Image-based flow visualization

Using the Probe tab, if the Probe Type in the Basic Settings is set to "Flow Image", then one can visualize flow in a planar slice as an animated texture.  When "Flow Image" is selected, all of the basic, layout, image, and appearance sections of the Probe tab are still available as discussed in the previous page; however the image in the probe is a texture based on a flow field.  The following animated sequence illustrates a cross section of vortices in hydrodynamic flow.  The color indicates the magnitude of the component of the flow in the direction orthogonal to the probe.

How it works:  The display of the flow uses a technique called “Image Based Flow Visualization” or IBFV. This is a technique introduced by Jarke Van Wijk, published in Proceedings, ACM Siggraph 2002. A random spot noise pattern is repeatedly advected by the vector field. The vector field is projected into the plane of the probe so as to always display the motion in the plane of the probe. Multiple such images are accumulated, resulting in the effect of smearing the pattern in the direction of the flow. This IBFV display is based on the flow field at the current time step; it does not show unsteady flow or how the flow moves in time.

IBFV Setup:  When the flow type is set to "Flow Image, there is a the section "Flow Image Control" at the top of the Probe Image settings. In this section:

• Steady field selectors are used to choose the variables that specify the X-, Y-, and Z-coordinates of the vector field that will be displayed. 
• The Alpha value (slider and text box) is a factor that specifies the fraction of each image that is added into the accumulated image. The default (0.12) works well to show motion that is fairly uniform.
• The Scale value is used to scale the values of the vector field. The field is initially scaled so that its average magnitude over the probe is 4.0 pixels per timestep when the Scale value is set to 1.0 (the default).

IBFV animation: 

• The pause ( ) and play(►) buttons are used to control the advance of the IBFV images.
• The button labeled "Start Capture Sequence" can be used to save the IBFV images to a sequence of jpeg files. When you click this button you are asked to specify the name of the first file of the sequence. Every subsequent image will be saved to file. When this button is clicked again, it terminates the saving of jpeg files.

IBFV limitations: 

• Note that the IBFV image in the 3D visualizer does not animate.  The animation only occurs in the Probe IBFV tab.
• When the 3D scene is animated, the IBFV cannot be animated.  The Probe image in the 3D scene will be a single frame from the IBFV in the tab at the appropriate time step.
• The IBFV in the Probe can only animate steady flow. 

Merging color/opacity of another variable into IBFV.  When using flow images in the probe, there is a checkbox in the Appearance parameters "Merge color and opacity of mapped variable into flow texture".  When checked, the probe hue and saturation as well as its opacity (in the 3D view) are determined by mapping the 3d variable of the probe using the Probe transfer function (in the Appearance). The value (intensity) in the probe texture is obtained from the IBFV texture.