Following the ideas in OpenGL ES iPhone - drawing anti aliased lines, I am trying to draw stroked anti-aliased lines and I am successful so far. After line is draw by the finger, I wish to fade the path, that is I need to change the opacity (color) of the entire path.
I have computed a large array of vertex positions, vertex colors, texture coordinates, and indices and then I give these to opengl but I would like reduce the opacity of all the drawn triangles without having to change each of the color coordinates. Normally I would use glColor4f(r,g,b,a) before calling drawElements, but it has no effect due to the color array.
I am working on Android, but I believe it shouldn't make the big difference, as long as it is OpenGL-ES 1.1 (or 1.0).
I have the following code :
gl.glEnable(GL10.GL_BLEND);
gl.glBlendFunc(GL10.GL_ONE, GL10.GL_ONE_MINUS_SRC_ALPHA);
gl.glEnableClientState(GL10.GL_COLOR_ARRAY);
gl.glShadeModel(GL10.GL_SMOOTH);
gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
gl.glEnableClientState(GL10.GL_TEXTURE_COORD_ARRAY);
gl.glEnable(GL10.GL_TEXTURE_2D);
// Should set rgb to greyish, and alpha to half-transparent, the greyish is
// just there to make the question more general its the alpha i'm interested in
gl.glColor4f(.75f, .75f, .75f, 0.5f);
gl.glVertexPointer(mVertexSize, GL10.GL_FLOAT, 0, mVertexBuffer);
gl.glColorPointer(4, GL10.GL_FLOAT, 0, mColorBuffer);
gl.glTexCoordPointer(2, GL10.GL_FLOAT, 0, mTexCoordBuffer);
gl.glDrawElements(GL10.GL_TRIANGLES, indexCount, GL10.GL_UNSIGNED_SHORT, mIndexBuffer.position(startIndex));
If I disable the color array gl.glEnableClientState(GL10.GL_COLOR_ARRAY);, then the glColor4f works, if I enable the color array it does nothing. Is there any way in OpenGl-ES to change the coloring without changing all the color coordinates?
I think that in OpenGl one might use a fragment shader, but it seems OpenGL does not have a fragment shader (not that I know how to use one).