Mesh Topology
Hello,
Simple question : why subdivide a quad into 2 tris? Example :
Last images showing wireframes
I do not own this image, it was taken from this thread which I recommend taking a look at : http://www.polycount.com/forum/showthread.php?p=1321690#post1321690
Thanks!
Simple question : why subdivide a quad into 2 tris? Example :
Last images showing wireframes
I do not own this image, it was taken from this thread which I recommend taking a look at : http://www.polycount.com/forum/showthread.php?p=1321690#post1321690
Thanks!
Replies
But isnt it 2 polygons rather than 1?
and arent you supposed to keep the Polycount low? see what I did there?
Polygon can be a misleading term when you're trying to size up a model. One polygon can have 3 verts, one polygon can have 4 verts, one polygon can have 59371 verts. Splitting a quad into two triangles wouldn't do anything on it's own to change the vertex count, so while it sounds like there are twice as many polygons, the mesh isn't 2x heavier.
Think of the term more like the word "coin". Triangles are like pennies in this case; it doesn't get any smaller as far as 3d goes. But there are other coins too (nickles, dimes, quarters, etc). If someone says they have 50 coins in their pocket, you don't know how much money they really have with that information. If they say they have 50 pennies, you know.
http://wiki.polycount.com/PolygonCount
a triangle is a polygon
a quadrilateral is a polygon
a pentagon is a polygon
a hexagon is a polygon
and so forth.
game engines render every "polygon" as a face with the lowest number of sides possible. which is a triangle (you can't have a two sided face). so even if your model is made up of quads and hexagons, or decagons... it will ALWAYS be broken down into triangles.
therefor, counting in triangles is by far and away more accurate than counting in polygons.
Invisible or hidden edges only really apply to modeling software. Often the direction of the hidden edges is determined by the program, for example Maya forces all hidden edges to flow in a certain direction so if you want to flip a hidden edge you have to force it to be visible. In Max you can flip hidden edges any way you like but there is a strong chance that they will be reinterpreted to flow one direction when imported into something else, FBX export (common UDK export method) complains about flipped edges, so you force them to be visible or triangulate the model before exporting.
Every mesh is triangulated for games and it will sometimes determine the direction of the hidden edges. Also games count triangles not polygons. If you remove all of the visible edges from a model you haven't saved anything but crated a nightmare because you have no idea how those hidden edges will be read.
Actually games count verts more than they count triangles. Each vert on a UV seam gets doubled, some shaders, material breaks and smoothing breaks also add to the overall vertex count above what is reported in max/maya. This is one reason why you want as few UV seams and as few materials per mesh as possible.
Recommended reading about splitting up meshes: http://www.ericchadwick.com/examples/provost/byf2.html
well if we want to be REALLY pedantic, polygon means many corners, not many sides...
so pffffft!
If you want to see how these hidden edges can affect how your geometry looks, make a single quad flat on the grid, grab two vertices diagonally opposite one another, and move them up or down. You can now watch your quad crease along that hidden edge.
Also, what Mark said. Verts. > Tris.
Imagine you got four side polygon, all vertexs are in the same z level. Then move on vertex up or down. Now you got two ways the shading can be, depending on which way you cut the polygon. Try it out and you will see. Especially with low low models, its often done to get the correct shading on models without any normal maps.
This is also important for deformation, like McGreed pointed out, you expect a poly to fold a certain way and if the hidden edge flips and it folds the opposite way, then it could deform undesirably, as well as alter the tanget for the connected verts and make the normal map incorrect like metalliandy pointed out.
Everything becomes a triangle at render time.
I mean, as a rule of thumb, yes, everything is triangles, but I was just wondering...
Are you sure that this minority of engines don't use the same trick that 3d apps use to render ngons?
I'm not sure how rendering as quads would alter the vert. counts of of a mesh, but perhaps they do something I am not aware of.
I would think that the reason that everything renders as triangles, is that its the simplest geometric form that can be rendered on screen (excluding verts.) and is only dividable into other triangles. This makes is much faster then rendering quads/ngons as you are rendering the simplest object that can be rendered over and over, rather than wasting overhead creating other n sided polygons which could be generated faster by creating the same shape out of triangles in the first place.
*Edit* Eric beat me to it. Ok, so 1 engine like quads :P