Orthophotos (orthographic photographs) are images that have been remapped to remove the effect of surface variations and camera position from a normal perspective photograph.
An orthomosaic is an orthophoto that was created from multiple source photographs. PhotoModeler can create both, and when this document refers to an ‘orthophoto’ it, in most cases, means an orthomosaic as well.
Orthophotos can be used for two purposes: 1) to extract a remapped photo for use in mapping or presentation, 2) to extract a planar texture for use with a rendering program.
Remapped orthophotos are useful aids for mapping and presentation because all the perspective distortion has been removed. This makes the photograph have the 2D geometry of a drawing. It can be useful also to change the view point (e.g. from a photo taken on the ground to one taken from a high-flying aircraft).
The strength of an orthographic drawing or photograph is that it is ‘to scale’ and can be used for measurement. That is if two objects are the same size in the orthophoto that means they are the same size on the real object (for objects parallel to the projection plane - see below). Orthophotos can be created with a specific scale so you can measure an object in the orthophoto and know how big it is in the real world. You cannot do that with perspective images (i.e. those taken by a real camera at an angle).
An orthophoto is a projection of the 3D model onto a plane using parallel projection. That is, all the rays of projection are parallel to each other and also perpendicular to the plane. The 3D model is assigned photographic data for every part of its surface and it is this photographic data that is projected. The result of this process is an image file that can be printed or saved on disk.
Many advanced 3D computer graphic rendering and animation programs accept texture images for mapping onto surfaces. The goal in using textures is to improve the realism of the rendering or animation. For example, a rendered building looks much better if the red brick pattern is a realistic one derived from photographs of real brick, or a rendered store front has a store sign from a photograph mapped onto its walls.
As well, GIS (Geographic Information Systems), often use orthophotos as backgrounds or sources for measurement and further mapping. The extraction of an image or a texture from a photograph used in PhotoModeler involves determining the relationship between the surface of the object and the photograph in 3D space. Once this 3D relationship is known, an inverse mapping can be done to extract an image that is orthographic (i.e. flat to the surface with no perspective distortions).
Note: one of the most important aspects to consider when generating accurate orthophotos is that the modeling surface should match the true surface.
If the object does not follow the surfaces or plane then the orthophoto will be distorted in those areas of mismatch. For example, if the modeling surface is the front of a building and the inset windows are not modeled, then the windows will not be true orthographic representations in the orthophoto (since they lie behind the modeling surface).
See the 3D Model Export section for a method for extracting textures tied directly to the 3D model.