TrInk Quick Start Guide by Aline Normoyle & Nancy Tsang

• Approximate Image-Based Tree-Modeling using Particle Flows by Neubert,  Franken, Deussen
• MoXi: Real-Time Ink Dispersion in Absorbent Paper by Chu and Tai

• Plug-in library, trink.mll, 
• MEL-based GUI, TrInkDialog.mel, 
• and related data files and paper textures

To create a tree with simple geometry, select "Create tree..." from the TrInk menu.  We can use the default settings to start, but because TrInk generates trees from source images, it's necessary to give TrInk a source image to guide the tree generation.  We'll talk more about how to create these images below, but let's start by browsing for one of the default images, such as "tree1.png", from the TrInk plug-in directory.  Now click the "Create" button.  After about 30 seconds, the created tree will appear in the scene.

Creating source images

Neubert et al.'s paper guided users in the preprocessing of photographs into source images for the tree generation algorithm.  This plugin assumes that images have already been preprocessed and will autoarrange the images inside a 3D bounding box before running the tree generation algorithm.  Source images should be non-transparent in areas containing tree branches.  The density of branches is  proportional to the opacities in the original images.  Optionally, main branches should be drawn over the density estimates with red lines. The red lines will guide the creation of the main branches and should be specified for best results.  These red lines must connect to be read in properly.  Some example source images appear below:

If the user specifies a single source file (or 3 source files), it will be arranged 3-dimensionally at 60 degree angles, producing a very symmetrical, circular tree.  If two images are specified, the images will be placed at a 90 degree angle.  In all cases, the images are aligned at their centers, for example,

Tree creation parameters

In this section, we discuss the tree creation parameters in the tree panel:

• Source Images (Required): The image set TrInk should use to guide the tree creation
• Build Parameters (Optional):
• Scale: Controls the size of the resulting tree.  The generated tree is constructed inside the unit cube and scaled to the given dimensions
• Number of particles: Controls the number of branches in the tree.  Specify more particles to create a denser tree with more branches and a thicker trunk.  Specify fewer to create a sparse tree.
• Density Grid Size: Controls the amount of detail retreived from the source images at the expense of speed.  Increase this number to better capture silhouettes.  Decrease this number to create a tree more quickly.
• Direction field Size: Not needed.  Do not change this value.

Ink Stroke Creation: Getting Started

The ink rendering feature allows the user to render NURBS curves as ink strokes. The user has the option of creating curves manually and assigning ink properties to each curve, or automatically generating curves shaped like branching plants and assigning ink values at creation time. In either case, ink properties can be changed or removed at any time.

Assigning Ink to NURBS curves

Ink Stroke parameters

In this section, we discuss the ink stroke creation parameters in the ink stroke panel:

• Auto Resize Brush: Automatically changes the brush size at render time according to the percieved curve length. Shorter curves result in smaller initial brush sizes, whereas longer curves result in a user specified initial brush size. Checking this feature will not change the user specified brush size.
• Gradient Color: Automatically render a gradation of the specified color. The specified color will be the darkest color used. Color is assigned to each curve based on their depth, where the furthest curve away from the camera eye recieves the darkest color.
• Size: Specify the brush size.
• Amt Water: Specify how much water the brush should contain. For best results, Amt Water should be greater than Amt Pigment.
• Pressure: Available pressure schemes for a curve. Brush size is automatically adjusted according the the scheme.
• Flow Spread: Specify the number of iterations the ink simulation should run. This affects the overall maximum radius of ink spread. A Flow Spread set to 1 will result in no fluid flow.
• Color: Specify color of the ink.
• Viscosity: Specify the viscosity of the ink (Doesn't really do anything).
• Amt Pigment: Specify the amout of pigment that will be applied to the stroke.
  

Rendering generated trees with ink strokes

TrInk allows the tree generation and ink features to be used together (with identical parameters):

1. Go to Menu->TrInk->Create ink tree...
2. Adjust to desired values
3. Hit "Create"

Known problems and limitations

TrInk has a few known problems and limitations, listed below:

• Tree generation parameters cannot be tweaked after creation.  Users must change parameters and recreate the tree.
• The user cannot manually specify the alignment of the source images.  The source images must be setup outside of Maya.
• The density estimate used for particle initialization is less accurate than it could be.
• Geometry creation for the tree simply extrudes circles along branch curves.  The resulting tree mesh is not continuous and only looks good from far away.
• The ink rendering is only supported for Maya Software (e.g. mental ray and renderman not supported)
• Ink strokes do not work with batch render.  However, a MEL script can be written to work around this problem.
• Ink strokes are not blended with non-ink objects.  The ink paper will cover other objects in the scene in the render view window, however, if the image is saved in a format that stores alpha values, it can be blended outside of Maya. Currently, the way that colors are written creates a paper color fringe around the brush strokes which may or may not be desireable.