I'm considering how to efficiently render 2D sprites. I'm trying to keep things forward compatible for OpenGL 3+, but I'm limited to OpenGL 2.

I'd have a mixture of static and dynamic sprites.

• Some sprites would be completely static; e.g. map tiles
• Moving sprites would have dynamic transforms
• Animated sprites would have dynamic texture coordinates; I'm using a texture atlas
• A sprite may be both moving and animated (of course)
• A sprite may be able to move but only does so infrequently; e.g. doors that swing only when opened
• The lifetime of a sprite may be dynamic; some sprites may exist for the whole duration of the game, others may be added and later removed from the scene mid-game

A strategy I've used is having a global VBO representing a single unit-sized quad. This unit quad is rendered multiple times for each sprite, where I provide my shader a transformation matrix as well as offset and scale uniforms for the texture coordinates.

I've read that batching sprites, where I get the world-space coordinates and final texture co-ordinates of all sprites and jam them into a single VBO, is normally the way to go performance-wise. The simplest(?) batching method that I understand is using a GL_STREAM_DRAW VBO that gets the vertex data of all sprites with a glBufferData call each frame, possibly using an additional GL_STATIC_DRAW VBO with all the sprites that I know are static and persistent.

Would sprite batching be significantly more performant than my unit-quad VBO approach? If so, is the method of sprite batching I described an efficient implementation of batching given my requirements for sprite behaviour?

The simplest(?) batching method that I understand is using a GL_STREAM_DRAW VBO that gets the vertex data of all sprites with a glBufferData call each frame, possibly using an additional GL_STATIC_DRAW VBO with all the sprites that I know are static and persistent.

Would sprite batching be significantly more performant than my unit-quad VBO approach?

Use glBufferSubData though - from what I understand, glBufferData destroys & re-creates the buffer each time with an overhead cost.

From what I can tell, since glBufferSubData doesn't allocate space (I think), I would effectively have a maximum size for my VBO?

Yup, so you just fill/draw, fill/draw, multiple times per frame if need be, in groups of that maximum size.

OK. That'll also save me from redefining the index buffer.

Though what's the proper way to fill the vertex buffer when I have less than the maximum number of sprites? Would I call glBufferSubData once to upload my remaining vertices and then call glBufferSubData a second time to fill the rest of the buffer with nulls? Would my index buffer be affected?

spriteVertex[] vertices = new spriteVertex[BATCHSIZE * 4];
int[] indices = new int[BATCHSIZE * 6];

MakeBuffers(ref vertBufferID, ref indBufferID, ref vaoID);

// fill the buffer with default (0s) on creation
glBufferData(BufferTarget.ArrayBuffer, sizeof(spriteVertex) * vertices.Length, vertices);

// fill index buffer with pre-calc'd values
PreCalcIndexBuffer();

...

class TileBatch
{
  public void Begin(Texture2D texture)
  {
    currentSprite = 0;
    currentTexture = texture;
  }

  public void Draw(Rect dest, Rect src, Color color)
  {
    // if we are out of room, flush (draw) the current batch
    if(currentSprite > BATCHSIZE)
    {
      flush();
      currentSprite = 0;
    }

    // calc all the vertex attributes for this sprite and store them in our CPU array
    int vertStart = currentSprite * 4;
    vertices[vertStart].position.X = dest.X;
    vertices[vertStart].texcoord.X = src.X;
    ...
    vertices[vertStart + 3].position.Y = destRect.Bottom;
    vertices[vertStart + 3].texcoord.Y = src.Bottom;
    
    currentSprite++;
  }

  public void End()
  {
    Flush();
  }

  private void Flush()
  {
    // set program uniforms and state
    shaderProgram.Texture = currentTexture;
    shaderProgram.mvpMatrix = currentMVP;
    ...

    int numberToDraw = currentSprite;

    BindVAO();
    
    // upload our CPU vertex data to GPU
    glBufferSubData(BufferTarget.ArrayBuffer, 0, sizeof(SpriteVertex) * numberToDraw, vertices);
    
    // draw the appropriate number of sprites
    DrawElements(BeginMode.Triangles, numberToDraw);
  }
}

MyBatch.Begin(myTexture);

MyBatch.Draw(Rect(0,0,32,32), Rect(50,50,32,32), Color.White);
MyBatch.Draw(...)
...

MyBatch.End();

specifying the count or whatever parameter it is to set the amount of data to upload

Oh yeah. I forgot about the count parameter in glDrawElements.

Thanks.