readme.md file for generated_undersampled folder in sharepoint

a3sm/readme.md

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+# Generated Undersampled PCM Example Data
+
+The files in this directory contain
+some undersampled PCM data generated from selected example ground-truth data
+in `photocurrent_data` folder.
+These files are intended for quick testing and comparison of algorithms
+so that every test can use the same undersampled data.
+
+Currently the structure is as follows:
+
+```
+generated_undersampled
+└── <randomisation-scheme>_random
+    └── <data-name>_<randomisation-scheme>
+        └── trial_<trial-index>
+            └── zero_filled
+                └── <data-name>_sample_<sampling_percentage>_percent_coarse_J=<coarse-J>_<randomisation-scheme>_random.npy
+```
+
+Where:
+
+- `<randomisation-scheme>` is one of `uniform` or `multilevel`
+
+- `<data-name>` is the name of the ground-truth data,
+for example `example_CIGS_256x256`
+
+- `<trial-index>` is an integer index for different random trials
+
+- `<sampling_percentage>` is the percentage of samples taken in the undersampling, for example `20.0` for 20%
+
+- `<coarse-J>` is the number of coarse samples taken in the J dimension, for example `6`
+
+
+The `.npy` files in the `zero_filled` folder contain the forward output of the generated undersampled PCM data.
+The undersampled measurement PCM data can be computed from this using the inverse WHT.