wdata-format-concept.md

@@ -138,13 +138,6 @@ dataV[ixyz + 1 * blocklength] = v_y;
 dataV[ixyz + 2 * blocklength] = v_z;
 ```
 
-## Time
-To compute the time associated with a given `icycle`, we use the formula
-```
-time = t0 + dt * icycle;
-``` 
-If `dt` is negative then `time` parameter has to be extracted from the additional binary file of the name `prefix__t.wdat`. For more information, see the implementation of function [`wdata_get_time`(...)](https://gitlab.fizyka.pw.edu.pl/wtools/wdata/-/blob/master/c/wdata.c#L562) from our C library.
-
 Let's get back to the `*wtxt` file.
 
 # Tags
@@ -205,7 +198,48 @@ The wtxt file can also contain info about additional text files associated with
 Note, beside this info in the wtxt file, C library do not provide any extra functionality for text files.  
 ```bash
 # This tag indicates that the file prefix_myfile.txt also belongs to the dataset
-# The tag is used only by VisIt for now
+# The tag is used only by VisIt plugin
 txt         myfile.txt
 ```
+# Domain
+## Coordinates
+```
+datadim                  3    # for 2d case, you can skip tags nz, dz, z0
+                              # for 1d case, you can skip tags ny,nz, dy,dz, y0,z0
+
+nx                       24   # number of points along the x-direction
+ny                       28   # number of points along the y-direction
+nz                       32   # number of points along the z-direction
+dx                        1   # spacing along the x-direction
+                              # if dx` is negative, then the x-coordinate should be extracted from the extra file `prefix__x.wdat`.
+dy                        1   # spacing along the y-direction
+                              # if dy` is negative, then the y-coordinate should be extracted from the extra file `prefix__y.wdat`.
+dz                        1   # spacing along the z-direction
+                              # if dz` is negative, then the z-coordinate should be extracted from the extra file `prefix__z.wdat`.
+x0                      -12   # origin of x-axis, if not specified, then default x0=0
+y0                      -14   # origin of y-axis, if not specified, then default y0=0
+z0                      -16   # origin of z-axis, if not specified, then default z0=0
+```
+
+The coordinates are converted into an array index using row-major prescription
+```c
+// lattice indicase
+int ix=...; // ix in [0,nx)
+int iy=...; // iy in [0,ny)
+int iz=...; // iz in [0,nz)
+
+// coordinate decoding
+double x = x0 + dx*ix;
+double y = y0 + dy*iy;
+double z = z0 + dz*iz;
+
+// array index: 3d case
+int ixyz = iz + nz*iy + nz*ny*ix; 
+```
 
+## Time
+To compute the time associated with a given `icycle`, we use the formula
+```
+time = t0 + dt * icycle;
+``` 
+If `dt` is negative, then the `time` parameter has to be extracted from the additional binary file of the name `prefix__t.wdat`. For more information, see the implementation of function [`wdata_get_time`(...)](https://gitlab.fizyka.pw.edu.pl/wtools/wdata/-/blob/master/c/wdata.c#L562) from our C library.
\ No newline at end of file