rendering a photoshop style brush in openGL

I have lines that are programmatically defined by my program. what I want to do is render a brush stroke along them.

the way I think the type of brush I want works is, it simply has a texture, mostly transparent, and what you do is, render this texture centered on EVE开发者_JAVA百科RY PIXEL in the path, and they blend together to create the stroke.

now assuming this even works, I'm going to make a bet that it will be WAY too expensive (targeting the ipad and other mobile chips, which HATE fillrate and alpha blending)

so, what other options are there?

if it could be done in realtime (that is, the path spline updating every frame) that would be ideal. but if not, within a fraction of a second on the ipad would be good too (the splines connect nodes, the user can drag nodes around thus transforming the spline, but it would be acceptable to revert to a simpler fill for the spline while it was moving around, then recalculate the brush once they release it)

for those wondering, I'm trying to get it so the thick lines look like they have been made with a pencil. it should look as real life as possible.

I considered just rendering the brushed spline to a texture, but as the spline can be any length, in any direction, dedicating a WHOLE rectangular texture to encompass the whole spline would be way to costly...

the spline is inevitably broken up into quads for rendering, so I thought of initially rendering the brush to a texture, then generating an optimized texture with each of the quads separated and packed as neatly as possible into the texture.

but two renders to texture... algorithm to create the optimized texture, making it so the quads still seamlessly blend with each other... sounds like a nightmare, and thats not even making it realtime.

so yeah, any ideas on how to draw thick, pencil like lines that follow a spline in real time on the ipad in openGL?

From my point of view, what you want is to render a line that:

• is textured
• has the edges fade off (i.e. no sharp edge to it)
• follows a spline

To achieve these goals I would first of all break the spline up into a series of line segments that closely approximate the curve (you can make it more or less fine-grained depending on how accurate you want it to be versus how fast you want it to render).

Once you have these, you will need to make each segment into 3 quads, one that goes over the middle of the line segment that serves as the fully opaque part of the line and one on each edge of the line that will fade out to be totally transparent.

You will need to use a little bit of math to make sure that you extrude the quads along a vector that bisects 2 segments equally (i.e. so that the angle between the each segment and the extrusion vector are equal). This will ensure that you don't have gaps in the obtuse part of the join and overlaps in the acute parts.

After all of this, you just need to use the vertex positions as the UV co-ordinates (probably scaled though) and allow the texture to wrap around.

Using this method, you should end up with a mesh that has a solid thick line running through the middle of your spline with "fins" that taper off into complete transparency. This should approximate the effect you want quite closely while only rendering to relevant pixels (i.e. no giant areas of completely transparent pixels) and with very litter memory overhead.

I've been a little vague here as its kind of hard to explain with text alone and without writing an in depth tutorial. If you need more info, just comment on what your stuck on and I'll elaborate further.