> Zdog is a pseudo-3D Vector Art engine. You define your geometry using 3D splines and blobs, which render as 2D SVG shapes for any arbitrary perspective.
> For nerds—this simple translation is made possible by assuming an orthographic projection (where drawing 3D splines becomes trivially reducible to flat 2D splines—not so with perspective projection). More obvious, the occlusion-culling is achieved with a trivial drawing order or by hiding entire 2D primitives when appropriate.
So does pretty much every single 3D engine that renders to a flat surface, like a monitor.
I think it is just a misconception stemming from the 90s (notice the influence of this engine) when anything non-polygonal (and sometimes, non-GPU accelerated, depending on who you asked and how misinformed they were) was considered as "pseudo 3D".
But there is nothing pseudo here, if your scene data structures use three dimensions, you are doing 3D.
In conventional 3D graphics each pixel is painted in 3D and a depth test applied. Here, 2D shapes are ordered discretely to create a 3D effect.
Instead of the per-pixel z-fighting you conventionally see in 3D graphics, here we see entire shapes cross each other. This method cannot correctly paint intersecting, or cyclically occluding shapes (without breaking them into smaller components). I think "pseudo" could refer to the lack of generality in the system's ability to render 3D geometry.
A depth buffer is something that you find common nowadays, especially with the GPUs providing it essentially for free, but again it is not what makes 3D graphics and a lot of otherwise full 3D polygonal games were made without it nor it is necessary to create 3D graphics. With other approaches to 3D rendering you do not even need a depth buffer to get correct results for penetrating objects (e.g. classic raytracing).
What is happening here is called "painter's algorithm" and was a very common approach for realtime graphics before depth buffers became hardware accelerated. Actually it is still often used even today for transparencies since those often cannot be rendered in arbitrary order and you need to depth sort them. One common approach to solve these issues - both then and now - is to subdivide/preprocess geometry so that wrong overlaps are not or rarely happening.
Indeed. I avoided the term depth buffer for this reason. A depth test is still necessary on a per-pixel basis for a correct solution to the visibility problem in the general case - even if that's done by raytracing or some other method.
My point above is that to call something 3D graphics does not depend on having per-pixel correct visibility and painter's algorithm (and others) is a valid method to render 3D graphics and was for many years. Applications using it either accepted the limitation (many games did that) or tried to overcome it by preprocessing the geometry to create clusters that are unlikely to overlap or by performing subdivision on the projected polygons.
The other thing - this engine does orthographic projection, which preserves (IIRC) the distance aspect between points without any distortion.
It's a commonly used projection method for when you don't want scaling issues, where the need for measurement accuracy is more important (such as for CAD drawings - which inherited it from the standard technique for 3D model projection from the regular 3-view top/plan/side of pencil/paper drafting).
Most ordinary 3D engines use a true perspective projection, where distance from the view camera (or view plane) is taken into account, which causes objects (and parts of objects) to look smaller as they recede from that plane.
Yes, but that again is not what would make something "not 3D" - orthographic projection is common in 3D graphics and even most game engines support it.
If it helps, think of it as an “actual 3D” scene where objects are laid out in three dimension, but within each bounding box is just a 2D painting of that object. So wherever objects intersect they just act like flat sprites hitting each other. They also have flat (2D) lighting.
It’s very similar to a sprite based 3D engine.
For other “pseudo-3D” engines see Wolfenstein, or even Super Mario which has “actual 3D” parallax but 2D drawing within each layer.
Another example is Heretic (a Doom engine variant) which could fake looking up and down by warping tiles, but distortion increased the higher up you look.
In every case there really is real 3D projection. But it just doesn’t go all the way. It retains some 2D algorithms for some part of the rendering.
Sadly, 2.5D has basically lost all its meaning. It's sometimes meant to represent sidescrollers that use 3D polygons, or even fully 3D games that just have a certain camera angle, or any number of other things. :(
Heretic is a 3D game, as was Doom, since the world is really 3D: in addition to the 2D vectors making up the walldefs, there is a height component in each sector and entity which adds the third dimension. That some parts of the game (entity-to-entity collision, but not hitscan-to-entity which knows about elevation) ignore the 3rd dimension or that the renderer took advantage of the way the world was represented to speed up wall rendering doesn't really change that the world itself is made up of three dimensions.
Entities being 2D sprites is also irrelevant as they are just billboards and many engines even today use billboards for various reasons (particles, foliage, LOD, etc). They are also placed in 3D space.
Wolfenstein 3D on the other hand, yes, that is a fully 2D game as there is no third dimension outside the implied wall height used during rendering.
I mean, it is really simple, there isn't need to justify some old misconceptions about "2.5D" or whatever that persist due to pop culture: if your graphics data has three dimensions, you are doing 3D graphics.
This is an element of how rasterization is performed and has no implication on if graphics are 3D or not though. PS1 hardware having no perspective correct texture mapping doesn't make the games that used it "not 3D" - and on the flip side, a side scrolling 2D platformer using a rasterizer capable of perspective correct texture mapping (like any platformer that uses OpenGL or Direct3D to render its worlds as triangles) doesn't make the game use 3D graphics.
