On 2023-08-07 06:35 (-4), BayashiPascal wrote:
> 
>> I am currently looking at a potential third issue: whether or not
>> generating SPDs by naively combining primary colors will lead to the
>> same fade_color/absorption primary color bias than RGB rendering has,
>> but I do not have results on that yet.
> 
> I don't understand what you mean here, but I guess you'll share here your
> results about it, so wait-and-see.

And it turns out that combining red, green, and blue SPDs does lead to
the same bias as RGB rendering.

The top right and top center images use SpectralRender, while the upper
right image is straight up RGB rendering.  Each image in the lower row
shows the SPD of the sphere in the image directly above.

You can see how the hue of the transparent prism in the top right image
very quickly drifts toward phosphor blue.  This is what drove me to
spectral rendering in the first place.  The top center image, which uses
macro MakeSp_RGB_Args() to combine red, green, and blue SPDs similarly
to D_RGB(), shows the same bias as the RGB render.  In the top left
image, which uses macro MakeSp_Meta_RGB_Args(), the transparent prism
maintains the hue of the sphere.

RGB composites are still useful for opaque pigments, as most hues cannot
be created at full 'value' with the metamer macros.  For example, the
blue in these images (H = 210) maxes out at 0.8214.  Aqua in the OP
maxes out at approximately 0.625.  But for colored transparent
materials, disconnecting the SPD from RGB modeling really makes a
difference.

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