# VTK Examples - Testing for the C++ examples now uses SSIM
**URL:** https://discourse.vtk.org/t/vtk-examples-testing-for-the-c-examples-now-uses-ssim/16405
**Category:** Announcements
**Created:** [May 4, 2026, 6:50am UTC](https://discourse.vtk.org/t/vtk-examples-testing-for-the-c-examples-now-uses-ssim/16405 "2026-05-04T06:50:07Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@amaclean](https://discourse.vtk.org/u/amaclean)
#### Post date: [May 4, 2026, 6:50am UTC](https://discourse.vtk.org/t/vtk-examples-testing-for-the-c-examples-now-uses-ssim/16405/1 "2026-05-04T06:50:07Z")
## Introduction
The VTK C++ Examples are now using the Structural Similarity Index (SSIM) which is a metric for measuring the perceived quality and similarity of two images.
Some images needed to be replaced due to the more stringent SSIM testing, mostly due to changes in color.
In summary, 98% or more tests pass in both Linux and Windows. The tests were done on the same machine using Debug builds for VTK and the VTK Examples. For Linux, two tests failed out of 1023 and for Windows using Visual Studio 2026, sixteen tests failed out of 1021. For windows ten failures generated this message on exit:
> Expression: cannot compare incompatible string\_view iterators for equality.
An issue for this has been raised, please see: [Bug in vtkBYUReader and vtkSLCReader](https://gitlab.kitware.com/vtk/vtk/-/work_items/20012).
If the image comparison fails, three files are generated, two PNG files consisting of the current image and a difference image, along with a VTI file. The VTI file has has three slices in the XY plane that can be viewed using ParaView called:
- Baseline, colored by Baseline Magnitude.
- Current, colored by Current Magnitude.
- ImageScalars, colored by Image Scalars Magnitude.
**Can anyone provide more information about this file?**
Also if you are interested, please see [Do we need a testing framework example?](https://discourse.vtk.org/t/do-we-need-a-testing-framework-example/16399).
## Linux
99% tests passed, 2 tests failed out of 1023.
- [IndividualVRML](https://examples.vtk.org/site/Cxx/IO/IndividualVRML/) now cannot run `teapot.wrl` as a test, it crashes for some reason, so I created `utah_teapot.wrl` from a recent version of `utah_teapot.obj` for testing purposes.
All tests pass except:
- Images-ImageContinuousDilate3D (Subprocess aborted)
- Images-ImageContinuousDilate3D (Subprocess aborted)
The crash happens at line 79 in vtkDataArrayValueRange\_AOS.h when the PNG file is used.
## Windows
98% tests passed, 16 tests failed out of 1021.
Nine failures are due to the usage of the `vtkBYUReader` and one due to the use of the `vtkSLCReader` please see: [Bug in vtkBYUReader and vtkSLCReader](https://gitlab.kitware.com/vtk/vtk/-/work_items/20012)
As in Linux, these two tests also fail:
- Images-ImageContinuousDilate3D (Subprocess aborted)
- Images-ImageContinuousDilate3D (Subprocess aborted)
The crash happens at line 79 in vtkDataArrayValueRange\_AOS.h when the PNG file is used.
Points-SignedDistance needs `VTK_TESTING_IMAGE_COMPARE_METHOD=LOOSE_VALID` in order to pass.
InfoVis-XGMLReader fails in windows even when `VTK_TESTING_IMAGE_COMPARE_METHOD=LEGACY_VALID` is set. `TIGHT_VALID` is Ok in Linux.
---
### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [May 4, 2026, 1:13pm UTC](https://discourse.vtk.org/t/vtk-examples-testing-for-the-c-examples-now-uses-ssim/16405/2 "2026-05-04T13:13:10Z")
> [@amaclean](#):
>
> [IndividualVRML](https://examples.vtk.org/site/Cxx/IO/IndividualVRML/) now cannot run `teapot.wrl` as a test, it crashes for some reason, so
Sounds like a bug to fix.
---
### Author:  [@amaclean](https://discourse.vtk.org/u/amaclean)
#### Post date: [May 4, 2026, 11:01pm UTC](https://discourse.vtk.org/t/vtk-examples-testing-for-the-c-examples-now-uses-ssim/16405/3 "2026-05-04T23:01:53Z")
Please see [IndividualVRML failure](https://gitlab.kitware.com/vtk/vtk/-/work_items/20039)