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slayemin

Rendering point sprites / textured quads purely on GPU using HLSL

3 posts in this topic

I'm trying to let my graphics card process all of my billboards and point sprites by using HLSL. I'm a bit of a newbie at it, so I need a bit of help.

 

Currently, what I'd like to do is send the vertex shader an array of point sprite coordinates (ie, a list of verticies). When the vertex shader gets a vertex, I want it to know that it is the center position of the point sprite and that it needs to create the four corners of the quad based on the viewing position of the camera. First question / area of confusion: How do I create these four new vertices out of one vertex in HLSL? I'm not sure about how to allocate the memory dynamically and pass it to the pixel shader, like I would within an application.

 

Second area of confusion: How do I efficiently invoke the shader within my application? Suppose I have a billboard or point sprite class. Normally, I'd manually create my four verticies in the application and then send them down to the video card by using a "DrawUserPrimitive()" method which has an array which contains the vertex data. Easy peasy... but now, I just have one Vector3 for position information, which is inside the world matrix, so I just send the world matrix to the video card. Now, when I try to activate the shader and render the point sprite, how do I do it?

 

Here is my render code for my billboard. I'm confused on how to trigger the shader if I don't actually send it any vertices. I should just have to call "pass.Apply()", right? But it doesn't seem to work.

public BillBoard(Vector3 position, float size, Texture2D texture)
        {
            m_position = position;
            m_size = size;
            m_mode = Mode.Sphere;
            m_effect = s_effect.Clone();
            m_texture = texture;
 
            Matrix world = Matrix.CreateWorld(m_position, Vector3.UnitX, Vector3.Up);
            m_effect.Parameters["xWorld"].SetValue(world);
            m_effect.Parameters["xTexture"].SetValue(m_texture);
 
        }
 
        public int Render(Camera3D currentCamera)
        {
 
            
            m_effect.Parameters["xProjection"].SetValue(currentCamera.Projection);
            m_effect.Parameters["xView"].SetValue(currentCamera.View);
            
            m_effect.CurrentTechnique = m_effect.Techniques["PointSprites"];
 
            foreach (EffectPass pass in m_effect.CurrentTechnique.Passes)
            {
                pass.Apply();
            }
 
            return 0;
        }
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How do I create these four new vertices out of one vertex in HLSL?

 

That's exactly what geometry shaders were made for. Unfortunately, they were introduced with D3D10 and are not available to XNA's D3D9.

 

You could create a vertex buffer of 4 vertices and annotate them so that the vertex shader knows which corner of the billboard it is currently handling. Then you can move the vertex from the center of the billboard to the corner's position.

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D3D9 does support instancing.  You could make one vertex buffer that contains a single quad (either 4 verts + index buffer, or 6 verts), and then store the per-instance billboard information in another vertex buffer.

 

This is exactly what I'm trying to do. Thanks for giving me the direction I was looking for.

In case anyone else finds this thread in the future and wants to know how to instance a bunch of similar geometries on the graphics card, here are two articles which perfectly describes the technique: 
XNA 3.1: http://www.float4x4.net/index.php/2010/06/hardware-instancing-in-xna/

XNA 4.0: http://www.float4x4.net/index.php/2011/07/hardware-instancing-for-pc-in-xna-4-with-textures/

 

Blog post by Shawn Hargreaves: http://blogs.msdn.com/b/shawnhar/archive/2010/06/17/drawinstancedprimitives-in-xna-game-studio-4-0.aspx

Edited by slayemin
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