How to use pixel_format="luminance_alpha" for the hdrfilm plugin?

[maxfrei750]

I'd like to render grayscale images with an alpha channel, so I set the Mitsuba variant to cuda_ad_mono and the pixel_format property of the hdrfilm to 'luminance_alpha', as per the docs. When loading the scene, I received the warning:

WARN  main  [HDRFilm] Monochrome mode enabled, setting film output pixel format to 'luminance' (was luminance_alpha).

Setting the Mitsuba variant to cuda_ad_rgb got rid of the warning, but now the rendered images have the shape (H, W, 4), instead of the desired (H, W, 2) and, more importantly, my color parameters have three elements, instead of just one.

What am I missing? Thanks!

[maxfrei750]

Never mind... Setting the Mitsuba variant to cuda_ad_rgb and pixel_format to luminance_alpha, does produce the desired (H, W, 2) output.

[njroussel]

Let me clear up a couple things:

The variant defines the color representation used throughout the entire rendering process. The pixel_format choice only defines how the result is then written to the film. This allows us to have an entire RGB simulation but only save the final luminance value of the RGB output in the film.

Using luminance_alpha should have been possible with a mono variant. I pushed a fix here:e3a9a3a

[maxfrei750]

Thanks for elaborating and the quick fix.