First I need to say I don't know how to program, let alone know how a PS2 or its emulation works. But I always had this question:
Why does PCSX2 has the Interlacing option at all?
I know the image of all the consoles of this era is interlaced because of the way the TVs, on which they would be connected, worked but PCSX2 is the only emulator out there which uses an interlacing option. I have used so many emulators, for the GameBoy, NES to the Gamecube and PS3 and none had a similar option as far as I can tell.
Why does PCSX2 needs an option like this?
And to add to the question: I was just watching this video of Modern Vintage Gamer
https://youtu.be/NyCyIqnQ27k
and he says at the 6:35 timestamp "..and even displayed interlacing, which actually surprised me. When I asked Polymega about this feature, they mentioned that
they developed an internal interlace effect."
So, again: Other emulators have to make their own interlace effects but PCSX2 has to have it for some reason no matter what?
Most other emulators that don't have deinterlacing settings usually have some simple process that takes care of deinterlacing silently in the background for you. Gameboys have never had interlacing at all, the first Gameboy was actually a progressive scan LCD display, and all future iterations (while not always LCD) were still progressive. Other consoles, most notably the Gamecube, had extremely predictable graphics code and dedicated memory space for graphics, allowing emulator developers to effectively hijack control of rendering code, and force progressive scan from within the game. Newer systems such as the PS3 or Xbox 360 will just inherently allow progressive scan on anything; the systems were made for HDMI output (though still component and composite capable for TV compatibility), and progressive scan is actually a requirement for HDMI to work at all.
The PS2 is a special kind of hell for any graphics developer because outside of commands for "draw a triangle" or "draw a point", there was not much limitation in the official development kit. This led to a whole lot of ingeniously evil tricks and twists to make the PS2 do things that Sony themselves never even thought of, with game developers sharing tips and tricks on mailing lists on how they pulled off their awesome effects. Furthermore, the PS2 had only 4 MB of dedicated video memory, and on top of this, system memory was shared between the CPUs and GS (the GPU). This led to a lot of designs that make us question how much glue some game developers inhaled while making their code.
Progressive scan was available but it was completely up to the developer to choose whether or not to use it. Furthermore, if not progressive scan, the developer still had free reign to choose how to store the interlaced video. They could choose to use one solid block of memory, or they could arrange it into a mesh of seemingly random locations and then stitch it back together later. They could do things like "clear the screen" using completely disproportionate memory sizes, and it worked. For a lot of the ingenious effects developers were doing, some of these evil tricks were necessary to work properly.
What does this mean for deinterlacing? There is no one routine that works every time. Instead we've over the years curated a number of deinterlacing techniques that, collectively, can handle just about every interlacing design we've encountered to date. Furthermore the Automatic mode is smart enough to detect which one to use, probably 95% of the time, with only a few games not automatically using the best mode. Unfortunately this is the best we are ever going to get, unless there is a serious breakthrough in progressive scan patching.
So, from that, what I understand is that ePSXe or Yabause or Dolphin or whatever, all still use a deinterlacing technique but keep it hidden from the user because they draw the screen in a more standardized way than PS2 and that we shouldn't bother changing it from Auto unless there is a problem with our games. Right?
And something more: Is there a possibility to add Vulkan support?
I cannot confirm for ePSXe since it is closed source but as a general rule, if the console could be used on a CRT TV, then it was capable of interlacing. If the emulator for it can output to your screen correctly without a setting for it, they are either deinterlacing quietly in the background, or forcing progressive scan in the game's code. Dolphin actually does the latter.
There is a possibility but it means basically starting over on graphics. So far the only benefit would be for AMD users, but they already have Direct3D11 as an option, so no one is really concerned right now.
Thank you for the answers!