fix(VtkTwoView): layer opacity has uniform 1 opacity transfer function

[PaulHax]

Removes voxel intensity based opacity transfer for layer slices. All layer voxels have opacity transfer function Y value of 1. Layer opacity controled by slice mesh opacity.

Good CT-PT dataset for testing https://demo.orthanc-server.com/app/explorer.html#study?uuid=6b9e19d9-62094390-5f9ddb01-4a191ae7-9766b715

Closes #355

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[floryst]

LGTM

[salimkanoun]

Hi there, the fusion is better but i think there still a problem, from 0.8 to 1 I don't see real change in opacity.

I see the opacity seems to be defined using gaussians, points or node while I think for PET/CT is should be a simple constant for all pixel in the image which balance with the CT opacity ( changing 0 to 1 with always Opacity CT + Opacity PT = 1)

For a layer are all voxel opacity a constant in the current code ?

[PaulHax]

@salimkanoun Your right, .8 to 1 does nothing. This PR was trying to preserve CT context with some intensity to opacity mapping. Uniform opacity across image would be too easy =)

But you've finaly gotten through to me. I'll make the PET/layer's opacity uniform across the image, and the slider will change the opacity of the layer uniformly.

[salimkanoun]

Sounds good, will try the new implementation, thanks !

[PaulHax]

I pushed up the simpler layer blending approach.

[salimkanoun]

Seems to me much better ! Thanks !

[floryst]

LGTM!

[floryst]

@PaulHax we can merge this once it's rebased.

[PaulHax]

Rebased!