The visual properties of CRTs are a surprisingly complex topic.
Still images:
- 240p and 480i (15 KHz) console games were meant to be played on SDTV CRTs. Most consumer TVs have relatively coarse dot pitch (white lines are actually composed of color dots/stripes spaced relatively far apart). This adds "texture" to game visuals, masking aliasing and blur to a degree.
- - I think low-res LCDs actually have a similar horizontal texture to aperture grille (phosphor stripe) CRTs, but dot/slot mask (dot/slot-shaped phosphors) CRTs look quite different from LCDs when seen close up.
- - VGA CRT computer monitors have a much finer dot pitch (adjacent phosphors of the same color closer together) and higher TVL (phosphors of the same color per screen height). This makes them look closer to LCDs, since the color dots/stripes are so close together they can disappear at a normal viewing distance. This is not necessarily a good look for 2D games, or even pre-HD (PS2/GC/Wii) 3D games.
- Unlike LCDs, CRTs have visible scanlines (lines of light traced by the three electron beams, horizontal on wide monitors and vertical on rotated/tall arcade monitors). On a properly calibrated CRT, the three colors of scanline are vertically aligned (convergence errors cause the scanlines and/or horizontal changes of brightness to be misaligned between the three colors). On many (not all?) CRTs, each color's scanline gets taller (vertically wider) when the color is brighter, creating a "blooming" effect utilized by games. The scanlines are usually narrow enough on SDTVs that there's a visible black gap between adjacent lines of light; modern emulators' "scanline" filters add black gaps between rows of pixels.
- Most consoles output a horizontally soft image (or composite video blurs it horizontally). In 480i on a SDTV, adjacent scanlines overlap. Both effects blur the image and reduces high image frequencies, acting as "hardware-level anti-aliasing to every game for free". Additionally, horizontal transitions between colors and black cause the scanline to become narrower as it fades out (on many TVs), which is another characteristic of the CRT look: https://twitter.com/CRTpixels/status/1599513805717135360
- Unlike SDTVs, high-quality properly-focused VGA monitors have scanlines so narrow that drawing 240 scanlines across a 17 inch monitor produces taller black gaps than scanlines, even at full brightness (which produces maximum scanline height). This is very much not an attractive appearance for 240p games. One solution is to display each line twice in vertically-adjacent 480p scanlines (line doubling). Alternatively you can send a high-resolution (960p or above) video signal to the VGA CRT, and add emulated scanlines in a GPU shader or hardware scaler (ideally RetroTink-5X).
- - Note that visible scanlines boosts high vertical frequencies up to (and to an extent, beyond) Nyquist, effectively acting as alias-boosting filters rather than anti-aliasing! *If you want phosphor dot texture and a smoothed on-screen image, which CRT TV/monitor you pick matters as much as playing on a CRT!*
- - Both 240p and 480i games were meant to be played on SDTVs. 240p games were meant to be seen with scanline gaps, while 480i games were not.
- - As an example of "alias-boosting", my 17-inch VX720 VGA monitor has visible scanlines when running at 480p. They're beautiful to look at, and work well in arcade-style games with large objects like Super Monkey Ball 2, or bloomy games like Mario Kart Wii. But they boost aliasing to the point that many GC/Wii games (especially Wind Waker with half-resolution DOF distance blur) look objectionably aliased (worse than a regular CRT or even a LCD display). As a result, when playing 480p games through my GBS-Control, I often configure it to upscale to 960p with vertical bilinear filtering ("line filter").
- I'm not actually sure what CRT monitors (dot pitch, beam sharpness, maximum resolution) PC games from various eras (DOS through 2000) were meant to be seen on; PC monitor resolutions have evolved substantially throughout the decade or so, while pre-HD consoles have universally targeted 15 KHz TVs. I'd appreciate input from someone who grew up in this era of PC gaming.
Interlacing:
- Console games since the PS2/GameCube era primarily output a 480i video signal, where every other frame (technically field)'s scanlines are vertically offset by half a line, causing solid colors to appear "filled" without gaps between scanlines. However, if your eyes are tracking an object moving up/down by half a line per field, then the scanlines line up again and gaps reappear. This is because each scanline is only illuminated for a small fraction of a field, and by the time the next field appears and the scanlines have moved half a line up/down, your eye has moved that distance as well.
- In 480i video mode, if even and odd scanlines are different average brightnesses in an area of the image, the image will flicker when displayed on an CRT. To address this issue, many video games enable "deflicker", which blends together (on Wii) 3 adjacent scanlines when outputting the video (technically when copying an image to XFB). This blurs the image vertically but eliminates flickering between fields.
- - Some GC/Wii games enable "deflicker" (vertical blur) even at 480p output (with no flickering), which softens the image vertically; Nintendont and Wii USB loaders come with optional hacks which hook games to prevent them from turning on vertical blur.
- - And for some reason, in certain translucent foggy/glowing/shadowed areas, some GC/Wii games actually output alternating brightness per scanline (like vertical dithering) and rely on the deflicker filter to smooth it out to a uniform color. Running without deflicker enabled results in horizontal stripes of alternating colors, which I'm (usually) fine with but some people find ugly.
Motion and latency:
- Video signals, both analog VGA and digital HDMI (and probably DisplayPort as well, unsure if DP's MST alternates screens or interleaves them), always deliver the screen's contents from top to bottom over the duration of a frame. Displays (both CRTs and flat-panels) update from top to bottom as the image arrives into the screen. As a result, the bottom of a screen has nearly a full frame of extra latency compared to the top of the screen (unless you configure the game to turn off both vsync and triple buffering, resulting in tearing and the bottom of the screen receiving a newer video frame).
- - You can reduce the latency at the bottom of a screen by delivering pixels faster, and spending more time after each frame not delivering pixels (HDMI Quick Frame Transport, https://forums.blurbusters.com/viewtopic.php?f=7&t=4064).
- LCDs and OLEDs update the state of each pixel once per frame (from top to bottom as the signal is transmitted from the video source), and pixels continue transmitting light for a full frame's duration until the next update arrives. (LCDs also have additional latency from an electrical signal to the image changing, because the liquid crystals respond slowly to electricity.) CRTs instead light up each point on the screen briefly once a frame, and the light output decays towards black (within https://en.wikipedia.org/wiki/Phosphor#Standard_phosphor_typ...)
- - As a result, CRTs flicker (bad). But when your eye is tracking a moving object, the entire screen is not smeared by the distance of a single frame like in LCDs, resulting in a sharper image in motion (noticeable in video games and even web browser scrolling). There are techniques (black frame insertion, backlight strobing) to produce CRT-like motion properties (complete with flicker) in LCDs and OLED screens (more info at https://blurbusters.com/gtg-versus-mprt-frequently-asked-que...).
- - Sadly, 30fps console games on CRT effectively lose out on this beautiful fluid motion property, because they display the same video frame twice in a row. As a result, when your eye is tracking a moving object, the object (and screen) appears as a "double image", with the two copies 1/60 of a second apart (at your eye's rate of motion). The same "double image" appears (but with a shorter distance, I don't know if it's perceptible) if you display a 60fps video at 120fps to avoid CRT flicker (or to feed 240p video into a 31-KHz-only monitor).
- - One idea I have is "motion adaptive black-frame insertion" (similar to motion adaptive deinterlacing), where an (ideally) OLED display displays a static dim image for nonmoving sections of the image (to avoid flicker), and a bright strobed image for moving sections (to avoid eye tracking smearing). I'm not aware of any monitors which perform this trick, and I'm not sure if the 1ms or so of added latency to perform motion detection (my best guess, judging by the minimum latency of a RetroTINK-5X Pro or GBS-Control) would make this product unpalatable to gamers.