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Shaders and VBOs, Performance and Relation

Daniel Coelho · 2013-04-05T16:13:15

I'm following some new tutorials about OpenGL, since the ones I had done before were way outdated (like 2000ish), and I have some questions about the Shader x VBO relation. My vertex shader code is this: #version 330 core layout(location = 0) in vec3 vertexPosition_modelspace; uniform mat4 MVP; void main() { //option 1 vec4 v = vec4(vertexPosition_modelspace, 1); gl_Position = MVP * v; //option 2 gl_Position.xyz = vertexPosition_modelspace; gl_Position.w = 1.0; } (Only one of the options is enabled, I just put both here to save some space) And the drawing code is this: glBindBuffer(GL_ARRAY_BUFFER, object->_VertexID); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, object->_VertexIndexID); glEnableClientState(GL_VERTEX_ARRAY); //glVertexPointer(3, GL_FLOAT, 0, 0); //I was using this originally glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0); glEnableVertexAttribArray(0); glDrawElements(GL_TRIANGLES, object->GetTotalIndexes(), GL_UNSIGNED_INT, 0); glDisableClientState(GL_VERTEX_ARRAY); Most VBOs tutorials I find use the glVertexPointer() function, but it seems that when I have more than one array of values in the shader (like vertices, colors and normals), I need to use glVertexAttribPointer() instead, and match the first value with the location = X in the shader code. I didn't understand very well what's this relation about, does this mean the values currently bound in the C++ code are sent to the layout(location) variable in my shader code? Also, I've read a bunch of tutorials about OpenGL and topics about it, and I see plenty of them talking about matrix calculations and such, but I never bothered to understand what they meant. I had always reached the results I desired without them, but seeing I'm finding more and more about them in newer tutorials, I'm curious. For example, there's a tutorial that shows how to calculate the "view" of each object with matrices (and GLM): mat4 mProjection = glm::perspective(45.0f, 4.0f / 3.0f, 0.1f, 100.0f); mat4 mView = glm::lookAt( glm::vec3(0, 0, 0), //Camera Position glm::vec3(0, 0, 0), //Looking to Position glm::vec3(0, 1, 0) //Head Up ); mat4 mModel = glm::mat4(1.0f); //model values mat4 mMVP = mProjection * mView * mModel; Then I send the mMVP to the shader, and using the option1 in the shader, I get the correct view for the object(s) on screen. However, I was achieving the same doing this: glLoadIdentity(); glTranslatef(-_camera.targetX, -_camera.targetY, -_camera.targetZ); //Looking to Position glRotatef(_camera.RotationX(), 1.0f, 0.0f, 0.0f); //0-360 values for rotations glRotatef(_camera.RotationY(), 0.0f, 1.0f, 0.0f); glTranslatef(-_camera.posX, -_camera.posY, -_camera.posZ); //camera position //Draw Objects And with this, I used the option2 in the shader. On my machine, option2 was about 25% faster. But I don't know if the shader calculations are done by the graphics card (meaning my processor was faster at calculating them) or not (then it's the reverse). Another option I was considering was not calculating the matrices in the C++ code, but just send them to the shader and let it calculate the final options, but I'm guessing this would be even slower. I've read that I shouldn't use FPS as a "performance" check because it's inconstant, but I don't know any other way to compare methods and figure which is better. So, what's the generally better accepted method? If you have arguments on why should one use matrices instead of the glFunctions (although I've read that glRotatef() is being marked as deprecated), I'd love to hear it as well. Most tutorials and books don't say which is better, it's usually "do this and you get the results you want". I plan on developing games for mobiles so I'm really concerned about performance and also for curiosity, I'd really like to understand this better.

General and Gameplay Programming Programming

Started by Danicco March 19, 2013 08:17 PM

11 comments, last by Ameise 11 years ago

noizex

1,456

March 20, 2013 10:34 AM

Downgrade? Don't treat my disagreement as attack to you or the things you said.

I just wanted to point out that both methods should result in equal GPU instructions and performance.

Except the performence difference all the other questions already had good answers.

We all know we should not use deprecated stuff and how to put a percent value in relation with a fixed number.

Solely for complete nonsense regarding FPS measurement. I don't downvote as response to an attack or anything.

As to both methods, I agree there shouldn't be any meaningful difference, which is why I'm pointing that measuring by FPS gives you completly wrong sense of performance. So it would be wise to start by picking correct measuring method, and based on that trying to find bottleneck or just ignoring this (which I'd do, seeing such a tiny difference in miliseconds between two methods).


Where are we and when are we and who are we?
How many people in how many places at how many times?

Danicco

449

Author

March 22, 2013 11:55 PM

First off, these two options are not equal, at least in terms of the source code you pasted. First one also calculates perspective projection and lookat - both which can be calculated lazily and only when certain parameters change. If you do this every frame then its one thing thats wrong. In second example you don't even mention lookat or perspective matrix - where are these calculations?

The way you measure it is also wrong, as you said you're using frames/s and you should be using ms/frame.

Using proper matrices and sending them as uniforms (or UBO) is the right way. Plus, as you mentioned, use of glRotate and similar calls is deprecated, and thats perfect reason not to use that kind of code.

In the second example I use the basic glLoadIdentity(), glTranslatef() and glRotate() as in the last code of block with my camera object to get where I'm looking at. In the 2nd example I don't think there's any perspective projection calculation in my draw function, other than the glutPerspective() call in my initialization.

My FPS for a teapot was at about 2500ish with the 2nd option and deprecated calls, and it dropped to 2000~2100ish with the new matrices calculations. I'm going to do what @wintertime's suggested and calculate the time per function so I get better measurements.

This site explains a bit of the advantages of using matrices (Q4), though I didn't really understand the last part as I still don't get matrices at all: "However, while this may seem extravagant, the savings come from processing each vertex. Using matrix multiplication, the savings made from processing just 4 vertices, will outweigh the additional set-up cost." (I think I understand the concept, but I don't quite see it in my code yet).

This is what I think it's going on my code:




void Game::Draw()
{
    glLoadIdentity();
    glTranslatef(); //go to camera target (center)
    glRotatef(); //rotate according to the camera view in X
    glRotatef(); //rotate according to the camera view in Y
    glTranslatef(); //go to the camera's position (eye)

    //loop to draw all objects
    //I just draw the object, each of their vertexes' xyz is already defined
}

I think of this as 5 matrix calculations, and then I just draw the objects, compared to the 1st option I mentioned:




mat4 mProjection = glm::perspective(45.0f, 4.0f / 3.0f, 0.1f, 100.0f);
void Game::Draw()
{
    mat4 mView = glm::lookAt(
        glm::vec3(0, 0, 0), //Camera Position
        glm::vec3(0, 0, 0), //Looking to Position
        glm::vec3(0, 1, 0) //Head Up
    );
    mat4 mModel = glm::mat4(1.0f); //model values
    mat4 mMVP = mProjection * mView * mModel;

    //loop to draw all objects
    //each vertex is multiplied by mMVP
}

This looks like at least 3 matrix calculations, but it's calculated for every single vertex of the objects, since I send it to the shader calculate the final xyz of each vertex by multiplying it for the MVP I sent.

It looks there's much more overhead in the 2nd example, but I can't be sure as I don't know what's happening when I draw after calling the (deprecated) glMatrixFunctions(), and this is what I'm asking in this thread.

Since it's deprecated now, I think it means my assumptions about being heavier are wrong, or the performance loss is negligible, but I'd like to understand what's going on.

It's that programmer "I need to understand what's happening!" thing, I think...