In the vertex shader program of a WebGL application, I am doing the following:
Calculate gl_Position P using a function f(t) that varies in time.
My question is:
Is it possible to store the updated P(t) computed in the vertex shader so I can use it in the next time step? This will be useful for performing some boundary tests.
I have read some information on how textures can be used to store and updated vextex positions, but is this feasible in WebGL, since even texture access from a vertex program is unsupported in OpenGL ES 1.0?
For a more concrete example, let us say that we are trying to move a point according to the equation R(t) = (k*t, 0, 0). These positions are updated in the vertex shader, which makes the point move. Now if I want to make the point bounce at the wall located at R = (C, 0, 0). To do that, we need the position of the point at t - dt. (previous time step).
Any ideas appreciated.
Regards
In addition to the previous answers, you can circumvent vertex texture fetch by PBOs, but I do not know, if they are supported in WebGL or GLES, as I have only desktop GL experience. You write the vertex positions into the framebuffer. But then, instead of using these as vertex texture, you copy them into a vertex buffer (which works best via PBOs) and use them as a usual vertex attribute. That's the old way of doing transform feedback, which I suppose is not supported.
There's no way to store anything in the vertex shader. You can only pass values from it to the fragment shader and write those to the framebuffer pixels. And as you said, vertex texture fetch isn't universally supported (for instance, ANGLE started supporting it only a few days ago), so even that is a bit unworkable.
You can do two things: either do all the position math in JS and pass in the p1 and p0 as uniforms. Or keep track of the previous time value and do the position math twice in the shader, both for t1 and t0 (shouldn't have much of a performance impact unless you're vertex shader -bound).
Is your dt a constant? if so you could retrieve the previous position for your point by evaluating
R(t-dt). If it is not a constant then you could use a uniform to pass it along on every rendering cycle.
Related
What is the final stage that is still possible to return the indexes that was not clipped or culled or occluded, and that are going to be rendered?
To answer the question asked, there isn't one. All vertex processing rendering stages happen before triangle clipping. As does transform feedback. And fragment shaders don't get vertex indices; they only get the per-vertex values from the vertex processing stage, after interpolation.
In theory, you could do something like this. Your VS outputs an integer index for the vertex, taken from gl_VertexID. You would need a GS that takes the three indices and packages them together into a flat uvec3. Each output vertex would be given the same values. And then, the fragment shader could get the uvec3 and write each of those indices out to a buffer via SSBO and an atomic counter.
Of course, you'll get the same index multiple times (assuming that triangles share indices). But you can do it.
It just doesn't serve much point. Rendering part of a mesh is a lot more trouble than it's worth. For performance, it's better to render either all of it or none of it, based on its visibility. And detecting that is best done via occlusion tests on a different, less complex shape.
This may seem like a basic question, but how can you work with/manipulate objects created with the help of shaders in OpenGL?
I am always in the need of the coordinates of different objects, to use in my host program, to create/manipulate different objects, based on those coordinates and then send them back to the vertex/fragment/geometry shader.
I have my initial vertex coordinates, that I have defined in my main program, but once they reach the geometry shader, the position is computed via:
gl_Position = projection_matrix * view_matrix * vec4(square_point,1);
EmitVertex();
And now, for example, I need to select and move them with the mouse, on the screen. But there is no easy way that I can think of getting the exact coordinates.
I've tried to do the position math in my main program, but I do not seem to get the same coordinates as the ones computed by the geometry shader. And calculating all on the CPU, is not really that optimal for the number of object that I have.
I've thought of doing some GPU->CPU data retrieve, via buffers, but there are so many object and so many coordinates, that it's relentless.
I imagine that there is another way to approach this, just that I may not have the proper knowledge of how OpenGL works.
You can use so called shader storage buffer objectsSSBO. You create them in your shader with the buffer qualifier. Then you do the necessary computation and download your data via glMapBufferRange and memcpy .
I send a VertexBuffer+IndexBuffer of GL_TRIANGLES via glDrawElements() to the GPU.
In the vertex shader I wanted snap some vertices to the same coordinates to simplify a large mesh on-the-fly.
As result I expeceted a major performance boost because a lot of triangle are collapsing to the same point and would be degenerated.
But I don't get any fps gain.
Due testing I set my vertex shader just to gl_Position(vec4(0)) to degenerate ALL triangles, but still no difference...
Is there any flag to "activate" the degeneration or what am I'm missing?
glQuery of GL_PRIMITIVES_GENERATED also prints always the number of all mesh faces.
What you're missing is how the optimization you're trying to use actually works.
The particular optimization you're talking about is post-caching of T&L. That is, if the same vertex is going to get processed twice, you only process it once and use the results twice.
What you don't understand is how "the same vertex" is actually determined. It isn't determined by anything your vertex shader could compute. Why? Well, the whole point of caching is to avoid running the vertex shader. If the vertex shader was used to determine if the value was already cached... you've saved nothing since you had to recompute it to determine that.
"The same vertex" is actually determined by matching the vertex index and vertex instance. Each vertex in the vertex array has a unique index associated with it. If you use the same index twice (only possible with indexed rendering of course), then the vertex shader would receive the same input data. And therefore, it would produce the same output data. So you can use the cached output data.
Instance ids also play into this, since when doing instanced rendering, the same vertex index does not necessarily mean the same inputs to the VS. But even then, if you get the same vertex index and the same instance id, then you would get the same VS inputs, and therefore the same VS outputs. So within an instance, the same vertex index represents the same value.
Both the instance count and the vertex indices are part of the rendering process. They don't come from anything the vertex shader can compute. The vertex shader could generate the same positions, normals, or anything else, but the actual post-transform cache is based on the vertex index and instance.
So if you want to "snap some vertices to the same coordinates to simplify a large mesh", you have to do that before your rendering command. If you want to do it "on the fly" in a shader, then you're going to need some kind of compute shader or geometry shader/transform feedback process that will compute the new mesh. Then you need to render this new mesh.
You can discard a primitive in a geometry shader. But you still had to do T&L on it. Plus, using a GS at all slows things down, so I highly doubt you'll gain much performance by doing this.
I'm using some standard GLSL (version 120) vertex and fragment shaders to simulate LIDAR. In other words, instead of just returning a color at each x,y position (each pixel, via the fragment shader), it should return color and distance.
I suppose I don't actually need all of the color bits, since I really only want the intensity; so I could store the distance in gl_FragColor.b, for example, and use .rg for the intensity. But then I'm not entirely clear on how I get the value back out again.
Is there a simple way to return values from the fragment shader? I've tried varying, but it seems like the fragment shader can't write variables other than gl_FragColor.
I understand that some people use the GLSL pipeline for general-purpose (non-graphics) GPU processing, and that might be an option — except I still do want to render my objects normally.
OpenGL already returns this "distance calculation" via the depth buffer, although it's not linear. You can simply create a frame buffer object (FBO), attach colour and depth buffers, render to it, and you have the result sitting in the depth buffer (although you'll have to undo the depth transformation). This is the easiest option to program provided you are familiar with the depth calculations.
Another method, as you suggest, is storing the value in a colour buffer. You don't have to use the main colour buffer because then you'd lose your colour or have to render twice. Instead, attach a second render target (texture) to your FBO (GL_COLOR_ATTACHMENT1) and use gl_FragData[0] for normal colour and gl_FragData[1] for your distance (for newer GL versions you should be declaring out variables in the fragment shader). It depends on the precision you need, but you'll probably want to make the distance texture 32 bit float (GL_R32F and write to gl_FragData[1].r).
- This is a decent place to start: http://www.opengl.org/wiki/Framebuffer_Object
Yes, GLSL can be used for compute purposes. Especially with ARB_image_load_store and nvidia's bindless graphics. You even have access to shared memory via compute shaders (though I've never got one faster than 5 times slower). As #Jherico says, fragment shaders generally output to a single place in a framebuffer attachment/render target, and recent features such as image units (ARB_image_load_store) allow you to write to arbitrary locations from a shader. It's probably overkill and slower but you could also write your distances to a buffer via image units .
Finally, if you want the data back on the host (CPU accessible) side, use glGetTexImage with your distance texture (or glMapBuffer if you decided to use image units).
Fragment shaders output to a rendering buffer. If you want to use the GPU for computing and fetching data back into host memory you have a few options
Create a framebuffer and attach a texture to it to hold your data. Once the image has been rendered you can read back information from the texture into host memory.
Use an CUDA, OpenCL or an OpenGL compute shader to write the memory into an arbitrary bound buffer, and read back the buffer contents
I have a list of vertices and their arrangement into triangles as well as the per-triangle normalized normal vectors.
Ideally, I'd like to do as little work as possible in somehow converting the (triangle,normal) pairs into (vertex,vertex_normal) pairs that I can stick into my VAO. Is there a way for OpenGL to deal with the face normals directly? Or do I have to keep track of each face a given vertex is involved in (which more or less happens already when I calculate the index buffers) and then manually calculate the averaged normal at the vertex?
Also, is there a way to skip per-vertex normal calculation altogether and just find a way to inform the fragment shader of the face-normal directly?
Edit: I'm using something that should be portable to ES devices so the fixed-function stuff is unusable
I can't necessarily speak as to the latest full-fat OpenGL specifications but certainly in ES you're going to have to do the work yourself.
Although the normal was modal under the old fixed pipeline like just about everything else, it was attached to each vertex. If you opted for the flat shading model then GL would use the colour at the first vertex on the face across the entire thing rather than interpolating it. There's no way to recreate that behaviour under ES.
Attributes are per vertex and uniforms are — at best — per batch. In ES there's no way to specify per-triangle properties and there's no stage of the rendering pipeline where you have an overview of the geometry when you could distribute them to each vertex individually. Each vertex is processed separately, varyings are interpolation and then each fragment is processed separately.