Question

How to create a fill effect? I have an irregular closed shape created using:

   glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
        (.......) 
   glVertexPointer(2, GL_FLOAT, 0, vertexBuffer);
   glDrawArrays(GL_POINTS, 0, vertexCount);
        (.......) 
   glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);

Now I would like to have an “fill/paint bucket” effect like in photoshop. The background inside the shape is white (for example) and by clicking inside the shape I want to change the color to red or green.

Can somebody give me some hints, please.

Answer 1

So you render a set of points and want OpenGL to “magically” fill the enclosed region. That’s not possible. OpenGL doesn’t realize that these points enclose some region. You don’t even draw a line strip, it’s just a set of points. Even a human has to put in a reasonable effort of thinking to see that the points “enclose” a region, let aside a computer, or just a simple interface for drawing points, lines and triangles onto the screen.

Instead of drawing points, just draw a polygon (use GL_POLYGON or GL_TRIANGLE_FAN instead of GL_POINTS). But if the enclosed region is non-convex that won’t work in all cases. What you always have to realize is, that OpenGL is nothing more than a drawing API. It just draws points, lines and triangles to screen. Yes, with fancy effects, but all in all it just draws simple primitives. It doesn’t manage any underlying graphics scene or something. The moment a primitive (like a single point, line or triangle) has been drawn, OpenGL doesn’t remember it anymore.

What you want to achieve (given you at least change the point set to a line loop that really encloses a region), is not to be achieved by simple means. In the simplest case you need some kind of flood fill algorithm that fills the region you enclosed by the lines. But for this you don’t profit from OpenGL in any way, as this requires you to analyse the image on the CPU and set individual pixels. And neither can shaders do this in a simple (or any?) way.