So according to many sources I've read, most lighting should be done in eye-space (which is camera space).
The book that I'm reading also claims to be using lighting in eye-space, and I took its examples into my application as well.
In my application, the only thing that I pass to the fragment shader which is related to the camera is the camera position, and it's a model-space coordinate.
The fragment shader also uses a normal matrix which I pass in as a uniform, and its only use is to transform local-space normal vectors to model-space normal vectors.
So why is my lighting implementation considered to be used in eye-space although I never passed a model transformation matrix multiplied by a camera matrix? Can anyone shed some light on this subject? I might be missing something here.
Related
I'm implementing the PHONG shading model in OpenGL. I need the normal, viewer direction, and the light direction for each fragment. A lot of demos pass in these vectors in world coordinates from vertex shader. Maybe it's because there isn't much difference between the normalized world coordinate vectors and the normalized perspective coordinate vectors?
I'm thinking for the "true" PHONG solution, these vectors should be transformed to be perspective coordinate system in vertex shader and then perform the .w divide in fragment shader because they are not the gl_position. Is this thinking correct?
Edit:
From this link seems to suggest OpenGl's varying qualifier requires the original 'Z' coordinate of the fragment to perform correct perspective interpolation. See https://www.opengl.org/wiki/Type_Qualifier_(GLSL)#Interpolation_qualifiers
So the question I'm wondering can OpenGL derive the Z-value from the depth value?
Edit: Yes it can. Getting the true z value from the depth buffer
First, you cannot forgo the division-by-W step. Why? Because it's hard-wired. It happens as part of OpenGL's fixed-functionality. The gl_Position your last vertex processing step generates will have its W component divided into the other three.
Now, you could try to trick your way around that, by sticking 1.0 in the gl_Position's W, and passing it as some unrelated output. But the W component is a crucial part of perspective-correct interpolation. By faking your transforms this way, you lose that.
And that's kinda important. So unless you intend to re-interpolate all of your per-vertex outputs in the FS and perform perspective-correct interpolation, this just isn't going to work.
Second, post-projective space, when using a perspective projection, is a non-linear transformation, relative to world space. This means that parallel lines are no longer parallel. This also means that vector directions don't point at what they used to point at. So your light direction doesn't necessarily point at where your light is.
Oh, and distances are not linear either. So light attenuation no longer makes sense, since the attenuation factors were designed in a space linearly equivalent to world space. And post-projection space is not.
Here's an image to give you an idea of what I'm talking about:
What you see on the left is a rendering in world space. What you see on the right is the same scene as on the left, only viewed in post-projection space.
That is not a reasonable space to do lighting in.
Hi im trying to create a shader for my 3D models with materials and fog. Everything works fine but the light direction. I'm not sure what to set it to so I used a fixed value, but when I rotate my 3D model (which is a simple textured sphere) the light rotates with it. I want to change my code so that my light stays in one place according to the camera and not the object itself. I have tried multiplying the view matrix by the input normals but the same result occurs.
Also, should I be setting the light direction according to the camera instead?
EDIT: removed pastebin link since that is against the rules...
Use camera depended values just for transforming vertex pos to view and projected position (needed in shaders for clipping and rasterizer stage). The video cards needs to know, where to draw your pixel.
For lighting you normally pass additional to the camera transformed value the world position of the vertex and the normal in world position to the needed shader stages (i.e pixel shader stage for phong lighting).
So you can set your light position, or better light direction in world space coordinate system as global variable to your shaders. With that the lighting is independent of the camera view position.
If you want to have a effect like using a flashlight. You can set the lightposition to camera position, and light direction to your look direction. So the bright parts are always in the center of your viewing frustum.
good luck
I can't seem to understand the OpenGL pipeline process from a vertex to a pixel.
Can anyone tell me how important are vertex normals on these two shading techinques? As far as i know, in gouraud, lighting is calculated at each vertex, then the result color is interpolated across the polygon between vertices (is this done in fragment operations, before rasterizing?), and phong shading consists of interpolating first the vertices normals and then calculating the illumination on each of these normals.
Another thing is when bump mapping is applied to lets say a plane (2 triangles) and a brick texture as diffuse with its respect bump map, all of this with gouraud shading.
Bump mapping consist on altering the normals by a gradient depending on a bump map. But what normals does it alter and when (at the fragment shader?) if there are only 4 normals (4 vertices = plane), and all 4 are the same. In Gouraud you interpolate the color of each vertex after the illumination calculation, but this calculation is done after altering the normals.
How does the lighting work?
Vertex normals are absoloutely essential for both Gouraud and Phong shading.
In Gouraud shading the lighting is calculated per vertex and then interpolated across the triangle.
In Phong shading the normal is interpolated across the triangle and then the calculation is done per-pixel/fragment.
Bump-mapping refers to a range of different technologies. When doing normal mapping (probably the most common variety these days) the normals, bi-tangent (often erroneously called bi-normal) and tangent are calculated per-vertex to build a basis matrix. This basis matrix is then interpolated across the triangle. The normal retrieved from the normal map is then transformed by this basis matrix and then the lighting is performed per pixel.
There are extensions to the normal mapping technique above that allow bumps to hide other bumps behind them. This is, usually, performed by storing a height map along with the normal map and then ray marching through the height map to find parts that are being obscured. This technique is called Relief Mapping.
There are other older forms such as DUDV bump mapping (Which was implemented in DirectX 6 as Environment Mapped, bump mapping or EMBM).
You also have emboss bump mapping which was a really early way of doing bump mapping
Edit: In answer to your comment, emboss bump mapping CAN be performed on gouraud shaded triangles. Other forms of bump-mapping are, necessarily, per-pixel (due to the fact they work by modifying the surface normals on a per-pixel (or, at least, per-texel) basis). I wouldn't be surprised if there were other methods that can be performed with per-vertex lighting but I can't think of any off the top of my head. The results will look pretty rubbish compared to doing it on a per-pixel basis, though.
Re: Tangents and Bi-Tangents are actually quite simple once you get your head round them (took me years though, tbh ;)). Any 3D coordinate frame can be defined by a set of vectors that form an orthogonal basis matrix. By setting up the normal, tangent and bi-tangent per vertex you are merely setting up the coordinate frame at each vertex. From this you have the ability to transform a world or object space vector into the triangle's own coordinate frame. From here you can simply translate a light vector (or position) into the coordinate frame of a given pixel on the surface of the triangle. This then means that the normals in the normal map don't need to be stored in the object's space and hence as those triangles move around (when being animated, for example) the normals are already being handled in their own local space.
Normal mapping, one of the techniques to simulate bumped surfaces basically perturbs the per-pixel normals before you compute the light equation on that pixel.
For example, one way to implement requires you to interpolate surface normals and binormal (2 of the tangent space basis) and compute the third per-pixel (2+1 vectors which are the tangent basis). This technique also requires to interpolate the light vector. With those 3 (2+1 computed) vectors (named tagent space basis) you have a way to change the light vector from object space into tagent space. This is because these 3 vectors can be arranged as a 3x3 matrix which can be used to change the basis of your light direction vector.
Then it is simply a matter of using that tagent-space light vector and compute the light equation per pixel, where it most basic form would be a dot product between the tagent-space light vector and the normal map (your bump texture).
This is how a normal maps looks like (the normal component is stored in each channel of the texture and is already in tangent space):
This is one way, you can compute things in view space but the above is more easy to understand.
Old bump mapping was way simpler and was also kind of a fake effect.
All bump mapping techniques operate at pixel level, as they perturb in one way or other, how the surface is rendered. Even the old emboss bump mapping did some computation per pixel.
EDIT: I added a few more clarifications, when I have some spare minutes I will try to add some math and examples. Although there are great resources out there that explain this in great detail.
First of all, you don't need to understand the whole graphics pipeline to write a simple shader :). But, of course, you should know whats going on. You could read the graphics pipeline chapter in real-time rendering, 3rd edition (möller, hofmann, akenine-moller). What you describe is per-vertex and per-fragment lighting. For both calculations the vertex normals are part of the equation. For the bump mapping shader you alter the interpolated normals. So after rasterization you have fragments where missing data has to be caculated to determine the final pixel color.
In OpenGL 2.1, I'm passing a position and normal vector to my vertex shader. The vertex shader then sets a varying to the normal vector, so in theory it's linearly interpolating the normals across each triangle. (Which I understand to be the foundation of Phong shading.)
In the fragment shader, I use the normal with Lambert's law to calculate the diffuse reflection. This works as expected, except that the interpolation between vertices looks funny. Specifically, I'm seeing a starburst affect, wherein there are noticeable "hot spots" along the edges between vertices.
Here's an example, not from my own rendering but demonstrating the exact same effect (see the gold sphere partway down the page):
http://pages.cpsc.ucalgary.ca/~slongay/pmwiki-2.2.1/pmwiki.php?n=CPSC453W11.Lab12
Wikipedia says this is a problem with Gauraud shading. But as I understand it, by interpolating the normals and running my lighting calculation per-fragment, I'm using the Phong model, not Gouraud. Is that right?
If I were to use a much finer mesh, I presume that these starbursts would be much less noticeable. But is adding more triangles the only way to solve this problem? I would think there would be a way to get smooth interpolation without the starburst effect. (I've certainly seen perfectly smooth shading on rough meshes elsewhere, such as in 3d Studio Max. But maybe they're doing something more sophisticated than just interpolating normals.)
It is not the exact same effect. What you are seeing is one of two things.
The result of not normalizing the normals before using them in your fragment shader.
An optical illusion created by the collision of linear gradients across the edges of triangles. Really.
The "Gradient Matters" section at the bottom of this page (note: in the interest of full disclosure, that's my tutorial) explains the phenomenon in detail. Simple Lambert diffuse reflectance using interpolated normals effectively creates a more-or-less linear light across a triangle. A triangle with a different set of normals will have a different gradient. It will be C0 continuous (the colors along the edges are the same), but not C1 continuous (the colors along the two gradients change at different rates).
Human vision picks up on gradient differences like these and makes them stand out. Thus, we see them as hard-edges when in fact they are not.
The only real solution here is to either tessellate the mesh further or use normal maps created from a finer version of the mesh instead of interpolated normals.
You don't show your code, so its impossible to tell, but the most likely problem would be unnormalized normals in your fragment shader. The normals calculated in your vertex shader are interpolated, which results in vectors that are not unit length -- so you need to renormalize them in the fragment shader before you calculate your fragment lighting.
I am trying to develop a basic Ray Tracer. So far i have calculated intersection with a plane and blinn-phong shading.i am working on a 500*500 window and my primary ray generation code is as follows
vec3 rayDirection = vec3( gl_FragCoord.x-250.0,gl_FragCoord.y-250.0 , 10.0);
Now i doubt that above method is right or wrong. Please give me some insights.
I am also having doubt that do we need to construct geometry in OpenGL code while rayTracing in GLSL. for example if i am trying to raytrace a plane do i need to construct plane in OpenGL code using glVertex2f ?
vec3 rayDirection = vec3( gl_FragCoord.x-250.0,gl_FragCoord.y-250.0 , 10.0);
Now i doubt that above method is right or wrong. Please give me some insights.
There's no right or wrong with projections. You could as well map viewport pixels to azimut and elevation angle. Actually your way of doing this is not so bad at all. I'd just pass the viewport dimensions in a additional uniform, instead of hardcoding, and normalize the vector. The Z component literally works like focal lengths.
I am also having doubt that do we need to construct geometry in OpenGL code while rayTracing in GLSL. for example if i am trying to raytrace a plane do i need to construct plane in OpenGL code using glVertex2f?
Raytracing works on a global description containing the full scene. OpenGL primitives however are purely local, i.e. just individual triangles, lines or points, and OpenGL doesn't maintain a scene database. So geometry passed using the usual OpenGL drawing function can not be raytraced (at least not that way).
This is about the biggest obstacle for doing raytracing with GLSL: You somehow need to implement a way to deliver the whole scene as some freely accessible buffer.
It is possible to use Ray Marching to render certain types of complex scenes in a single fragment shader. Here are some examples: (use Chrome or FireFox, requires WebGL)
Gift boxes: http://glsl.heroku.com/e#820.2
Torus Journey: http://glsl.heroku.com/e#794.0
Christmas tree: http://glsl.heroku.com/e#729.0
Modutropolis: http://glsl.heroku.com/e#327.0
The key to making this stuff work is writing "distance functions" that tell the ray marcher how far it is from the surface of an object. For more info on distance functions, see:
http://www.iquilezles.org/www/articles/distfunctions/distfunctions.htm