How to manipulate polygons and vertices realtime

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Gnarlyman 101 Jan 14, 2012 at 17:27

Hey hey,

I’m new at all this, and while I understand everything quite well in theory, I’m not too good with verbiage yet. I.e., conceptually, I really understand all the bits and pieces. However, if I use such labels as “pipeline” a bit out of place, forgive. Also, I don’t know if the following Q will sound odd, but I really do know what I’m doing with it, and I’d like to be able to pull it off. So…

I’m tinkering around with polygons and such on the rendering pipeline, and I wanted to know how you could go about manipulating the individual vectors from coordinate to coordinate that form the polygon faces. What I’d like to do is have the polygon matrices all lined up (i.e., the world all the way to the projection, the final “2D” image if you will) and then mess around with the polygons vectors in between the vertices. Actually, I would like to mess around with the vector lines before that stage in the pipeline if that’s possible.

Basically, I would like to find out how to interface with the lines drawn between vertices on any stage of the pipeline, do my own math on them real-time, and also manipulate the art styles of the vertex-to-vertex vectors as well.

Thanks for any help out there!

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Reedbeta 167 Jan 14, 2012 at 18:30

I’m having a hard time understanding what you’re talking about. Maybe some kind of a mocked-up screenshot would help communicate what you’re trying to do?

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Gnarlyman 101 Jan 14, 2012 at 18:50

Perhaps this is a better explanation…

Say you take the vertices 4,3 and, say, 2,1 of a square and the vector line drawn between them. Is there a way to say to the engine, “take these vertices, place one more in between them that I will give to you by this mathematical function, and draw two vectors in lieu of one”? And, is there a way to add artistic “adjustments” to vectors drawn between coordinates, not just using the polygon edges as placeholders, if you will, for the spaces they describe?

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Reedbeta 167 Jan 14, 2012 at 23:52

Hmm. It sounds like what you want to do is along the lines of subdividing a mesh - cutting up its polygons (or in your case, apparently, cutting up lines) into smaller ones and then applying displacements to the new vertices generated by this. There are a lot of ways to do this; which is the best way will depend on what you’re trying to do. One way is to do it all on the CPU and include all the subdivided, displaced vertices in the vertex buffer you submit to the GPU. If the geometry must be animated, you would use a dynamic vertex buffer that’s refilled each frame. Alternatively, you might use a geometry shader on the GPU, or even the hardware tesselation shaders if you have quite recent video hardware.

I still don’t know what you mean by “artistic adjustments to vectors” (I think by vectors you really mean line segments) or “placeholders for the spaces they describe”.

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Gnarlyman 101 Jan 15, 2012 at 04:44

Thanks Reedbeta, def going in the right direction. Excellent. Really appreciate your great help and advice. I’m getting the gist of what you’re saying, and I think that’s pretty much what I’m getting at. Half of my problem is figuring out the exact terminology and classifications of what I’m seeking to do, which is very defined and thought-out conceptually in my mind and on paper.

I’m looking to do quite a lot with what must be called “displaced vertices” and such; basically a lot of math somewhere directly prior to the actual frame display. The thing is, I want as few polygons as possible, but I’d like to do adjustments to the polygons somewhere along the render pipeline (and where is something I’m trying to figure out). One of the things I’m struggling with is dealing with what are inherently “structural” things and not actual display things. I.e., usually, the line segments (sorry! That’s what I meant by “vectors”, learning the correct lingo here) of meshes exist in the undisplayed mesh matrix world of the engine, but not in the actual frame on the screen, where they’re “colored over” and not necessarily used as lines themselves.

Firstly, I’d like to use certain line segments (“outline” ones, basically”) to draw comic-book style “ink” lines. This would obviously vary frame to frame and require (I’m assuming) algorithms to determine which polygon vertices and line segments are actually the current outline ones. It would also mean, of course, that I’m seeking to directly use the usually non-displayed polygon vertices as art tools themselves (black lines), and that I’m seeking to have artistic control over how they’re displayed (what I meant by artistic adjustment to vectors). I’d also have to develop algorithms of some sort to “color in” the spaces made by the outlines (what I meant by placeholders basically), and I’d like to use the polygon points in between the “outline” line segments to determine the shading and such, as those points would obviously contain info about how far various portions of the in-between-the-lines segments protrude inwards or outwards in the mesh in that particular frame. I’m also seeking to figure out how to dynamically create polygon “points” that would generate temporary polygons over a certain area of a mesh. I.e., if for a brief moment a mesh needs to bend or morph in a certain section of its body, I want to be able to create said “points” and have either shading data and/or literal polygons and vertices be created, if only temporarily for the movement.

Hope that all makes sense…? If it doesn’t, let me know and I’ll go at it again; I know exactly what I’m going at, but I don’t know if it’s coming across very clearly…
Thanks so much for your help!
I’m really appreciating devmaster.net and the forums; by far the best dev forum I’ve been on. Thanks again. Great stuff here.

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Reedbeta 167 Jan 15, 2012 at 08:44

Ahh, outlining. Something you may want to google is “non-photorealistic rendering” or NPR. There’s actually a good amount of research out there about rendering in artistic styles, making images look painted, inked and so forth.

One pretty simple way to do outlining is to draw two copies of the mesh; one is drawn normally, with full color, lighting shading, and the second copy is drawn all in black, with backface culling reversed (so that it draws back faces but not front faces) and is slightly expanded by pushing the vertices outward along their normal vectors. This offsetting can be done dynamically in the vertex shader quite easily. The result is that since the front faces of the second draw are missing, it doesn’t occlude the surfaces from the first draw, but at the edges you can see a bit of the back side of the expanded mesh.

Doing it this way, the outline would automatically update based on camera position and movement of the object. You could alter the color and thickness of the outline per-vertex as well, if that’s what you wanted to do. I think you would need pretty highly tesselated meshes to make this look good from all angles, though.

Another option for outlines is to run an image-space postprocess that detects edges using a Sobel filter and darkens them. That can also be done in a pixel shader using what’s called a full-screen pass.

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Gnarlyman 101 Jan 15, 2012 at 19:34

Ah ha, thanks a bunch Reedbeta. That’s terrific stuff to chew on. Def going to track all that stuff down. I hadn’t thought about approaching my aims with those techniques. I’m still curious, is there a way to “draw” line segments themselves? Use them as art lines, if you will, themselves, and manipulate the art? I gather that’s not necessarily a common aim, but I do figure that there’s a way to do it.

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Gnarlyman 101 Jan 15, 2012 at 19:36

PS–in reference to your above post, I’m assuming that those are essentially the core techniques of the art of a game like Borderlands? That game is fairly close to the look I want. I just want a few additional abilities regarding line segments and art control.

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Reedbeta 167 Jan 15, 2012 at 20:28

Yes, the GPU can draw line segments directly and even run pixel shaders on the lines. I don’t know specifically how Borderlands works but after doing a google search and reading some forum posts, it seems that people think it is being done as a postprocess. I did not find anyone from the company explaining how it’s done, though, so people could be mistaken.

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Gnarlyman 101 Jan 16, 2012 at 00:15

Thanks for the research Reedbeta, appreciate it. It seems that you are indeed affirming what I’ve hoped for, that I can use the line segments are art lines (like “brush strokes”), and that I can do some art on them too. That, combined with the other techniques you’ve mentioned, give me a lot to work with. Do you have any idea with sort of functions in APIs deal directly with line segments? Or do you just take vertex and line segment data from polygon data and..I dunno, write your own quasi-wrappers…algorithms…whatever?

Thanks again for all your help. This place is great.

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Reedbeta 167 Jan 16, 2012 at 00:57

Well, making lines look like brush strokes is going to be challenging. When I say GPUs can draw lines, I mean that they basically give you what the line tool does in MSPaint. They’re a fixed width and you have to give a list of vertices which the lines will traverse, connect-the-dots style, and each line is basically a long, thin rectangle. While you can run a pixel shader to generate different colors along the length of the line, it doesn’t sound like that’s enough to do what you want - although it’s hard to judge without seeing something concrete, like a Photoshop mockup of the kind of image you hope to create. You might have to do your brush-strokes by drawing quad strips and mapping a texture along the strip, or something like that. Basically, literal points and lines are second-class citizens in the GPU world, and you get the greatest flexibility by using polygons, which is what it’s really designed for.

Before going too much further with your project, I’d strongly suggest that you roll up your sleeves and dive into the NeHe tutorials and work through those until you have a more concrete idea about how 3D graphics APIs work. There’s just so many details and concepts that I won’t be able to explain in the space of a forum post, and which you’ll learn much faster by trying out and doing yourself than by reading about them in generalities.

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TheNut 179 Jan 16, 2012 at 04:00

It’s been a while since I touched cell shading techniques and felt like whipping up a quick demo illustrating sobel edge detection. You can view the shader source here (nothing special really). Both rendering techniques, post-process edge detection and back-face rendering, have flaws though. Accurate edge detection requires and image with high contrast. If your image lacks contrast, you can end up with some less than desirable results. In my particular demo, I rendered the first pass using full ambient lighting, which is why only the silhouette edges are rendered and not the interior edges. Had I included lighting in the first pass, some of the 3D models with low polygons would end up looking like wireframes once the edges are rendered. It just requires some careful maths to get around. Rendering the edges with back-face polygons is another technique that works well all-around, but you can run into problems with low detail models or viewing models at certain angles.

I think it’s possible to do clean looking brush strokes if you can render the edges as lines instead of reverse engineering that from models or images. You can view a couple neat brush algorithms on Mr. Doob’s Harmony web app.

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Gnarlyman 101 Jan 16, 2012 at 05:23

Reedbeta:
I see. Yeah, that’s basically what I’m trying to do. I simply want to have total access to line segments so I can take the data and write a heck of a lot of my own shaders to do a bunch of things to them. And yes, I’m looking into those NeHe tutorials. They look awesome, planning to rip through most of them shortly.

T.N.:
Wowzers, thanks! That’s a lot of great info to take in. Ideally, I’d like to do what you said, take some algorithms a la Mr. Doob’s example there and apply them to outline line segments in order to achieve a comic pen/ink/brush look with slight imperfections and variable width.

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Gnarlyman 101 Jan 16, 2012 at 05:29

PS–how do I uploaded images to a post here from my comp? I’m trying to load a few images from my comics… In the meantime, my reference styles, very close to my own, are the B&W linework from Calvin and Hobbes (i.e., what I’d like to develop line segment shader algorithms for) and, as regards the coloration, both the watercolor images Bill Watterson did in his books and the matte coloration found in the Tintin comics. Sort of a medium between the two.

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Reedbeta 167 Jan 16, 2012 at 06:08

You can upload your images to ImageShack or another free image host, and then insert them in a forum post using that URL (there’s an image button on the toolbar here).

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geon 101 Jan 16, 2012 at 10:54

I’d recommend to use imgur.com. It works better than the others, with less hassle.

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Gnarlyman 101 Jan 16, 2012 at 23:00

Sweet, thanks.

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