Draw multiple shapes in one vbo - c++

Question

I want to render multiple 3D cubes from one vbo. Each cube has a uniform color.
At this time, I create a vbo where each vertex has a color information.
Is it posible to upload only one color for a one shape (list of verticies)?
I'm also want to mix GL_TRIANGLES and GL_LINES in the glDrawElements-method of the same shader. Is it posible?
//Edit : I only have OpenGL 2.1. Later I want to build this project on Android.
//Edit 2:
I want to render a large count of cubes (up to 150.000). One cube has 24 verticies of geometry and color and 34 indices. Now my idea is to create some vbo's (maybe 50) and share out the cubes to the vbo's. I hope that this minimizes the overhead.

Answer 1

Drawing lots of cubes
Yes, if you want to draw a bunch of cubes, you can specify the color for each cube once.
Create a VBO containing the vertexes for one cube.
// cube = 36 vertexes with glDrawArrays(GL_TRIANGLES)
vbo1 = [v1] [v2] [v3] ... [v36]
Create another VBO with the view matrix and color for each cube, and use an attribute divisor of 1. (You can use the same vbo, but I would use a separate one.)
vbo2 = [cube 1 mat, color] [cube 2 mat, color] ... [cube N mat, color]
Call glDrawElementsInstanced() or glDrawArraysInstanced(). This will draw the cube over and over again.
Alternatively, you can use glUniform() for each cube, but this will limit the number of cubes you can draw. The above method will let you draw thousands, easily.
Mixing GL_TRIANGLES and GL_LINES
You will have to call glDraw????() once for each type of primitive. You can use the same shader for both times, if you like.

Answer 2

Regarding your questions :
Is it possible to upload only one color for one shape ?
Yes , you can use a uniform instead of a vertex attribute(ofc this means changes in more places). However, you will need to set the uniform for each shape, and have a different drawcall for each differently colored shape .
Is it possible to mix GL_TRIANGLES and GL_LINES in the glDrawElements ?
Yes and no. Yes , but you will need a new drawcall (which is obvious). You cannot do on the same drawcall some shapes with GL_TRIANGLES and some shapes with GL_LINES.
In pseudocode this will look like this :
draw shapes 1,2,10 from the vbo using color red and GL_TRIANGLES
draw shapes 3,4,6 from the vbo using color blue and GL_LINES
draw shapes 7,8,9 from the vb using color blue and GL_TRIANGLES

Answer 3

With OpenGL 2.1, I don't think there's a reasonable way of specifying the color only once per cube, and still draw everything in a single draw call.
The most direct approach is that, instead of having the color attribute in a VBO, you specify it directly before the draw call. Assuming that you're using generic vertex attributes, where you would currently have:
glEnableVertexAttribArray(colorLoc);
glVertexAttripPointer(colorLoc, ...);
you do this:
glDisableVertexAttribArray(colorLoc);
glVertexAttrib3f(colorLoc, r, g, b);
where glDisableVertexAttribArray() is only needed if the array was previously enabled for the location.
The big disadvantage is that you can only draw cubes with the same color in one draw call. In the extreme case, that's one draw call per cube. Of course if you have multiple cubes with the same color, you could still batch those into a single draw call.
You wonder whether this is more efficient than having a color for each vertex in the VBO? Impossible to say in general. You'll always get the same answer in cases like this: Try both, and benchmark. I'm skeptical that you will find it beneficial. In my experience, it's fairly rare for fetching vertex data to be a major performance bottleneck. So cutting out one attribute will likely no give you much of a gain. On the other hand, making many small draw calls absolutely can (and often will) hurt performance.
There is one option you can use that is sort of a hybrid. I'm not necessarily recommending it, but just in the interest of brainstorming. If you use a fairly limited number of colors, you can use a single scalar attribute in the VBO that encodes a "color index". Then in the vertex shader, you can use a texture lookup to translate the "color index" to the actual color.
The really good options are beyond OpenGL 2.1. #DietrichEpp nicely explained instanced rendering, which is an elegant solution for cases like this.
And no, you can not have lines and triangles in the same draw call. Even the most flexible draw calls in OpenGL 4.x, like glDrawElementsIndirect(), still take only one primitive type.