backwards compatible half-float PNG test app source+examples

I've found a lossless and trivally invertible transform that takes
half-float HDR values (typically read from .HDR or .EXR images) and packs
them to 16-bit unsigned pixels that are completely compatible with existing
non-HDR aware PNG software. It uses a simple invertible and lossless global
tone mapping operator that operates directly on the half-float values. Old
readers view and see these PNG's as 16-bit PNG's (so usually 48bpp per
pixel for RGB).

The high bytes are tone mapped so the 16bpp images appear passable to
existing readers. New readers can parse a small ~257 byte ancillary chunk
which contains a byte remapping table used to remap the high bytes of the
16-bit components in the PNG image back to half-floats. (It's a little bit
more complex than this to deal with signed floats and very low values, but
that's the gist of it.) It's lossless for all valid half-float values
(normals, denormals, signed). I filter out any NaN's/Inf"s in this test.

You can see a bunch of example 48-bpp PNG's packed from .EXR images in this
way here:
https://github.com/richgel999/png16/tree/main/bin

The C++ source to the example "hdrpng" tool is here. The example app
excluding image reading/writing is only ~450 lines of code. (It currently
compiles with VS 2022 under Windows - I'll add a Linux cmake file next.)
https://github.com/richgel999/png16/

It uses the popular open source lodepng library, unmodified, to write and
read 16-bit PNG files and manipulate the ancillary "hdRa" chunk.
hdrpng supports packing and tone mapping .EXR images to .PNG, unpacking HDR
.PNG to .EXR, and an .EXR file comparison mode to verify that the half
float values can be 100% recovered from the PNG file with no loss. The tone
mapping in this example is automatic.

I've tested the resulting 48bpp PNG files with pngcheck, Windows Explorer,
Chrome, Paint Shop Pro, several tools, and the Windows Photo viewer app. So
far, so good - they all look fine.

Here are a couple example 48bpp PNG files. (Not sure what gmail will do to
them, but this is what they look like.) These are how existing PNG readers
view these HDR files. The half-float data is 100% preserved in these .PNG
files, so HDR capable viewers are able to retrieve the original half-float
pixels and do their own tone mapping or HDR processing.

[image: Kapaa.png]
[image: memorial.png]

These PNG's validate successfully using pngcheck, because they are
completely standard PNG files that any reader can load:

[image: image.png]