VTK/ProgrammableMultiVolumeRendering

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The goal of this page is to document the shader replacement functionality available in vtkGPUVolumeRaycastMapper starting with version (9?) of VTK.

Fragment shader variables

Global variables

Variables prefixed with "g_"

Name	Type	Space	Description
g_dataPos	vec3	Normalized volume	Current ray integration position in texture space defined by the first volume, recomputed at every iteration along the ray
g_terminatePos	vec3	Normalized volume	Position where we terminate ray integration in texture coordinates of the first volume. This position is computed from the input depth buffer.
g_eyePosObj	vec4	data	position of the camera in the data space (space of first volume)
g_fragColor	vec4	None	Variable where color is accumulated along the ray
g_srcColor	vec4	None	Result of the color computation at g_dataPos (current position).
g_exit	bool	None	Terminate ray integration.
g_skip	bool	None	Skip computation of current iteration step if true

Interpolated variables

Variables prefixed with "ip_". These variables contains values computed in the vertex shader stage and interpolated for each fragment.

Name	Type	Space	Description
ip_textureCoords	vec3	Normalized volume	Interpolated texture coord
ip_vertexPos	vec3	?	Interpolated vertex position at current fragment
ip_inverseTextureDataAdjusted	mat4	Volume 0 to Texture 0	Transforms from data space (volume 0) to texture space (volume 0). The matrix is adjusted to take into account the offset between geometry and texture space.

Uniform variables

Variables prefixed with "in_". The value of these variables is set from the c++ code by the mapper. It is now possible for users to define custom uniform variables by using the class vtkOpenGLUniforms. The vtkOpenGLGPURaycastMapper class contains one instance of this class for each type of shader. The objects can be accessed with vtkOpenGLGPURaycastMapper::GetFragmentCustomUniforms and vtkOpenGLGPURaycastMapper::GetVertexCustomUniforms functions. They allow to define uniform variable of any GLSL type and set their values at any moment. These variables can then be used in custom shader code.

Input volumes

Name	Type	Space	Description
in_volume	sampler3D[n]	None	One sampler for each of the input volumes (n). Allows to sample the 3D texture that contains the volume
in_volume_scale	vec4[n]	None	Scale to apply to the sampled scalar value of each input volume
in_volume_bias	vec4[n]	None	Offset to apply to the sampled scalar value of each input volume
in_noOfComponents	int	None	In the single input volume case, it is possible to feed the mapper with a volume that has multiple components per voxel. This variable contains the number of components.
in_independentComponents	int	None	When a single volume contains multiple components: whether we should treat components as independent. This affects, amongst other things, if a single gradient is computed for all components (dependent) or if one gradient is computed for each component (independent).

Other textures

Name	Type	Space	Description
in_noiseSampler	sampler2D	None	Texture that contains noise and that is used to slightly modify the path of each ray an avoid sampling artefacts.
in_depthSampler	sampler2D	None	Texture that contains the depth buffer generated by the rendering of polygonal geometry.

Camera

Name	Type	Space	Description
in_cameraPos	vec3	World	Same as vtkCamera::GetPosition().
in_projectionMatrix	mat4	World to Clip	Projection matrix including model view. Transform vertices from world space to clip space.
in_inverseProjectionMatrix	mat4	Clip to World	Inverse of projection matrix including model view. Transform vertices from world space to clip space.
in_modelViewMatrix	mat4	World to Camera	Transforms vertices from world space to camera space
in_inverseModelViewMatrix	mat4	Camera to World	Transforms vertices from camera space to World space
in_windowLowerLeftCorner	vec2	screen pixels	Lower left corner of the viewport
in_inverseOriginalWindowSize	vec2	screen pixels	1/(viewport size).
in_inverseWindowSize	vec2	screen pixels	1/(viewport size).

Vertex shader variables

Uniform variables

Name	Type	Space	Description
in_cellSpacing	vec3[n]	data?	Distance between voxels for each volume in volume space
in_modelViewMatrix	mat4	camera to world	Transforms between camera space and world space
in_projectionMatrix	mat4	camera to clip	Projects data from camera space to clip space
in_volumeMatrix	mat4[n]	data to world	Transforms bounding box of each of the volumes to world coordinates
in_inverseTextureDatasetMatrix	mat4[n]	data to texture	Transforms from data space to texture space for each volume
in_cellToPoint	mat4[n]	---	Corrects for offset between points and OpenGL texture coordinates for each volume