Add preprocessing description

Author: cliu72

templates/2_Pixie_Cluster_Pixels.ipynb

@@ -345,11 +345,13 @@
    "source": [
     "During pixel preprocessing, the following is done for each FOV:\n",
     "\n",
-    "* Gaussian blur each channel separately\n",
+    "* Gaussian blur each channel\n",
     "* Remove empty pixels\n",
-    "* For the remaining pixels, normalize each pixel by the sum of all the channels\n",
+    "* Channel normalization and pixel-sum normalization\n",
     "* Subset a `subset_proportion` fraction of non-empty, normalized pixels. This creates the subsetted dataset for training\n",
     "\n",
+    "Since the publication of the Pixie paper <a href=https://www.nature.com/articles/s41467-023-40068-5>(Liu et al., Nature Communications 2023)</a>, in addition to the preprocessing steps as described in the paper, we added an additional channel normalization step and threshold for filtering out low-expressing pixels. We first perform an initial channel normalization. We then filter out any pixel with a total sum value less than a threshold value (default threshold is the mean 5th percentile value of the total sum for all pixels across all FOVs). Then, we perform  pixel-sum normalization and another channel normalization before clustering as described in the paper. We found that these additional steps helped clustering performance for channels with widely varying intensity ranges. The channel normalization percentiles are controlled by the parameters `channel_percentile_pre_rownorm` and `channel_percentile_post_rownorm`. For a full set of parameters for `create_pixel_matrix`, please consult the <a href=https://ark-analysis.readthedocs.io/en/latest/_markdown/ark.phenotyping.html#ark.phenotyping.pixie_preprocessing.create_pixel_matrix>pixel training docs</a>.\n",
+    "\n",
     "Note: if you get integer overflow errors loading in your data, try changing the `dtype` argument to a larger type."
    ]
   },