What is the meaning of the term Y-shearing?

[doomlayman]

A thread I have opened about Doom graphics was closed and I couldn't reply to OpenRift talking about the concept of Y-shearing. The purpose of this post is to help me and others understand what is "Y-shearing" exactly.

 

Kyka said "You can have 2D sprites that exist in the real world, for example. This does not make the real world 2D." and for me personally it's a nice summing of the thread, but here I just want to make sure I understand what is Y-shearing. Again I don't have any above-fundamental knowledge in computer graphics, physics or math so please excuse my ignorance or any false assumption. I do know what are X and Y and Z axels or what is the difference between width, height, length and depth.

 

I have tried to look in Google Images for images that would explain what is Y-shearing but I failed to understand the concept even from the images.

 

What I did notice when testing in Heretic is that when I look up at maximum or down at maximum and shoot with the Eleven Wand projectiles are not being shoot at the ceiling or on the floor, rather on the closest point to it on the wall (one point where the wall and the ceiling coincide or where the wall and the floor coincide). Does this have anything to do with "Y-shearing"?

 

Any "accessible" explanation of the term would really help.

Edited May 30, 2023 by doomlayman

[OpenRift]

Okay, so Y-shearing is essentially where all walls that are perpendicular to the ground still show as perfectly vertical regardless of how high or low you look.

 

Here's an example from Heretic (taken in Crispy Heretic)

 

And here's a picture of a similar angle in Quake (taken in WinQuake):

 

You'll notice how in Quake, there are no vertical lines from this angle in the geometry, where as in Heretic, all vertical walls remain perfectly vertical, no matter how much you look up or down. It's a bit more noticeable in practice, but this is the best example I can think of.

[dististik]

Google is your friend. Effectively the scene is rendered "taller" than what is shown to the player and when looking up or down the game view just scrolls the rendered image due to limitations in the Doom engine's renderer.

[Ludi]

OpenRift's pics with a visualizer:

 

The lines in Quake will eventually converge, but lines in the Doom engine must stay parallel.

 

Hope this helps!

Edited May 30, 2023 by Ludi

[doomlayman]

Thanks OpenRift, I understand it from the picture and description you wrote. I have noticed the walls overlap vertically or on the Y axis |||||.

 

I understand that the height of all walls in a room in Heretic will be the same (room here is very abstract).

Edited May 30, 2023 by doomlayman