# Maximum Intensity Projection
# ============================

# Maximum Intensity Projection (MIP) is a rendering technique for point
# clouds that reorders vertex depth so points with higher scalar values
# are always rendered in front, regardless of their actual distance from
# the camera.
# This is useful for dense point cloud visualization where high-value
# data points would otherwise be hidden behind lower-value points that
# happen to be closer to the viewer. The technique was proposed by
# Cowan (2014) for visualizing grade data in mining applications, where
# it is referred to as "X-ray plunge projection."
# MIP works by replacing the z-coordinate in OpenGL clip space with the
# negated, normalized scalar value via a custom vertex shader. This
# means that depth ordering is driven entirely by scalar magnitude
# rather than spatial position.
import numpy as np
import pyvista as pv
from pyvista import examples

# Normal vs. MIP Rendering
# ========================

# Using a sample of the knee dataset, we compare normal rendering
# (left) where closer points occlude farther ones, with MIP rendering
# (right) where the highest scalar values punch through to the front.
vol = examples.download_knee_full()

sample_n = 50000
rng = np.random.default_rng(0)
indices = rng.integers(0, vol.n_points, sample_n)
pts = vol.extract_points(indices, adjacent_cells=False, include_cells=False)

display = dict(
    cmap='jet', clim=[0, 100], style='points', point_size=5, show_scalar_bar=False
)

pl = pv.Plotter(shape=(1, 2))
pl.enable_parallel_projection()

pl.subplot(0, 0)
pl.add_mesh(pts, **display)
pl.add_text('Normal Projection', font_size=12)

pl.subplot(0, 1)
mip_actor = pl.add_mesh(pts, **display)
mip_actor.enable_maximum_intensity_projection()
pl.add_text('Maximum Intensity Projection', font_size=12)

pl.link_views()
pl.show()

# MIP with Circle Point Sprites
# =============================

# MIP modifies the vertex shader while point sprites modify the
# fragment shader, so both features compose cleanly on the same actor.
# Using circle sprites with MIP produces a cleaner visualization than
# the default square points.
combined_display = dict(
    cmap='jet', clim=[0, 100], style='points', point_size=15, show_scalar_bar=False
)

pl = pv.Plotter(shape=(1, 2))
pl.enable_parallel_projection()

pl.subplot(0, 0)
actor_squares = pl.add_mesh(
    pts,
    render_points_as_spheres=False,
    **combined_display,
)
actor_squares.enable_maximum_intensity_projection()
pl.add_text('MIP (squares)', font_size=12)

pl.subplot(0, 1)
actor_circles = pl.add_mesh(
    pts,
    point_shape='circle',
    **combined_display,
)
actor_circles.enable_maximum_intensity_projection()
pl.add_text('MIP + circle sprites', font_size=12)

pl.link_views()
pl.show()

# NOTE:
#     MIP requires VTK >= 9.3.

# NOTE:
#     MIP does not work correctly with `opacity < 1` unless depth
#     peeling is enabled. See `pyvista.Plotter.enable_depth_peeling()`.

# References
# ==========

# Cowan, E.J., 2014. 'X-ray Plunge Projection', Understanding
# Structural Geology from Grade Data. AusIMM Monograph 30, 207-220.

# ----------------------------------------------------------------------
# Generated by sphinx-examples-as-code https://github.com/pyvista/sphinx-examples-as-code