• Load Pic3D, Create segmentation, Activate 2D Fast Marching
• Place a seed point somewhere
• Adjust settings in such way, that only the pixel of the seed point is highlighted
• Place more Seed points:

You will see a 0.5 offsett in the conversion of the Seed point world coordinates into the image pixel coordinates. Only, when you click in a pixel's top-right corner, you will select that pixel. Otherwise, a neighboring pixel is selected.