Marker Placement (Basic)

This page explains how you should position HemoVision markers inside your scene to be able to analyze the images in HemoVision. This page describes the basics for documenting a single zone within your crime scene. When you want to combine multiple zones into a room, have a look at the advanced marker placement page.

Detail Marker Placement

When documenting part of a crime scene (referred to as a zone), HemoVision requires one overview marker and multiple detail markers to be placed. Detail markers indicate regions that you want to photograph from close by, with more detail. You can place as many detail markers as you like, making sure that each marker has a unique number. Detail markers can be placed on any flat surface, in any orientation. The image below shows two scenes with detail markers. In the left scene, all stains (and hence all markers) are placed on the same vertical wall. In the right scene, the bloodstains are spread across different surfaces, with different orientations. Hence, we placed markers on each surface containing interesting bloodstains.

Overview Marker Placement

Next, we need to position the overview marker. As explained before, the reference cross on the overview marker defines the world origin. This means that, by default, the height of the reference point with respect to the floor is zero. When loaded into HemoVision, a reconstruction will therefore look something like this:

Notice how most of the images and all stain trajectories fall below the floor. Obviously, we want to translate our whole scene so that the reference point is at the correct height. If you have a 3D scan of the scene, you can simply rotate and / or translate your scene so that it matches up with your 3D scan, as shown below:

If you do not have a 3D scanner, however, you still want to be able express coordinates with respect to some predefined origin (e.g, the corner of the room). To make this easy, HemoVision offers three options for placing the overview marker. Each of these is discussed next. Note that in the below examples, the position and orientation of the detail markers is irrelevant.

Overview Marker on a Vertical Wall

The overview marker is placed horizontally on a vertical wall. By placing the marker horizontally, we can easily measure the height of the overview marker, and use this height to offset the origin (y-offset). This is illustrated in figure (a) below. Note how our scene has been moved up with respect to the origin. The origin now lies straight below the reference point of the overview marker.

If you want to further offset the scene parallel to the wall (e.g, so that the origin falls in the corner of the room), you can measure the horizontal distance from the reference point to the wall, as shown by the red line in the figure below (b).

To Do:

1. Place marker horizontally on wall
2. Y-offset: Measure distance from floor to reference point (a)
3. X-offset (optional): Measure distance from reference point to corner (b)

Overview Marker on the Floor

The overview marker is placed flat on the floor. HemoVision implicitly rotates the marker from its standard upright position to a horizontal position, with its front facing up. The height of the marker (y-offset) is automatically set to zero. The origin falls in the overview marker reference point. No measurements are needed, as shown in figure (a) below.

If we want to express our results with respect to the wall (we want to have the origin in the wall), we measure the shortest distance from the reference point to the wall. In this case, make sure that the overview marker's long side is parallel to the wall. By specifying the z-offset, the origin now starts at the wall (figure (b)).

We can further move the origin towards the corner of the room, by measuring along the wall (x-offset). Again, make sure that the marker's long side is parallel to the wall. This is illustrated in figure (c) below.

To Do:

1. Place marker flat on floor (a)
2. Z-offset (optional): Place marker on floor parallel to wall, and measure shortest distance (b)
3. X-offset (optional): Place marker on floor parallel to wall, and measure to corner (c)

Overview Marker on the Ceiling

The overview marker is placed flat on the ceiling. HemoVision implicitly rotates the marker from its standard upright position to a horizontal position, with its front facing down. The height of the marker with respect to the floor need to be measured as y-offset (the height of the ceiling, in other words). X and z-offsets can optionally be measured, similar to when the overview marker is place on the floor.

Multiple Zones

The three methods above allow you to quickly place your markers, and easily define your own origin in world coordinate space. If you are want to learn how to combine multiple zones into a room, without the need for a 3D scanner, read the advanced marker placement page.