While trying to write code to support loading 16bit files for targa datatype I realized two things. First, I've never tried working with 16bit so I know very little other than bit shifting is required. And second, I didn't have any valid 16bit targa image data for testing.
I discovered that there are two basic types of 16bit rgb data. There is rgb555 and rgb565 respectively. Apparently targa uses the rgb555 variety where red, green and blue are each 5bits. The remaining 1 bit can be discarded during decoding.
So I had the basic method but no sample data. Then I realized two things. Windows bitmap images can be 16bit and our bmp datatype can decode 16bit data. However, bmp is supposed to use rgb565.
But I had a graphics utility that could import and export a few image types but it provided bitdepth and conversion options. For exporting 24bit bmp it provided 3 options: rgb555 or rgb565 or 24bit. So I chose to export the 16bit bmp as rgb555 to match the supposed targa format.
When importing 24bit then exporting targa no options were provided. It simply saved 24bit targa. But when I imported the 16bit bmp I just converted then tried to export as targa it gave the same three options as for bmp. So I chose rgb555 to be consistent.
So here's the interesting part if you enjoy this type of thing. Using my favorite hex editor I compared the two files. The 16bit pixel data was identical. So now I could make 16bit test files.
Microsoft provided some code for converting 16bit to 24bit rgb. Now the rest is just writing the 16bit conversion code then inserting it into the datatype and testing it. Fun stuff.

BTW it seems PaintDotNet can't display 8bit and 16bit targa images correctly. It forgets to swap the red and blue pixel elements per scanline for 16bit to make BGR into RGB. For 8bit it also fails to swap red and blue for colormap values with the same result. The images appear blue or oddly colored. But GIMP displays 8bit and 16bit targa images just fine. And soon our MultiView will display them as well using the targa datatype.