# OpenGL [Design Hiccup] Particles and Transform feedback

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noodleBowl    713

I am currently trying to finish up my particle system. The core of it is built, but I keep running into a major hiccup!

For my system I am targeting OpenGL 3.3 and using transform feedback in order to create a State Particle System.

Each of my particles in this current point in time have the following attributes: Current Position, Deviation from the Starting Position, Velocity, Starting Velocity, Acceleration, Current Lifetime, and Lifespan (starting lifetime). In addition to this, each particle is to be represented as a quad and have texture coords for their image.

My hiccup or major block comes in with the transform feedback and the output varyings.

In my particle update area, my transform feedback is using GL_POINTS to iterate over each particle where it outputs the new velocity, acceleration, etc. The major drawback/part in this update is that one run of GL_POINTS (one particle update) will output 4 positions (one for each corner of a quad). I do this to prevent floating point discrepancies between the positions, since they should be all the same and a single difference means I get a graphics glitch. After everything is said and done my VBO ends up looking like:

//All particle data is stored in the same buffer
//Particle1                                                                 //Particle2
[Position1, Position2, Position3, Position4, Velocity...][Position1, Position2, Position3, Position4, Velocity...]


My mega block / kicker

My hiccup comes in right here. While I can update the buffer above perfectly, the other major thing I need to do is read it for rendering.

This is the part I cannot seem to get down. Due to the VBO's data and its makeup, my shader cannot properly read in the data. It gets confused and ends up using a Velocity, Acceleration, etc as a position causing my particles to get rendered incorrectly

I thought about just having extra dummy data in my VBO, so that my Vertex Pointers would see my VBO data correctly.

Basically I decided to pad the attributes other than position:

//All particle data is stored in the same buffer
//Particle1                                                                                                                                                         //Particle2
[Position1, Position2, Position3, Position4, Velocity, DummyVelocity1, DummyVelocity2, DummyVelocity3...][Position1, Position2, Position3...


But I get left hooked by my transform feedback, since output varyings can only write the exact way they are configured and have no ability to offset or skip certain blocks of data. My attributes run over each other when updating, making me run into the same problem above.

I thought to combat this I could just write more/double up on my output varyings. But I get right hooked by the GL_MAX_TRANSFORM_FEEDBACK_INTERLEAVED_COMPONENTS limit as I easily am already close to the maximum.

Once I start using texture coords or even if I decide my particles need another attribute I will go over the max. Making this solution a no go

Now, I'm left kind of in the dark as to what to do. I thought about doing a double update pass, where I run though my "non render attributes" and update them, outputing the results to a VBO. Then running a secondary update, using results from the first update to update each particle's position and the output from this update would go to a third buffer and get used in the render. Something like:


//First pass: updating the velocity, acceleration, etc. Uses its own varyings
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, nonRenderVBO);
glBeginTransformFeedback(GL_POINTS);
glDrawArrays(GL_POINTS, 0, particleCount);
glEndTransformFeedback();

//Second pass: updating the positions. Uses its own varyings
glBindBuffer(GL_ARRAY_BUFFER, nonRenderVBO);
glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, positionVBO);
glBeginTransformFeedback(GL_POINTS);
glDrawArrays(GL_POINTS, 0, particleCount);
glEndTransformFeedback();

/*Later on: use the positionVBO in my render method*/



BUT I feel like this is a very horrible idea / poor solution.....

Any ideas on what I could do so I can get over this roadblock? Any help would be appreciated

Edited by noodleBowl

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