The aim of this subtask was to develop a visually appealing representation of bleeding for the hysteroscopic simulator. This plays a key role in surgical interventions, since bleeding primarily determines the visibility of the surgical scene. Also, critical cases that cause heavy bleeding have to be handled properly by the surgeon.
In order to meet the real-time requirements of the simulator, an efficient method has been implemented. The blood is simulated as a stream of particles. The movement of the particles is affected by the blood source and the flow field caused by the distension fluid circulation (see hydrometra simulation). The particles are rendered as alpha-blended billboards. The size of the billboards and the texture applied on them changes dynamically to achieve more realistic appearance. Due to numerous tuning parameters versatile bleeding from focal to diffuse can be generated.
The primary objective is to achieve a visually convincing appearance of the interaction of blood and distension fluid. Non-Newtonian properties of the blood are therefore neglected. The following key components can be identified:
The interaction equations have to be solved efficiently from both computational and rendering point of view. As a solution, we have chosen to use particles with dynamically textured billboards. The particles enable the realistic movement and the interaction with the environment, while the billboards with changing texture ensure an efficient yet configurable and versatile visual appearance.
A bleeding source is considered as an emitter, which emits particles during the period of a bleeding. In general, the direction of the initial velocity is identical to the surface normal at the emitter point. The rectangular polygon which represents the particle has an initial minimal width and height. In the scene, the polygons are oriented according to the so called billboarding technique, ie. they always face the camera.
The polygons are textured dynamically. A 3D texture block is stored in
the texture memory and represents an expanding Gaussian blob. Passing through this 3D texture along the time coordinate provides a changing texture sequence. As time elapses, the size of the polygon is gradually increased up to a maximal width and height. The
size and the texture alteration of the polygons together contribute to the
changing appearance. The textured billboards are then alpha-blended during the
The influence of the emitter with its initial velocity is decaying by the time, while the influence of the external field induced by the distension
fluid is increasing. After a particle exceeds its lifetime, the particle gets a velocity component with
arbitrary direction and magnitude. This represents the spatial dissolving of the blood
in the distension fluid. In each timestep the
particles with positions post are advected by a simple Euler-step:
The resulting vparticle is the sum of three components (emitter, dissolving, external field), with weights changing during the lifetime of a particle.
The way a bleeding stops can be different depending on the type of bleeding. In the most simple case it stops after a given time, however it can also be related to a coagulation procedure or pressure increase by a change of the hydrometra.
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