raycasting initially renders at low resolution

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Hi.
I have a very similar problem.
I am using the latest VTK v8.2.0 from a Java application.
I am rendering a volume through vtkFixedPointVolumeRayCastMapper.

Even if I set:

======
// Initialize the mapper
volumeMapper = new vtkFixedPointVolumeRayCastMapper();
volumeMapper.AutoAdjustSampleDistancesOff();
volumeMapper.LockSampleDistanceToInputSpacingOn();
volumeMapper.SetBlendModeToComposite();
volumeMapper.SetImageSampleDistance(1);
volumeMapper.SetInteractiveSampleDistance(1);

  GetRenderWindow().SetDesiredUpdateRate(0.001);

======

the very first rendering of my volume appears very low resolution:

CPUVolRend_LowResFirstRendering
CPUVolRend_LowResFirstRendering.PNG1072×816 238 KB

Then, the first time I interact with the view (mouse drag to rotate the camera), then my rendering restores to high resolution, and remains hi-res from now on:

CPUVolRend_HiResAfterInteraction
CPUVolRend_HiResAfterInteraction.PNG1081×804 823 KB

Can you guess why the very first rendering of my CPU volumer mapper is so low res? How to fix this?

Thanks in advance.
Best regards,

Marco Sambin