cocos2d for iPhone

A fast, easy to use, free, and community supported 2D game engine

1. 

   Lars
   Member

   Hi there,
   I'm experimenting with modifing textures in OpenGL in a cocos2s environment: I'm rendering a grid from triangles (gl_triangle_strips + texture coordinates) to display an image on the screen.

   The user can modify the texture by touching and moving on the screen (water ripples, etc.). For this operation, I modify the texture coordinates of the grid. In most cases this looks nice (fig. 2), but for some directions there are ugly "stairs" in the texture (see fig. 1): http://dev.nulldesign.de/fluids.png

   I'm pretty sure that I would expect stairs everytime I modify the texture, because the coordinates get distorted a lot. So I'm just wondering: Is there a better way to modify the texture or somehow smooth the result?

   Cheers,
   Lars

   Posted 1 year ago #
2. 

   patrickC
   Member

   When deforming the vertices you need to make sure no edges overlap.
   At the moment you have hundreds of overlapping faces and that's what's causing those "steps" or artifacts.

   You'll need to find a way so that every vertex "pushes" the next one in line rather than freely dragging vertices around.

   I did something similar using a series of bodies and elastic distance joints in box2d, but the mesh was much less detailed and already performance was suffering too much.

   Two possible solutions:
   A) Write a custom FFD (searching for "2d grid ffd" on google gives a decent starting point)
   B) Write a custom verlet integrator to calculate forces between vertices in the grid so no edges overlap

   Depending on what you're trying to achieve, I think A would be easier and faster to compute, but solution B would allow you to add "forces", which might be what you're after since that looks like a fluid simulation

   Good luck ;)

   Posted 1 year ago #
3. 

   Lars
   Member

   Thanks Patrick,
   I thought already that I have to put a bit more effort into this ;). I'll do some research and think about implementing some sort of FFD. Right now my fluid simulation is running with 60fps on the 3GS and I'd like it to stay at this framerate...

   Posted 1 year ago #
4. 

   Lam
   Member

   @patrickC and @Lars
   Wow thanks for the hint for solution A!

   I also implemented fluids from Jos Stam's article on Real-Time Fluid Dynamics for Games by accessing pixel values on the texture directly but it was too slow for large texture sizes.

   So I threw that in the back burner and will probably release the code sometime as a demo.

   But now the ffd has got me thinking again. I posted a more efficient deCasteljau bezier spline class a few weeks or so to the forum and now I think I can make use of the function to create the mesh ffd. Maybe I should play around with a fluid solver using texture mapping.

   Posted 1 year ago #
5. 

   patrickC
   Member

   Definitely the way to go Lam.
   I'm actually very interested in the topic as I'd been planning for a while to use a similar technique to render backgrounds in "the" game that I have planned for "the" future.

   Being able to create a class that handles a textured grid and manipulate the vertices with a fast FFD would allow us to create all sorts of neat visual effects with little performance costs.

   If you're planning on having a go at it and could do with some support let me know.

   Posted 1 year ago #
6. 

   manucorporat
   Member

   How do you draw a grid with primitives that you can deforme?
   Could you release a little of your code? I don't go to use it for develop a similar app. Don't worry.

   Posted 1 year ago #
7. 

   Steve Oldmeadow
   Administrator

   cocos2d already has code that deforms texture grids - that is how the effects work. What slows down effects is that they update the texture that they are attached to. If you don't update the texture the speed is very good. Riq has a feature called "lightweight effects" on the horizon which is basically an effect attached to a static texture.

   Posted 1 year ago #
8. 

   manucorporat
   Member

   yeah, I don't want to use FBO, only deforms a texture.

   Posted 1 year ago #
9. 

   Steve Oldmeadow
   Administrator

   The code is the same for deforming the vertices. An fbo is attached to a texture. All the code you need is sitting in cocos2d right now.

   Posted 1 year ago #

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