CODEWITHSUNDEEP
opengl-esopengl-es-2.0webglvertex-shader
AlvinfromDiaspar
AlvinfromDiaspar

Reputation: 6834

Is it okay to send an array of objects to vertex shader?

Due to performance issues drawing thousands of similar triangles (with different attributes), I would like to draw all of these using a single call to drawElements. But in order to draw each triangle with their respective attributes (ex: world location, color, orientation) I believe i need to send an array buffer to my vertex shader, where the array is a list of all the triangle attributes.

If this approach is indeed the standard way to do it, then i am delighted to know i have the theory correct. I just need to know how to send an array buffer to the shader. Note that currently i know how to send a multiple attributes and uniforms (though they are not contiguous in memory, which is what im looking for).

If not, i'd appreciate it if a resident expert can point me in the right direction.

I have a related question because I am having trouble actually implementing a VBO. How to include model matrix to a VBO?

Upvotes: 0

Views: 283

Answers (1)

Matic Oblak
Matic Oblak

Reputation: 16794

You do have the theory correct, let me just clear a few things...

You can only "send vertex array" to the vertex shader via pointers using attributes so this part of the code stays pretty much the same. What you do seem to be looking for are 2 optimisations, putting the whole buffer on the GPU and using interleaved vertex data.

To put the whole buffer on the GPU you need to use VBOs as already mentioned in the comment. By doing that you create a raw buffer on the GPU to which you can put any data you want and even modify them in runtime if needed. In your case that would be vertex data. To use non interleaved data you would probably create a buffer for each, position, colour, orientation...

To use interleaved data you need to put them into the buffer sequentially, if possible it is best to create a data structure that holds all the vertex data (of a single vertex, not the whole array) and send those to the buffer (a simple primitive array will work as well). A C example of such structure:

typedef union {
    struct {
        float x,y,z; //position
        float r,g,b,a; //color
        float ox,oy,oz; //orientation
    };
    struct {
        float position[3];
        float color[4];
        float orientation[3];
    };
}Vertex;

What you need to do then is set the correct pointers when using this data. In the VBO you start with NULL (0) and that would represent the position in this case, to set colour you would have to then use ((float *)NULL)+3 or use some conveniences such as offsetof(Vertex, color) in C. With that you also need to set the stride, that would be the size of the Vertex structure so you could use sizeof(Vertex) or hardcoded sizeof(float)*(3+4+3).

After this all you need to watch for is correct buffer binding/unbinding while rest of your code should be exactly the same.

Upvotes: 1

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