Seurat Merged objects tSNE - How to paint on original IDs? I am working with single-cell RNA-seq data, using the R package "Seurat" to cluster and visual data-points.

I had two single cell datasets from which I generated two Seurat objects. I then combined the two using MergeSeurat. I did differential gene expression analysis, performed clustering, and ran a tSNE plot.

Now what I want to do is create that same tSNE plot, but color each cell according to the original object from which it came. How do I do this?
Below a few lines of code that accompany BC Wang's answer.
After using MergeSeurat the sample name will be added to meta data under orig.ident. this can then be used to color the tSNE either using group.by or pt.shape. The former will show colors for each sample, the latter will color each cluster and give sample id another shape.

path1 &lt;- file.path("path to 10x sample 1")
path2 &lt;- file.path("path to 10x sample 2")

S1 &lt;- Read10X(data.dir = path1)
S2 &lt;- Read10X(data.dir = path2)
S1 &lt;- CreateSeuratObject(raw.data = S1, min.cells = 3, project = "S1")
S2 &lt;- CreateSeuratObject(raw.data = S2, min.cells = 3, project = "S2")

s.combined &lt;- MergeSeurat(object1 = S1, object2 = S2, add.cell.id1 = "S1", 
            add.cell.id2 = "S2", project = "2samples")


All data analysis goes here:
RunTSNE etc...

Plot the data

TSNEPlot(s.combined, group.by ="orig.ident")
TSNEPlot(s.combined, pt.shape="orig.ident")