My question is closely related with this one
So this seems like I can't use glDrawElements when I want flat shading for instance(or making one face in the mesh with specific color for ex.), coz I need different normals(colors) for the same vertex (one normal value for each triange(face) in which this vertex participate), right? So glDrawArrays(banch of tris) - the only way for such things, right?
Thx.
You need to repeat the vertices or remove that face from your glDrawArrays and run a second one with only just that face (but with different normals). There is not flat shading in OpenGL ES 2.0
Related
I've been trying to render silhouettes on CAD models with webgl. The closest i got to the desired result was with fwidth and a dot between the normal and the eye vector. I found it difficult to control the width though.
I saw another web based viewer and it's capable of doing something like this:
I started digging through the shaders, and the most i could figure out is that this is analytical - an actual line entity is drawn and that the width is achieved by rendering a quad instead of default webgl lines. There is a bunch of logic in the shader and my best guess is that the vertex positions are simply updated on every render.
This is a procedural model, so i guess that for cones and cylinders, two lines can always be allocated, silhouette points computed, and the lines updated.
If that is the case, would it be a good idea to try and do something like this in the shader (maybe it's already happening and i didn't understand it). I can see a cylinder being written to attributes or uniforms and the points computed.
Is there an approach like this already documented somewhere?
edit 8/15/17
I have not found any papers or documented techniques about this. But it got a couple of votes.
Given that i do have information about cylinders and cones, my idea is to sample the normal of that parametric surface from the vertex, push the surface out by some factor that would cover some amount of pixels in screen space, stencil it, and draw a thick line thus clipping it with the actual shape of the surface.
The traditional shader-based method is Gooch shading. The original paper is here:
http://artis.imag.fr/~Cyril.Soler/DEA/NonPhotoRealisticRendering/Papers/p447-gooch.pdf
The old fashing OpenGL technique from Jeff Lander
So, I am working on an OpenGL ES 2.0 terrain rendering program still.
I have weird drawing happening at the tops of ridges. I am guessing this is due to surface normals not being applied.
So, I have calculated normals.
I know that in other versions of OpenGL you can just activate the normal array and it will be used for culling.
To use normals in OpenGL ES can I just activate the normals or do I have to use a lighting algorithm within the shader?
Thanks
OpenGL doesn't use normals for culling. It culls based on whether the projected triangle has its vertices arranged clockwise or anticlockwise. The specific decision is based on (i) which way around you said was considered to be front-facing via glFrontFace; (ii) which of front-facing and/or back-facing triangles you asked to be culled via glCullFace; and (iii) whether culling is enabled at all via glEnable/glDisable.
Culling is identical in both ES 1.x and 2.x. It's a fixed hardware feature. It's external to the programmable pipeline (and, indeed, would be hard to reproduce within the ES 2.x programmable pipeline because there's no shader with per-triangle oversight).
If you don't have culling enabled then you are more likely to see depth-buffer fighting at ridges as the face with its back to the camera and the face with its front to the camera have very similar depths close to the ridge and limited precision can make them impossible to distinguish correctly.
Lighting in ES 1.x is calculated from the normals. Per-vertex lighting can produce weird problems at hard ridges because normals at vertices are usually the average of those at the faces that join at that vertex, so e.g. a fixed mesh shaped like \/\/\/\/\ ends up with exactly the same normal at every vertex. But if you're not using 1.x then that won't be what's happening.
To implement lighting in ES 2.x you need to do so within your shader. As a result of that, and of normals not being used for any other purpose, there is no formal way to specify normals as anything special. They're just another vertex attribute and you can do with them as you wish.
Some background:
I am very new to OGL. My application concerns itself with 2D only. All objects are normal to the viewing direction, and I use orthographic projection. I find that the performance of the system is limited by the number of draw* calls indicating that I need to batch more.
There is only one object that I need to draw, but it consists of thousands of triangles that potentially overlap. I have the ability to pre-compute geometry in my particular application and order the triangles back to front since they have varying degrees of transparency. The vertex attribute consists of the color (only) including alpha that is used in the fragment program.
What I've done:
All the primitives are triangles and I assign the 3 vertices of each triangle the same color since the color is constant across a face. I put all of the vertices, for all triangles, and their colors into a single VBO (16-bit; there aren't that many vertices). The index buffer orders the triangles back to front and I issue a single draw call. I use alpha blending (SRC_ALPHA, ONE_MINUS_SRC_ALPHA).
Result:
I see that the result is correctly blended and rendered on the only machine that I possess and test on. I have not tried it on others. I've searched for quite some time, but in vain, for some definitive answer. BTW, the only reference is in the VBO extension spec where there is a mention of a "sequence of primitives" but it does not address what happens when the primitives overlap.
Question:
Is this the guaranteed behavior? That is will the result be the same as issuing multiple calls within glBegin(...) and glEnd(...) in immediate mode (which is guaranteed by the standard)?
Note: Depth buffer and stencil buffer are turned off.
It is guaranteed by the OpenGL specification that primitives will be rendered in the order provided. Each primitive pulled from a glDraw* command will be rendered in the order specified by its component vertices.
So yes: if you put the triangles in an order, that's the order you'll get them out when you render them.
Is this easy to do? I don't want to use texture images. I want to create a rectangle, probably of two polygons, and then set a color on this. A friend who claims to know OpenGL a little bit said that I must always use triangles for everything and that I must use textures for everything when I want it colored. Can't imagine that is true.
You can set per-vertex colors (which can all be the same) and draw quads. The tricky part about OpenGL ES is that they don't support immediate mode, so you have a much steeper initial learning curve compared to OpenGL.
This question covers the differences between OpenGL and ES:
OpenGL vs OpenGL ES 2.0 - Can an OpenGL Application Be Easily Ported?
With OpenGL ES 2.0, you do have to use a shader, which (among other things) normally sets the color. As long as you want one solid color for the whole thing, you can do it in the vertex shader.
I want to convert mouse's current X and Y coordinates into the 3D space I have drawn in the viewport. I need to do this on the OpenGL ES platform. I found following possible solutions implemented in OpenGL, but none fits what I am looking for.
I found NeHe's tutorial on doing exactly this, but in traditional OpenGL way. It uses gluUnProject.
http://nehe.gamedev.net/data/articles/article.asp?article=13
Although gluUnProject is not available in OpenGL ES, its implementation seems simple enough to port back. But before calling it, we need to call glReadPixels with GL_DEPTH_COMPONENT and that is not possible in OpenGL ES. (The reason I found in this thread: http://www.khronos.org/message_boards/viewtopic.php?f=4&t=771)
What I want to do is similar to picking, except that I don't want to select the object but I want exact coordinates so that I can recognize particular portion of the object that is currently under mouse cursor. I went through the Picking tutorials in this answer.
https://stackoverflow.com/posts/2211312/revisions
But they need glRenderMode, which I believe is absent in OpenGL ES.
If you know how to solve this problem in OpenGL ES, please let me know.
Thanks.
I think the general solution is to figure out where in world space the clicked coordinate falls, assuming the screen is a plane in the world (at the camera's location). Then you shoot a ray perpendicular to the plane, into your scene.
This requires "world-space" code to figure out which object(s) the ray intersects with; the solutions you mention as being unsuitable for OpenGL ES seem to be image-based, i.e. depend on the pixels generated when rendering the scene.
With OpenGL ES 2.0 you could use a FBO and render the depth values to a texture. Obviously, this wouldn't be exactly cheap (just a way around the restriction of glReadPixels)...
Further, since - as I understand it - you want to pick certain parts of your object you might want to do some sort of color-picking where each selectable portion of the object has an unique color (note that the Lighthouse 3D tutorial only shows the general idea behind color-picking, your implementation would probably be different). You could optimize a little by performing a ray/bounding-box intersection beforehand and only rendering the relevant candidates to the texture used for picking.