Re: backwards compatible half-float PNG test app source+examples

If you want to see these example HDR .PNG's, unpacked to HDR .EXR files,
using an in-browser HDR viewer, I've unpacked them (using the example
hdrpng tool) to github here:
https://github.com/richgel999/png16/tree/main/bin/unpacked

The EXR viewer app (it's pretty good - I use it for testing on SDR
monitors):
https://viewer.openhdr.org/


On Fri, Nov 10, 2023 at 4:34 AM Richard Geldreich <rich@binomial.info>
wrote:

> 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]
>