I'm wondering if it is possible to do pattern recognition with QGIS. I have 5 NDVI Landsat images and I try to find areas automatically (constant pattern). Is there a tool in QGIS?
The pic below shows three different raster layers from different years. You can recognize typical areas (e.g. bright or dark blue pattern) which I want to identify automatically. 
Yes, you can do object-based pattern recognition with the Orfeo Toolbox (OTB) plugin in QGIS. It's not a native plugin to QGIS, so you will have to download it (here) and then follow the installation instructions. Besides segmentation, there are a lot of other powerful tools within OTB that you can use, such as their machine learning algorithms for image classification.
