Fully transparent colors are not transparent in hdr

[VlaDexa]

(it isn't fully opaque as the screenshot suggests, but it's baaarely transparent)

Left is my hdr monitor (DP-2), right one is my non-hdr monitor (HDMI-A-1)
hyprland.conf.txt

Code for that triangle is a bit complicated, because I'm playing around with vulkan. I'll probably come up with something shorter later
tranparent.tar.gz

[VlaDexa]

tried a simple sdl program

aaand now it's white

#define SDL_MAIN_USE_CALLBACKS
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>

SDL_Window *window;
SDL_Renderer *renderer;

SDL_AppResult SDL_AppInit(void **appstate, int argc, char **argv) {
  // create a window
  SDL_CreateWindowAndRenderer("Transparent", 960, 540,
                              SDL_WINDOW_RESIZABLE | SDL_WINDOW_TRANSPARENT,
                              &window, &renderer);

  return SDL_APP_CONTINUE;
}

SDL_AppResult SDL_AppIterate(void *appstate) {
  int width = 0, height = 0;
  SDL_GetWindowSize(window, &width, &height);

  SDL_SetRenderDrawColor(renderer, 0, 0, 0, SDL_ALPHA_TRANSPARENT);
  SDL_RenderClear(renderer);

  SDL_SetRenderDrawColor(renderer, 255, 0, 0, SDL_ALPHA_OPAQUE);
  // const SDL_FRect rect = {100, 100, 100, 100};
  const SDL_FRect rect = {width / 4., height / 4., width / 2., height / 2.};
  SDL_RenderRect(renderer, &rect);
  SDL_RenderFillRect(renderer, &rect);
  SDL_RenderPresent(renderer);

  return SDL_APP_CONTINUE;
}

SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event) {
  // close the window on request
  if (event->type == SDL_EVENT_WINDOW_CLOSE_REQUESTED) {
    return SDL_APP_SUCCESS;
  }

  return SDL_APP_CONTINUE;
}

void SDL_AppQuit(void *appstate, SDL_AppResult result) {
  // destroy the window
  SDL_DestroyRenderer(renderer);
  SDL_DestroyWindow(window);
}

tranparent-sdl.tar.gz

[njdom24]

Are you seeing similar behavior for colors anywhere between transparent and opaque?

[VlaDexa]

vulkan with clear value of [0.1 0.1 0.1 0.1]

sdl with clear value of r: 10, g: 10, b: 10, a: 10

[UjinT34]

SDR and HDR black levels are different by default and HL will adapt SDR blacks when displayed in HDR mode (make them brighter). Shouldn't happen with alpha=0 though.
Vulkan app might show black because premultiplied/non-premultiplied alpha is handled incorrectly or alpha mode isn't set correctly. Brighter color in SDL is expected. sdr_min_luminance should help with that.

[njdom24]

I expect black level raise without sdr_min_luminance, but what's happening here with alpha seems excessive. Haven't tested the cases in this discussion myself, but I do see my transparent windows/layers get their alpha significantly raised across the board with cm=hdr.

(IIRC I've seen this issue before I started poking around with my other changes.)