SoVolumeRenderingQuality Class Reference
[Nodes]

Volume rendering quality property mode More...

#include <VolumeViz/nodes/SoVolumeRenderingQuality.h>

Inheritance diagram for SoVolumeRenderingQuality:

Public Types
enum	GradientQuality { LOW = SoVolumeShader::GRADIENT_FORWARD_DIFF, MEDIUM = SoVolumeShader::GRADIENT_CENTRAL_DIFF, HIGH = SoVolumeShader::GRADIENT_SOBEL }
enum	LightingModel { OIV6 = SoVolumeShader::OIV6, OPENGL = SoVolumeShader::OPENGL }
enum	EdgeDetect2DMethod { LUMINANCE = 1, DEPTH = 1 << 1, GRADIENT = 1 << 2 }
Public Member Functions
virtual SoType	getTypeId () const
	SoVolumeRenderingQuality ()
Static Public Member Functions
static SoType	getClassTypeId ()
Public Attributes
SoSFBool	lighting
SoSFBool	preIntegrated
SoSFBool	jittering
SoSFFloat	gradientThreshold
SoSFBool	edgeColoring
SoSFColor	edgeColor
SoSFFloat	edgeThreshold
SoSFBool	boundaryOpacity
SoSFFloat	boundaryOpacityIntensity
SoSFFloat	boundaryOpacityThreshold
SoSFBool	edgeDetect2D
SoSFFloat	edgeDetect2DInnerThreshold
SoSFFloat	edgeDetect2DOuterThreshold
SoSFBitMask	edgeDetect2DMethod
SoSFEnum	gradientQuality
SoSFEnum	lightingModel
SoSFBool	colorInterpolation
SoSFFloat	surfaceScalarExponent
SoSFFloat	unnormalizedGradientExponent
SoSFBool	segmentedInterpolation
SoSFFloat	segmentedInterpolationThreshold
SoSFBool	voxelizedRendering
SoSFBool	voxelOutline
SoSFFloat	voxelOutlineThreshold
SoSFFloat	voxelOutlineWidth
SoSFColor	voxelOutlineColor
SoSFBool	ambientOcclusion
SoSFBool	deferredLighting
Deprecated

SoDEPRECATED SoSFBool	cubicInterpolation

Detailed Description

Volume rendering quality property mode

This property node causes subsequent SoVolumeRender nodes to be drawn with different rendering effects and/or levels of quality.

NOTE:

Only one (or none) of the following nodes can be active at a time: SoVolumeShader or SoVolumeRenderingQuality. However since Open Inventor 7.1, both SoVolumeRenderingQuality and SoVolumeIsosurface may applied to the same SoVolumeRender node.

If the application simply needs to replace one or more of the VolumeViz shader rendering functions with a customized shader, you can use either SoVolumeShader or SoVolumeRenderingQuality. However if the application needs the advanced rendering features of SoVolumeRenderingQuality, e.g. lighting, in addition to the customized behavior, then you should use this node (which is derived from SoVolumeShader). Using the shader rendering framework is explained on the SoVolumeShader page.

The SoVolumeRenderingQuality node must be before the SoVolumeRender and after the SoTransferFunction.

This is a shader node!
The effect will usually be undesirable if it is applied to standard geometry (polygons, lines, etc). Therefore applications should generally keep the volume visualization nodes and standard geometry nodes separate in the scene graph (i.e. under different SoSeparator nodes).

Please read the comments for each field.
Some options have no effect in ray-casting mode (the default) and some options only apply in ray-casting mode. Similarly, some options only apply to gradient lighting and have no effect on deferred lighting.

Pre-integrated and lighted rendering

Pre-integrated volume rendering
Lighted volume rendering

The pre-integrated mode (preIntegrated field) generally provides higher image quality for the same number of slices. Most applications should enable this field. However, note that pre-integration integrates between color map values. For typical scalar data containing sampled values, this provides a beneficial smoothing. It is especially useful when the color changes sharply between adjacent color map entries. However, when the color changes sharply between adjacent voxels, it can can cause values that are not actually in the original data to be displayed. This is undesireable for some data, for example "label" volumes resulting from segmentation. Pre-integration is not recommended for such data. Also note:

The preIntegrated field has no effect on SoVolumeIsosurface rendering.

The preIntegrated field is only considered when SoVolumeRendering::renderMode is set to VOLUME_RENDERING (the default).

When lighting is enabled for volume rendering, VolumeViz applies the same lighting equation used for polygonal geometry, including (optionally) specular highlights. The base voxel color comes from the SoTransferFunction node or (optionally) from a custom shader function provided by the application. This color is modified using the current material (SoMaterial), a vector simulating a "normal vector" and one or more directional light nodes (SoDirectionalLight) found in the scene graph. Voxels can also cast and receive shadows (see SoShadowGroup).

Limitation: Pre-integrated and lighted rendering are not supported if using custom fragment shaders and redefining FRAGMENT_COMPUTE_COLOR slot.

VolumeViz supports two lighting modes using either the lighting field or the deferredLighting field. In both cases lighting is computed on the GPU as part of the rendering process. (Do NOT use the lighting field in the SoVolumeRendering node. This field enables a CPU lighting computation that is slow.)

Gradient lighting
The lighting field enables gradient based lighting, where the "normal vector" at each sample along the ray is a gradient vector computed from the data values in the volume. Gradient based lighting only supports a single light source and only the first light in the scene graph (typically the viewer's "headlight") is used. No other lights affect the volume in this case. The lightingModel field controls the lighting equation to be used. Using the OPENGL model (not the default) is recommended. The gradientQuality field controls the algorithm used to compute gradient vectors. Several other fields affect the gradient computation including gradientThreshold, surfaceScalarExponent and unnormalizedGradientExponent. Setting surfaceScalarExponent to a small value, for example 2.0, is recommended.

Deferred lighting
The deferredLighting field enables screen space lighting, where the "normal vector" is computed from the final image depth buffer. Deferred lighting is faster and supports multiple light sources, but works best when the transfer function makes each data value either opaque or transparent. Deferred lighting is not affected by the lightingModel field or any of the gradient related fields.

Unlike other primitives (including other VolumeViz primitives), volume lighting is not affected by an SoLightModel node. Also unlike other primitives, if lighting is enabled and there are no light nodes in the scene graph, the voxel color is taken from the transfer function or custom shader function "as is" (other primitives would be black in this case).

Each light node's direction and intensity fields are used, but the color field is not currently used. The current SoMaterial specifies the ambient, specular, diffuse, and emissive color values for the lighting equation. Note that the default diffuse color is "light gray" (0.8), not full white. This allows specular lighting to push the color toward full white (as expected). If specular lighting is not desired, then changing this to full white (1.0) is recommended in order to see the true colors specified in the transfer function.

The following figures show the same volume data:

Default volume rendering	Pre-integrated volume rendering	Lighted pre-integrated volume rendering

Quality enhancement parameters

Jittering

jittering

SoVolumeRender::samplingAlignment

No Jittering	With Jittering

Gradient quality: When gradient lighting (lighting field) is enabled, the gradientQuality field allows you to choose between various gradient computation techniques. The computational cost increases with the quality. Has no effect on deferred lighting.

Low quality	Medium quality	High quality

Surface scalar: When gradient lighting (lighting field) is enabled or edgeDetect2DMethod is GRADIENT, the surfaceScalarExponent field disables lighting (or edge detection) on uniform surfaces in order to avoid noise in these area. This field should not be mixed with unnormalizedGradientExponent. The default value is zero, but a small value, for example 2.0, is recommended. Has no effect on deferred lighting.

Surface Scalar disabled	Surface Scalar enabled

Unnormalized gradient : When gradient lighting (lighting field) is enabled, if unnormalizedGradientExponent is not 0, voxels with small gradients will get more contribution from the ambient light than voxels with high gradients. It is similar to surfaceScalarExponent but uses the ambient light instead of the transfer function color for uniform surfaces. Has no effect on deferred lighting.

Lighting model: When gradient lighting (lighting field) is enabled, the lightingModel field allows backward compatibility with the lighting model used in Open Inventor 6 or using the standard OpenGL lighting model. Setting this field to OPENGL is recommended. The following picture shows that in the case of OIV6, the volume is much brighter than the sphere. By setting the lighting model to OPENGL, the brightness of the volume matches the brightness of the sphere. Has no effect on deferred lighting.

OIV6	OPENGL

Gradient threshold: When gradient lighting is enabled, gradients with a length less than gradientThreshold are ignored during the lighting computation. This avoids doing lighting on noise while still lighting important data. In the following screenshots, setting a threshold of 0.1 removed lighting on the noise surrounding the spheres. Has no effect on deferred lighting.

With gradientThreshold set to 0	With gradientThreshold set to 0.1

Image enhancement parameters

SoTransferFunction

Ambient occlusion
The ambientOcclusion field enables a rendering technique that simulates self-shadowing of the volume. In other words, the amount of ambient (global) light in the scene reaching each sample is reduced by neighboring voxels. This effect makes it much easier to see the relative depth of features in the volume. Generally we recommend using this effect rather than the following effects.
Both lighting and/or shadow casting may also be enabled, but neither is required to use ambient occlusion. This effect works best when the volume data contains surfaces (region boundaries with relatively sharp gradients) or in voxelized rendering mode. In both cases it works best when voxels are either transparent or nearly opaque. If you use ambientOcclusion, you should set the SoVolumeRender::samplingAlignment field to BOUNDARY_ALIGNED to reduce "slicing" artifacts.

Boundary opacity
boundaryOpacity increases opacity depending on the length of the gradient vector. Areas with large gradient changes will have their opacity increased according to the boundaryOpacityIntensity. Note that this option has a significant performance penalty.

Edge coloring
When edgeColoring is on, the color of each voxel may be mixed with the edgeColor. Areas where the normal (computed from the gradient) is facing the camera will have an unmodified color, whereas areas where the normal is more perpendicular to the view direction will tend towards edgeColor.

Edge detection
When edgeDetect2D is enabled, an image space filter is applied on the volume rendering image in order to detect edges, which will be highlighted. The results are affected by the edgeDetect2DInnerThreshold and edgeDetect2DOuterThreshold fields. The edgeDetect2DMethod bitmask allows to apply the edge detection on the image's luminance, depth and/or gradient. The gradient method may give noisy result, the surfaceScalarExponent may help to improve the result in this case. The gradient method has a significant performance penalty.

The following table shows the available edge detection techniques (explanation of faux shading is in SoTransferFunction):

No Edges	Boundary Opacity	Edge 2D

Edge Coloring	Faux Shading (see SoTransferFunction)

Because this node is derived from SoVolumeShader, IVVR_FIRST_RESERVED_TEXTURE_UNIT applies to it. See SoVolumeShader for more information.

Volume projection

SoProjection

FILE FORMAT/DEFAULT

lighting	FALSE
preIntegrated	FALSE
jittering	FALSE
gradientThreshold	0.0001
edgeColoring	FALSE
edgeColor	(0, 0, 0)
edgeThreshold	0.2
boundaryOpacity	FALSE
boundaryOpacityIntensity	1.5
boundaryOpacityThreshold	1.5
edgeDetect2D	FALSE
edgeDetect2DInnerThreshold	0.1
edgeDetect2DOuterThreshold	0.1
edgeDetect2DMethod	LUMINANCE
gradientQuality	MEDIUM
lightingModel	OIV6
colorInterpolation	TRUE
unnormalizedGradientExponent	0
surfaceScalarExponent	0
segmentedInterpolation	FALSE
segmentedInterpolationThreshold	0.5
voxelizedRendering	FALSE
voxelOutline	FALSE
voxelOutlineThreshold	2.
voxelOutlineWidth	2.
voxelOutlineColor	(0, 0, 0)
ambientOcclusion	FALSE
deferredLighting	FALSE
interpolateOnMove	FALSE

ACTION BEHAVIOR

SoGLRenderAction

Member Enumeration Documentation

enum SoVolumeRenderingQuality::EdgeDetect2DMethod

The edge detection algorithm used when edgeDetect2D is TRUE, can work on the image luminance, depth buffer and/or gradient.

These enums can be combined in the field edgeDetect2DMethod.

Enumerator:

LUMINANCE

Edge detection will use the image's luminance.

This method has a very little impact on speed.

DEPTH

Edge detection will use the image's depth buffer.

GRADIENT

Edge detection will use the volume's gradient.

These gradients are affected by the surfaceScalarExponent fied. This method is the most computationally expensive.

enum SoVolumeRenderingQuality::GradientQuality

Gradient Quality mode.

Enumerator:

LOW

Use the forward difference technique to compute the gradient.

This is a fast technique but it gives a lower quality gradient.

MEDIUM

Use the central difference technique to compute the gradient.

This is a compromise between quality and speed.

HIGH

Use a Sobel filter to compute the gradient.

This gives hiqh quality gradients at the expense of speed.

enum SoVolumeRenderingQuality::LightingModel

Available Lighting Model mode.

Enumerator:

OIV6

Use Open Inventor 6 lighting model for volume and isosurface lighting.

OPENGL

Use the equation specified by OpenGL to do volume and isosurface lighting.

This model should be preferred because it responds to SoMaterial in the usual Open Inventor way.

Constructor & Destructor Documentation

SoVolumeRenderingQuality::SoVolumeRenderingQuality ( )

Constructor.

Member Function Documentation

static SoType SoVolumeRenderingQuality::getClassTypeId ( ) [static]

Returns the type identifier for this class.

Reimplemented from SoVolumeShader.

virtual SoType SoVolumeRenderingQuality::getTypeId ( ) const [virtual]

Returns the type identifier for this specific instance.

Reimplemented from SoVolumeShader.

Member Data Documentation

SoSFBool SoVolumeRenderingQuality::ambientOcclusion

If true, apply an ambient occlusion effect (default is FALSE).

Ambient occlusion is an shading effect that approximates attenuation of light due to neighboring voxels. It works best when the volume data contains surfaces (region boundaries with relatively sharp gradients), or in voxelized rendering mode, and those voxels are opaque.

If you use ambientOcclusion, you should set the SoVolumeRender::samplingAlignment field to BOUNDARY_ALIGNED to reduce "slicing" artifacts.

NOTE: Only available in raycasting mode. NOTE: field available since Open Inventor 9.1

SoSFBool SoVolumeRenderingQuality::boundaryOpacity

Enable boundary opacity.

If TRUE, increases boundary opacity based on the gradient magnitude. Default is FALSE.

This effect has a significant performance penalty.

SoVolumeRender::subdivideTile will be disabled if set to TRUE.
NOTE: field available since Open Inventor 7.0

SoSFFloat SoVolumeRenderingQuality::boundaryOpacityIntensity

Applies when boundaryOpacity is TRUE.

If > 1, this increases the boundary opacity globally. If < 1, decreases it. Default is 1.5.
NOTE: field available since Open Inventor 7.0

SoSFFloat SoVolumeRenderingQuality::boundaryOpacityThreshold

Applies when boundaryOpacity is TRUE.

If this value is low (near 0), only regions with a high gradient will be enhanced. Default is 1.5.
NOTE: field available since Open Inventor 7.0

SoSFBool SoVolumeRenderingQuality::colorInterpolation

Controls interpolation of data values used for color lookup.

If FALSE, interpolation is not done on data values used to access the colormap but is still done when computing gradient for lighting. This may be used to render a segmented dataset where interpolation is not appropriate. Default is TRUE. Only affects gradient based lighting (lighting field).

Generally it's better to set the SoVolumeRender::interpolation field to NEAREST, unless you really want to use gradient based lighting.

NOTE: field available since Open Inventor 7.2

SoDEPRECATED SoSFBool SoVolumeRenderingQuality::cubicInterpolation

Enable cubic interpolation of data values.

Warning:: Heavy GPU usage. Primarily useful for still image rendering. Default is FALSE.
NOTE: field available since Open Inventor 7.0

Deprecated:: Deprecated since Open Inventor 9300
Use SoVolumeShape::interpolation field instead.

SoSFBool SoVolumeRenderingQuality::deferredLighting

Enable screen space lighting (computed on the GPU).

Default is FALSE. Deferred lighting is computed based on the final image depth buffer instead using the data gradients. It is much faster than gradient based lighting (see the lighting field) and supports multiple light sources (up to 8). Because it is not based on data gradients, it does not have problems with small/random gradients and it gives a much better result when using clipping nodes like SoVolumeClippingGroup or SoUniformGridClipping. Light sources are defined by Open Inventor SoDirectionalLight nodes.

Notes:

Since Open Inventor 9.3, deferred lighting supports up to 8 light sources.

Both deferred lighting and gradient lighting may be enabled at the same time, but the intent is that the application should use one or the other.

Gradient related fields, e.g. surfaceScalarExponent, have no effect on deferred lighting.

In any case, do not use the SoVolumeRender::lighting field.

LIMITATIONS

Only directional lights (SoDirectionalLight) are supported.

If you use deferredLighting, you should set the SoVolumeRender::samplingAlignment field to BOUNDARY_ALIGNED or SMOOTH_BOUNDARY_ALIGNED to reduce "slicing" artifacts.

The light color is taken into account, but not the light intensity field. You can get the same effect as reduced intensity by reducing the light color values.

Deferred lighting works best when
- The volume data can be considered to contain "surfaces", for example bones (etc) in medical data or metal parts in NDT data, and
- The transfer function makes the surface material relatively opaque and the surrounding material relatively transparent.

Only available in raycasting mode (the default).

NOTE: field available since Open Inventor 9.2

SoSFColor SoVolumeRenderingQuality::edgeColor

Color used to draw edges when edgeColoring or edgeDetect2D is TRUE.

Default is black (0,0,0).
NOTE: field available since Open Inventor 7.0

SoSFBool SoVolumeRenderingQuality::edgeColoring

Enable edge coloring.

If TRUE, changes the color based on the gradient direction (normal). Edges will be highlighted with the color specified in edgeColor. Areas where the normal (computed from the gradient) is facing the camera will have an unmodified color, whereas areas where the normal is more perpendicular to the view direction will tend towards edgeColor. Default is FALSE.
NOTE: field available since Open Inventor 7.0

SoSFBool SoVolumeRenderingQuality::edgeDetect2D

Enable 2D edge detection.

If this field is TRUE, a 2D edge detection algorithm is used to highlight edges in the rendered image of the volume. Default is FALSE.
NOTE: field available since Open Inventor 7.0

SoSFFloat SoVolumeRenderingQuality::edgeDetect2DInnerThreshold

Increase this value to decrease noise on silhouette edges.

1 = remove all edges, 0 = remove nothing. Default is 0.1
NOTE: field available since Open Inventor 7.0

SoSFBitMask SoVolumeRenderingQuality::edgeDetect2DMethod

Method used for 2D edge detection.

Specifies the edge detection algorithm used when edgeDetect2D is TRUE. Edge detection can work on the image luminance, depth buffer and/or gradient. For example, gradients may be too noisy to give interesting edges and may be ignored with this field. The gradient method is also the slowest (luminance being the fastest) because it needs more texture fetchs. See EdgeDetect2DMethod. Default is LUMINANCE and GRADIENT.
NOTE: field available since Open Inventor 8.1

SoSFFloat SoVolumeRenderingQuality::edgeDetect2DOuterThreshold

Increase this value to decrease noise on edges in the volume.

1 = remove all edges, 0 = remove nothing. Default is 0.1
NOTE: field available since Open Inventor 7.0

SoSFFloat SoVolumeRenderingQuality::edgeThreshold

If this value is low, more edges will be detected.

Default is 0.2. Min value is 0. There is no max, but most of the time a value between 0 and 1 is good.
NOTE: field available since Open Inventor 7.0

SoSFEnum SoVolumeRenderingQuality::gradientQuality

Specifies the algorithm used to compute gradients, for example, normals for lighting.

Only affects gradient based lighting (lighting field).

Use enum GradientQuality. Default is MEDIUM.
NOTE: field available since Open Inventor 7.0

SoSFFloat SoVolumeRenderingQuality::gradientThreshold

Ignore all gradients with a magnitude less than the specified threshold.

Default is 0.0001, meaning that all gradients are used in the lighting computation. Maximum useful value is 1.0, because gradient vectors are normalized. Only affects gradient based lighting (lighting field).
NOTE: field available since Open Inventor 7.0

SoSFBool SoVolumeRenderingQuality::jittering

Enable jittering.

If set to TRUE, a random offset is added to texture coordinates in order to decrease ringing artifacts (boundary edge artifacts) without the cost of drawing a higher number of slices. Note that this creates a "noisy" image. Instead we recommend setting the SoVolumeRender::samplingAlignment field to BOUNDARY_ALIGNED. Note: Jittering is only available when pre-integrated rendering is enabled. Default is FALSE.
NOTE: field available since Open Inventor 7.0

SoSFBool SoVolumeRenderingQuality::lighting

Enable gradient based lighting (computed on the GPU).

Default is FALSE. Gradient based lighting is computed using the direction and magnitude of gradient vectors computed from the data values in place of "normal vectors" in the lighting equation. A gradient vector is computed for each sample point along the rays cast through the volume.

VolumeViz also supports screen space lighting (see the deferredLighting field).

Notes:

The result of the gradient lighting computation is affected by how the gradient vectors are computed. See the gradientQuality field.

Gradient lighting may produce "noisy" results when the gradient magnitudes are small and/or the gradient directions are not consistent. This often happens, for example, in relatively homogeneous regions of a volume. Several fields are provided to compensate for this effect. See gradientThreshold and surfaceScalarExponent.

Both gradient lighting and deferred lighting may be enabled at the same time, but the intent is that the application should use one or the other.

In any case, do not use the SoVolumeRender::lighting field.

LIMITATIONS

Only directional lights (SoDirectionalLight) are supported.

Only one light source, the first one traversed in the scene graph, is supported.

SoSFEnum SoVolumeRenderingQuality::lightingModel

Sets the lighting model.

For backward compatibility with Open Inventor v6. Applications should set this field to OPENGL. Only affects gradient based lighting (lighting field).

Use enum LightingModel. Default is OIV6.
NOTE: field available since Open Inventor 7.0

SoSFBool SoVolumeRenderingQuality::preIntegrated

Enable pre-integrated volume rendering.

Pre-integrated rendering can significantly increase image quality (at the cost of slightly lower performance). Default is FALSE.

Limitations:

Pre-integration integrates between color map values. For typical scalar data containing sampled values, this provides a beneficial smoothing. When the color changes sharply between adjacent voxels, it can can cause values that are not actually in the original data to be displayed. This is undesireable for some data, for example "label" volumes resulting from segmentation. Pre-integration is not recommended for such data.

The preIntegrated field has no effect on SoVolumeIsosurface rendering.

The preIntegrated field is only considered when SoVolumeRendering::renderMode is set to VOLUME_RENDERING (the default).

SoSFBool SoVolumeRenderingQuality::segmentedInterpolation

Allow correct interpolation when rendering segmented data.

NOTE: Only available on SoVolumeIsosurface.
NOTE: field available since Open Inventor 8.5

SoSFFloat SoVolumeRenderingQuality::segmentedInterpolationThreshold

Defines the iso distance used when segmentedInterpolation is true.

Default is 0.5. value must be in the range [0 .. 1] NOTE: Only available on SoVolumeIsosurface.
NOTE: field available since Open Inventor 8.6

SoSFFloat SoVolumeRenderingQuality::surfaceScalarExponent

If this field is greater than or equal to 1, a surface with a small gradient will be less lighted than a surface with a high gradient.

The resulting color is a mix between the lighted voxel color and the non-lighted voxel color (ie: the corresponding entry in the colormap). If the value is less than 1, the field is ignored. Values higher than 256 will apply lighting on almost all surfaces. For most datasets, values between 2 and 16 should be enough. Default is 0.
Only affects gradient based lighting (lighting field). NOTE: field available since Open Inventor 8.1

SoSFFloat SoVolumeRenderingQuality::unnormalizedGradientExponent

When unnormalizedGradientExponent is not 0, surfaces with high gradients will be more lighted than surfaces with small gradients.

Voxels with small gradients will have less diffuse and specular than other with high gradients (ie: ambient color will be the biggest contributor). This lighting contribution can be controlled with this field. If this value is high, smaller gradients will contribute to lighting. Values higher than 256 will apply lighting on almost all surfaces. For most dataset, values between 2 and 16 should be enough. Default is 0.
Only affects gradient based lighting (lighting field). NOTE: field available since Open Inventor 8.1

SoSFBool SoVolumeRenderingQuality::voxelizedRendering

If true, SoVolumeRender displays voxels as individual cubes.

Default is false.

Setting true will force using the raycasting render mode (see SoVolumeShader::raycasting) for volume rendering (but that is the default mode anyway).

NOTE: field available since Open Inventor 9.0

SoSFBool SoVolumeRenderingQuality::voxelOutline

If true, draw the outline of voxels (default is false).

Notes:

Since Open Inventor 9.6, this mode applies to volume slice primitives (SoOrthoSlice etc) in addition to volume rendering.

In the case of SoObliqueSlice, you may see triangles, quadrilaterals and even irregular pentagons in some cases. These lines are correct and correspond to the intersections between the voxels and the oblique slice. For example: