Marker Placement
Correct marker placement is critical. Markers serve as geometric anchors that enable HemoVision’s reconstruction pipeline to work in a robust and accurate manner.
Poor placement may result in weak geometric coverage and unstable reconstructions.
Detail Marker Placement
Detail markers should be placed:
- On flat surfaces
- Near stains or patterns of interest
- Distributed across all relevant surfaces
Detail markers may be placed in any orientation. Their rotation does not affect reconstruction.
Each detail marker must have a unique ID within the entire crime scene. If multiple reconstructions are created, marker IDs must remain unique across them. An example of detail marker placement can be seen in the image below.
Tip
You cannot place too many detail markers, but you can place too few.
Info
You do not need to place a detail marker for each bloodstain. As a rule of thumb, a detail marker can be used to cover a planar area of roughly 50cm x 50cm (this area can be larger or smaller depending on camera quality and image resolution).
Overview Marker Placement
The overview marker is used to position a reconstruction in 3D space inside HemoVision. By default, the marker’s reference cross is placed at the origin of the world coordinate system, with the marker’s long and short sides aligned with the X- and Y-axes respectively, as illustrated in the screenshot below.
After the reconstruction has been created, the user must position it in 3D space so that it corresponds to the correct location on the crime scene. In HemoVision, the user has full control over this process. Reconstructions can be translated and rotated freely in all directions.
In principle, this means that the overview marker can be placed in any position and orientation on the crime scene, as this configuration can be recreated in the software. However, this approach is only practical when a reference is available. For example, if a 3D scan of the scene is available and imported into HemoVision, the reconstruction can simply be aligned with that scan.
When no such reference exists, it is helpful to pay more attention to how the overview marker is placed at the scene. By placing the marker in a predictable way and taking a few simple measurements, the reconstruction can later be positioned manually in 3D space. To simplify this process, we recommend three common marker placements. HemoVision provides presets for each of these setups.
On a Vertical Wall
Place the marker horizontally (level) on a vertical wall. By default, the marker will be placed upright at the origin of the world space, as illustrated in the left example below. Most of the marker (and everything located below it) will fall below the zero-plane (which is probably not what you want).
Measure the vertical distance from the floor to the reference cross to determine the height of the marker (the Y-offset in a Y-up coordinate system). If this is the only measurement taken, the world origin will lie directly below the marker’s reference cross, and the reconstruction will be positioned at the correct height. This is shown in the middle of the figure below.
Optionally, horizontal distances can be measured to define X and Z offsets. For example, you may measure distances to a corner of the room, as shown on the right in the image below.
The figure below shows a more concrete example of the overview marker being placed on a vertical wall:
- No Measurement: The marker will be positioned at the origin in its default position. So in this example, half of the scene will fall below the horizontal X-axis.
- Y-Offset: By providing a Y-offset (vertical), we move the marker upwards with respect to the world origin. Consequently, the world origin will lie directly below the reference cross.
- X- and Y-Offset: By providing two measurements, we move the marker upwards and sideways. In this case, the origin was chosen to be at the corner of the room.
On the Floor
Place the marker flat on the floor.
If no measurements are taken, the marker’s reference cross will lie at the world origin. The marker’s long and short sides will be aligned with the X- and Z-axes (assuming a Y-up coordinate system), and the marker’s height will be zero, corresponding to the floor level in the scene. This is shown on the left in the image below.
If desired, horizontal offsets can be measured to position the marker relative to a reference point in the room, such as a corner. This is shown in the center of the figure below.
You could even add a vertical offset, for example, if the overview marker is placed flat on top of a table. This is illustrated on the right in the figure below.
The figure below shows a more concrete example of the overview marker being placed on the floor:
- No Measurement: The marker will be positioned at the origin in its default position. The origin of the coordinate system will be in the middle of the room.
- X-Offset: By providing a X-offset, we move the marker sideways with respect to the world origin. Consequently, the world origin will be at the wall.
- X- and Z-Offset: By providing two measurements, we move the marker sideways in two directions. In this case, the origin was chosen to be at the corner of the room.
On the Ceiling
Place the marker flat on the ceiling, facing downward.
Measure the height of the ceiling to define the vertical position of the marker. The marker’s reference cross will then be placed above the world origin at the measured ceiling height.
As with the floor setup, optional horizontal offsets can be added to position the marker relative to a reference point in the room, such as a corner.
Mistakes to Avoid
Below are some commonly made mistakes you should avoid:
- Don't place markers on curved surfaces
- Don't tape over the marker (A, B)
- Don't use printed markers (B, C)
- Don't use the same ID twice within one scene
- Don't angle the marker against a surface (D)
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Forentrics - Bringing Evidence into Focus