stb_image_write.h: Radiance .hdr saved incorrectly when source exceeds Rec709 color space

[AnotherCommander]

Describe the bug The Radiance .hdr format supported by stbi is RGB9_E5 . This means that no negative numbers can be used when encoding an image. When working in the scRGB color space, this implies that any non-Rec709 colors, (i.e. colors with negative R,G or B component values) cannot be represented.

Attempting to save to an .hdr file using stb_image_write when the source contains DCI-P3 or BT2020 colors will result in a broken .hdr. Broken here means that any non-Rec709 colors appear with completely wrong hues and the result is unusable.

To Reproduce Steps to reproduce the behavior:

1. Download and build the example exr2rgbe from https://github.com/syoyo/tinyexr. It is a very simple program to build and uses tinyexr and stb_image_write to do its read/write operations.
2. Get an .exr image (lets call it inputimg.exr and be sure that it contains colors in the DCI-P3/BT2020 spaces. Place it in the exr2rgbe folder together with the executable you created at step 1.
3. Run exr2rgbe inputimg.exr ouputimg.hdr
4. Now try to open the resulting .hdr. You will immediately notice that it is wrong.

Expected behavior The resulting .hdr should look very close to the source on screen. It is expected that DCI-P3 and/or BT2020 colors will be lost during write since the format does not allow for negative numbers, but at least it won't be blatantly wrong.

Screenshots I have already uploaded my results in a zip file available from here for those who can't be bothered to follow the steps to reproduce: https://drive.google.com/file/d/15fF3VWsKjHNrgzhYg-Ce3WrofQBCIwnp/view?usp=sharing

Some comments: You will note that the zip linked above contains three files: my original .exr, the bad result I get from stb_image_write.h v1.16 and one more file, which is the .hdr fixed to display properly all colors in Rec709. The fix is actually very simple: make sure that the rgb components of the buffers being written are clamped to 0.0f. The proposed patch for the fix is this:

--- 638,656 ----

  static void stbiw__linear_to_rgbe(unsigned char *rgbe, float *linear)
  {
!    int exponent, i;
     float maxcomp = stbiw__max(linear[0], stbiw__max(linear[1], linear[2]));

     if (maxcomp < 1e-32f) {
        rgbe[0] = rgbe[1] = rgbe[2] = rgbe[3] = 0;
     } else {
        float normalize = (float) frexp(maxcomp, &exponent) * 256.0f/maxcomp;
+
+         // ensure that any non-Rec709 colors are clamped to Rec709
+         for (i = 0; i < 3; i++)
+         {
+                 if (linear[i] < 0.0f)  linear[i] = 0.0f;
+         }

        rgbe[0] = (unsigned char)(linear[0] * normalize);
        rgbe[1] = (unsigned char)(linear[1] * normalize);
***************

[nothings]

Serious question, which is better, "fixing" it or adding documentation "it's the client's job to make sure the colors lie in the correct colorspace", given that we can't actually fix it, just clamp it.

[rygorous]

Given that we literally produce complete garbage when values are negative, clamping seems the minimum.