# Stitching existing data

If your data have already been acquired and not pre-processed via `syncAndCrunch`, you should handle them as follows. Each acquisition session should have its own directory [set up like this](/new-users/the-experiment-directory-structure.md). i.e. It needs:

* A meta-data file (the "recipe" file).
* A sub-directory called `rawData` that houses the raw data directories. Move them here if they're not already here. 

With the above in place you can use `stitchAllChannels` (see `help stitchAllChannels`) to automate the process of conducting all the pre-stitching analyses and then stitching. e.g.

```
>> cd /mnt/myLocalData/SampleXYZ
>> ls -l
>> stitchAllChannels
>> downsampleAllChannels(25) %Generate 25 micron volumes for stitchit.sampleSplitter
```

Note that you should be using `stitchit.sampleSplitter` once data have been stitched to crop out the sample from the larger acquisition area. In higher resolution acquisitions this can save many tens of GB.  

If you look inside `stitchAllChannels` you'll see it really doesn't do very much. Instead of using `stitchAllChannels` you can run the required functions yourself one at a time like this:

```
>> cd /mnt/myLocalData/mySample
>> generateTileIndex %Indexes tiles, determining where each fits in the final volume
>> preProcessTiles([],'illumChans',2,3) %Calculate coefficients for correcting artifacts from channels 2 and 3
>> collateAverageImages %Calculate grand average tiles
>> stitchSection([],2) %Stitch all physical sections from channel 2
```

That's it: those are the core commands in *StitchIt*. So what was going on there? `generateTileIndex` indexes the tiles to map their names on to their position in the final volume. Then the `preProcessTiles` command calculates coefficients for the comb correction and average tiles for illumination correction. The first input argument tells it to only work on directories that already haven’t been processed, the second and third arguments tell it to calculate bidirectional scanning coefficients and illumination correction on channels 1 and 2. Average image data for illumination correction are stored for each physical section within that section’s raw data directory. If interrupted, `preProcessTiles` can resume where it left off. `collateAverageImages` produces the grand average images used for illumination correction (data from the whole specimen contribute to the illumination correction). The above steps is most of what `syncAndCrunch` does too.

In case you're wondering what these various corrections are good for, here's an example of the same section before and after illumination correction by dividing out the average tile.

![](/files/-MUQV4ogb9Mg2oZdrsAM)

As you can see, illumination correction is a big improvement but is not perfect. Want it perfect(ish)? [Post-processing steps to the rescue!](/advanced-usage/post-processing.md)


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