimum average similarity under which we consider the test as failing
  • min-lowest-priority: (default 1): The minimum 'lowest' similarity under which we consider the test as failing
  • reference-images-dir: Define the directory in which the files to be compared can be found
  • result-output-dir: The folder in which to store resulting grey scale images when the test failed. In that folder you will find images with the structural difference between the expected result and the actual result.
  • output-video-format: The format in which you want the images to be saved
  • reference-video-format: The format in which the reference images are stored
  • check-recurrence: The recurrence in seconds (as float) the frames should be dumped and checked.By default it is GST_CLOCK_TIME_NONE, meaning each and every frame is checked. Not that in any case, after a discontinuity in the stream (after a seek or a change in the video format for example) a check is done. And if recurrence == 0, images will be checked only after such discontinuity
  • is-config: Property letting the plugin know that the config line is exclusively used to configure the following configuration expressions. In practice this means that it will change the default values for the other configuration expressions.
  • framerate: (GstFraction): The framerate to use to compute frame number from timestamp, allowing to compare frames by 'frame number' instead of trying to match timestamp between reference images and output images.


Let's take a special configuration where we want to compare frames that are outputted by a video decoder with the ones after a agingtv element we would call my_agingtv. We force to check one frame every 5.0 seconds only (with check-recurrence=5.0) so the test is fast.

The configuration file:

 core, action=set-property, target-element-klass=Sink, property-name=sync, property-value=false

 validatessim, is-config=true, output-video-format="I420", reference-video-format="I420"
 validatessim, element-classification="Video/Decoder", output-dir=/tmp/test/before-agingtv/
 validatessim, element-name=my_agingtv, output-dir=/tmp/test/after-agingtv/, \
       reference-images-dir=/tmp/test/before-agingtv/, \
       result-output-dir=/tmp/test/failures, check-recurrence=5.0

Save that content in a file called check_agingtv_ssim.config

Launch the pipeline

   GST_VALIDATE_CONFIG=check_agingtv_ssim.config gst-validate-1.0-debug uridecodebin uri=file://a/file ! videoconvert ! agingtv name=my_agingtv ! videoconvert ! autovideosink

The results of the search are