sjt SiroVision 3D image format

The SiroVision .sjt 3D image format is just a tiff file with CSIROs custom tags. Indeed in the early versions of SiroVision the file was just saved with the .tif extension, the image data could (and still can) be read with any tif reader which will skip over the unknown tags added by CSIRO. The custom tags contain the x,y,z vertex data of a regular mesh, the coordinates each in their own chunk. There can be multiple meshes in one file. The image data is stored as normal tiff layers, one per mesh contained in the file. Since the mesh is a regular gridded rectangle, the texture coordinates simply range from 0 to 1 across the width and height of the mesh.

Unfortunately CSIRO and Datamine chooses not to publish the details of the format, forcing developers to reverse engineer it. Fortunately it isn't difficult, but that makes it doubly shameful that CSIRO and Datamine don't facilitate with some documentation.

The author has developed a utility called siro2mesh that will extract the meshes and save as OBJ files with corresponding MTL files. One can extract the texture layers for each mesh with imagemagick as follows.

   cp somefilename.sjt somefilename.tif
   convert somefilename.tif somefilename_%d.jpg

There are of course other options of separating a multilayer tiff into separate images.

Each mesh section is stored in a separate IDF tag. The mesh and associated image both have their own dimensions

Processing TIFF File CompositeImage.sjt
	File size = 127751282
	IFDs: 2

Processing IFD 0 
TIFF data
	Bits Per Sample = 8 x 8 x 8
	Samples Per Pixel = 3 
	Width = 5320
	Length = 2071 
Siro 3D data
	Pixels Per Point = 3
	Data Rows = 691
	Data Cols = 1774
	Top Left Image X = 1
	Top Left Image Y = 1
	Siro Spatial Data Points: 1225834

Processing IFD 1 
TIFF data
	Bits Per Sample = 8 x 8 x 8
	Samples Per Pixel = 3 
	Width = 5203
	Length = 1945 
Siro 3D data
	Pixels Per Point = 3
	Data Rows = 649
	Data Cols = 1735
	Top Left Image X = 1
	Top Left Image Y = 1
	Siro Spatial Data Points: 1126015

An example of a .sjt 3D model is shown below, untextured mesh on the left and textured version on the right.

The spatial data is arranged simply in planar format, the (u,v) being uniform across the mesh.

typedef struct {
   uint32 nx;   // Will generally be nrows * ncolumns
   uint32 ny;   // These will usually all be the same
   uint32 nz;
   double *xvalues;     
   double *yvalues;    
   double *zvalues;   
} SIRO_Spatial_Struct;

The vertices of the mesh are arranged in columns and then by rows.

   for (j=0;j<nrows;j++) {
      for (i=0;i<ncolumns;i++) {
         index = i * nrows + j;
         p.x = xvalues[index];
         p.y = yvalues[index];
         p.z = zvalues[index];
         u = i/(double)ncolumns;
         v = 1 - j/(double)nrows;
         // do something with vertex p with texture coordinates (u,v)
      }
   }

A more recent addition to the SiroVision software uses the extension .sjtx, to date the details of that format is unknown. The file is no longer a valid tif file. Please contact the author if you have information on this (possibly eXtended) variation.