Keep float values when saving screenshots to .exr or .hdr

[stuartparmenter]

Objective

A screenshot of a float render target, for example an Hdr camera rendering to an Rgba16Float image, cannot be saved. Image::try_into_dynamic has no arm for Rgba16Float or Rgba32Float, so save_to_disk logs "screen format cannot be understood" and writes nothing.

Solution

try_into_dynamic gets the two float arms. Both produce DynamicImage::ImageRgba32F.

For a float source, save_to_disk keeps the float range in .exr and .hdr. For any other format it clips to 0..=1, sRGB encodes, and warns.

Non-float screenshots save as before. Image::convert, which TextureAtlasBuilder uses, filters out float results, so a float source still returns None.

Testing

Unit tests cover both conversions. An Hdr camera rendering to an Rgba16Float image was captured and saved. .exr and .hdr keep the range, and .png clips to 8-bit sRGB with the warning.

Showcase

let target = images.add(Image::new_target_texture(512, 512, TextureFormat::Rgba16Float, None));
commands.spawn((
    Camera3d::default(),
    Hdr,
    RenderTarget::Image(target.clone().into()),
));

// Later, when a frame is wanted.
commands
    .spawn(Screenshot::image(target))
    .observe(save_to_disk("frame.exr"));

What comes next

Later PRs in the HDR series add decoding of HDR transfer encodings and HDR PNG output.

---

This PR was built by me with the assistance of Claude Code w/ Fable 5.1

[JMS55]

I don't understand what the issue is here.

[stuartparmenter]

This is here to prevent a behavior change in TextureAtlasBuilder, which also relies on try_into_dynamic through convert. Before this PR, Rgba16Float and Rgba32Float images failed in try_into_dynamic, so convert returned None and the atlas builder skipped them. If we let them flow through now, convert would turn them into 8-bit textures by clamping and scaling, with no sRGB encode, and call it a Rgba8UnormSrgb which would be broken. The filter keeps convert returning None for those two formats, as before.

Possibly longer term there is a more correct fix here for TextureAtlasBuilder but it would need to tonemap and encode HDR content, which is generally out of scope for me just trying to get HDR screenshots working and I don't think convert is the right place for that to happen

[JMS55]

"Float screenshots" -> I think we should be more specific about what this means

[stuartparmenter]

i'll update the comment to be more clear that we're talking about a screenshot of an Rgba16Float or Rgba32Float render target (i.e. what you get from a Hdr camera rendering to an Image)

[JMS55]

Explain to me what's going on here too.

[stuartparmenter]

This was found due to a panic during testing, and I would have spent a while trying to figure it out w/o Claude.

Basically The .hdr format stores each pixel as three 8-bit color values plus one shared 8-bit exponent, so the range it can hold is limited. image's encoder works out that exponent with floor(log2(max)) + 1 as an i32 and then stores it as (exp + 128) as u8, and it never checks that the value fits. If a channel is infinite, which is easy to get in a float render target, log2 is infinite, the cast to i32 saturates, and the + 1 overflows. If a channel is 2^127 or bigger, or smaller than 2^-128, the exponent byte wraps around and the pixel decodes to garbage. So before encoding we clamp every channel and set every NaN and subnormal value to zero.

I'll update the comment to be more clear.

I'll see if I can find an upstream issue and file on if not.

[beicause]

If it's an issue with TextureAtlasBuilder it should be fixed in that, rather than having this function skip the formats.

[stuartparmenter]

As I noted above, convert already returns None for float images, and TextureAtlasBuilder logs an error and leaves that rect blank. This PR keeps that behaviour. The filter is there so convert doesn't start clamping linear float data to 8 bits with no sRGB encode.

Handling HDR sources in TextureAtlasBuilder properly, whether by clamping or tonemapping, is unrelated to screenshots and belongs in its own PR (if someone wants to do it in the future)