.hdr images decoded as LDR.

[pema99]

In my renderer, I was using the following code to load radiance HDR image (.hdr extension), but noticed that all values were getting clamped to [0; 1] range:

pub fn load_dynamic_image(path: &str) -> Option<DynamicImage> {
    image::io::Reader::open(path).ok()?.decode().ok()
}

I was able to fix it by adding a special case:

pub fn load_dynamic_image(path: &str) -> Option<DynamicImage> {
    // Image crate does not by default decode .hdr images as HDR for some reason
    if path.ends_with(".hdr") {
        let hdr_decoder = image::codecs::hdr::HdrDecoder::new(std::io::BufReader::new(
            std::fs::File::open(&path).unwrap(),
        )).ok()?;
        let width = hdr_decoder.metadata().width;
        let height = hdr_decoder.metadata().height;
        let buffer = hdr_decoder.read_image_hdr().ok()?;
        return Some(DynamicImage::ImageRgb32F(image::ImageBuffer::from_vec(
            width,
            height,
            buffer.into_iter().flat_map(|c| vec![c[0], c[1], c[2]]).collect(),
        )?));
    }

    image::io::Reader::open(path).ok()?.decode().ok()
}

I noticed it seems to be caused by this line HdrAdapter::read_image_data:

fn read_image_data(&mut self, buf: &mut [u8]) -> ImageResult<()> {
        assert_eq!(u64::try_from(buf.len()), Ok(self.total_bytes()));
        match self.inner.take() {
            Some(decoder) => {
                let img: Vec<Rgb<u8>> = decoder.read_image_ldr()?; // <-- clamp to LDR
                ...

I guess the simple fix would just be to swap the call to read_image_hdr instead. I'm mainly opening this bug report to verify if this is intentional or indeed a bug. It seems like too obvious of a mistake to me.

Expected

.hdr image is decoded as HDR values, nothing is clamped.

Actual behaviour

.hdr image is decoded such that all pixels above 1.0 are clamped to 1.0.

Reproduction steps

Run the code from the first snippet, passing a path to an .hdr image. Read the pixel values and observe all will be clamped. Attaching the image file I used to test.
https://dl.polyhaven.org/file/ph-assets/HDRIs/hdr/2k/clarens_midday_2k.hdr

[fintelia]

This is a bug, but one that existing code may be unintentionally relying on. I think the right approach is probably to change when we release 0.25.0. Honestly, I'm not really sure why we support LDR decoding of .hdr images at all. Might make sense just to deprecate it and leave only the HDR decoding logic.

[pema99]

I submitted a patch, will 0.25 be the next release?

[fintelia]

We'll see. There may be a 0.24.7 release first