It happens when you try to convert a normal map/displacement map to another format, like JPG. in this type of convertion, the image loses a lot o information, and can cause a lot of artifacts in the material.
Try to use the Critical Map Detector tool, in the addon tab before starting the conversion. in this panel, you will be able to ignore the displacement/normal map, keep the format or keep the size. and now these maps should works correctly in your scene.
You lose a lot of information when converting a 16 Bits image to 8 Bits, and sometimes this can cause artifacts in the converted image.
When you have to convert a 16 Bits to 8 Bits image, you can add the dithering effect in the post processing panel, this will avoid these artifacts in this type of conversion.
The addon uses the power of blender's natives render engines to generate new images with the user's selected settings (New Size, New Format)
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These are some features that aren't fully supported. it's works! but sometimes it will not work 100% as expected
Blender does not support advanced scaling filters like external image converters.
This means that when you downsize an image with FormatSwap, the image may get more pixelated than using an external image resizer.
If you need to use a resized texture (specially textures resized to 1K resolution or bellow) in a close-up render, it still recommended that you use an external image converter/resizer for best quality or keep the original texture size.
FormatSwap Reduces the Render loading time, the process that happens before the rendering process truly starts. If you have a scene with hundreds of 4K textures, all this textures will be loaded before the render starts.
By converting or reducing the texture size, this loading time will be significantly slower, resulting in a faster render time.
Blender UI Freezes when any operator takes more than 5 segunds to run. in the FormatSwap case, this process can take minutes or more (talking about a scene with more than 100 textures).
This time will depends on your hardware, but you can check all the progress in the blender console.
Nothing Happens! the conversion process will be simply stopped. the replacing texture process only starts when a textures were converted