I think I understand what you’re getting at regarding curvature “options”. With most shaders that support curvature you get a straightforward option of “X” curvature and “Y” curvature values, but - and I’m not a mathematician here - these options result in a “linear” curve… you don’t have any control over the shape/profile of the curve. Royale gives you additional controls to modify the curvature profile… like, for example, moving the control handles of a bezier curve in a vector graphics program like Adobe Illustrator.

But what are these curves supposed to represent in the first place? Well, they’re supposed to give the the viewer the “illusion” of looking at a 3-dimensional CRT tube/screen with perspective (depth). On our modern flatscreen LCD displays, these curves are physically placed upon a flat orthographic plane (since the screen is flat) and the curves exist on the X and Y axes only. There is no actual depth, so you can only simulate depth. On a real CRT television, the tube/screen itself is curved and so the “curves” exist on not only the X and Y axes, but also Z (depth). An advanced shader such as Royale may give you options to adjust the effect of depth/perspective by calculating its maths in 3 dimensions, rather than two, by taking the Z axis into account.

Also, it’s worth bearing in mind that there are 2 primary types of CRT screens out there, and they have different types of curvature: aperture-grille (Trinitron) style CRTs and shadow mask (other/generic) style CRTs. Aperture-grille CRTs use “cylindrical projection” tubes/screens and shadow mask CRTs tend to use “spherical projection” tubes/screens. This will result in much different-looking curves. A “cylindrical projection” screen, in real life, doesn’t actually have any curvature on either the X or Y axis… but it does have curvature on the Z (depth) axis. Think of it as a straight-edged 4:3 rectangular box “projected” onto a curved glass cylinder. The way to “fake” this appearance in a 2D flatscreen shader is to disable curvature on the X (width axis) and add just a bit of curvature on the Y (height) axis… the higher the Y axis curvature value, the deeper perspective/depth you are simulating… but, of course, having additional Z axis controls in an advanced shader like Royale allows you simulate this effect even more accurately by, for example, adjusting the radius of the cylindrical CRT tube.

On shadow mask style CRT with spherical projection, think of the CRT tube/screen as being cut out of a large-radius glass ball/sphere (rather than a cylinder). The greater the spherical radius, the less apparent screen curvature. In the real world, the curvature along the edges of the this type of screen exist on all 3 axes: X (width) Y (height) and Z (depth). Simulating this effect in a shader on a flatscreen monitor, you definitely want to use curvature on at least the X and Y axes (the default option in most shaders). Any advanced shaders that give you more options will allow you to dial in the additional perspective/depth effects.

Long story short, if you are trying to *accurately* represent a real-world CRT effect on your flatscreen monitor, you are currently more-or-less limited to simulating an aperture-grille Trinitron style cylindrical projection CRT (Trinitrons/PVMs/BVMs - that sort of thing). This means you’ll turn off any curvature on the X axis, and use just a bit of curvature on the Y axis. If you have access to the depth axis in an advanced shader, you can choose to tweak that a bit too.

If you’ve got a bit higher-end monitor, such as a 4K HDR-capable display (you need a lot of resolution and brightness!), you can now also approximate a shadow mask style CRT with… passable (though still highly-compromised)… result, depending on how OCD you are about these sort of things. In this case, use both X and Y curvature in your shader (and additional tweaks if you are using Royale).

And finally, if you don’t care too much about accuracy, or you’ve never owned or seen a CRT TV (hey, there are adults born after 2k these days!)… well, then… there are no “rules”. Do whatever looks good to you (I see TONS of “CRT-style” screenshots with heavy usage of both X and Y curvature, using aperture-grille style masks/scanlines… so what if this combo isn’t something you really see!?).

I realise this was a bit of a deep dive on CRT, but hopefully there’s some useful info to consider. As many other forum goers have commented over the years, ironically it’s much more straightforward to emulate a high-quality/enthusiast/professional grade late 90s-to-early2k model CRT than it is to emulate some off-the-shelf mid-80s-era bargain-bin-grade CRT!