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Block triangulation data

The following sections describe data within an *.ibt block triangulation file.

Camera parameters
Image points
Object points
Point cloud colors
Observations
Meta data

Camera parameters

An *.ica file is a binary file with 12 64-bit double values per image/frame. These values represent a 3x4 projection matrix, P, such that x = P * X, where x is a 3x1 vector with homogeneous image coordinates and X is a 4x1 vector with homogeneous object coordinates.

There are only 11 independent parameters in the P matrix. These 11 camera parameters can be divided into five interior parameters and six exeterior parameters.

Interior parameters:

Assuming square pixels and perpendicular camera axises f=fx=fy and s=0, reducing the number of interior parameters to three; f, x0, and y0.

Exterior parameters:

Image points

An *.ipt file contains 2D image coordinates. The file is binary with four 32-bit floats per image point. The four values define (column,row,x,y), where (x,y) are normalized image coordinates in the range [-0.5..+0.5]. In IBT files with only two images, the image point lists in the two images and also the object coordinate list will match. The image point file name is determined from the video file name with a frame number at the end for video projects and from the image file name for image projects.

Object points

An *.iob file contains 3D object coordinates. The file is binary with four 32-bit float values per object point. The four values define (X,Y,Z,W), where W typically is 1 (one). The object coordinates are typically provided in an arbitrary free 3D coordinate system without georeferencing.

Point cloud colors

An *.irgb file is a binary file with colors red, green, blue, alpha saved as four 8-bit unsigned chars per color entry. There are as many entries in an *.irgb file as there are object points in an *.iob file (and colors and object points are in the same order). The *.irgb file typically contains colors from an image, but may contain colors from a height coded palette. The colors are used to display point clouds in the block triangulation view.

Observations

An *.iix file is a binary file with observations. This file is optional if there are only two images in the *.ibt block file. If there is no *.iix observation file then it is assumed that there are the same number of points in the two image point files and in the object point file and that the points are ordered so that e.g. the first point in each of the image point files refer to the first point in the object point file. If there is an *.iix file, the it is binary and contains as many observations as there are object points. Each observation starts with an unsigned int defining, n, the number of images the object point was observed in followed by n triplets of (int a, int b, bool c) defining the image/camera number, a, the image point index, b, and a boolean variable which is true if the points is "active"/in use or false if the point should be ignored.

Meta data

An *.imx file is an XML text file with meta data, which describe the block triangulation data in more detail.

The file must start with an iconic-block root node, which contains a version attribute with version number.

A type child node contains a text, which is either "float" or "double" and determines the format of all image and object files.

An images child tag must contain at least one image child node, which in turn contains at least three attributes:

Currently, there is at the most one additional meta file per frame/image, but this may change in the future to e.g. enable meta files with different purposes.

Figure 4.3: Sample meta data XML (*.imx) file (included in the sample ibt file in the block triangulation section). This meta data file tells us that (i) the meta data file version is 1.0, (ii) image and object coordinates are saved in float format, (iii) for each of the two images, there is an *.rpc file attached in the archive for the purpose of georeferencing.