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Part of the confusion is the confusing terminology. "Shader" is a bad name. How do you "shade" a vertex? That implies color, when in fact vertex "shading" is

Part of the confusion is the confusing terminology. "Shader" is a bad name. How do you "shade" a vertex? That implies color, when in fact vertex "shading" is really about deforming the position of vertices. It has nothing to do with color! "Vertex program" is a better term. That brings us to "pixel shader." That's actually a good name in order for beginners to learn the concept, but it's imprecise. OpenGL insists on calling it a "fragment program" because with certain forms of antialiasing, there are multiple "fragments" per pixel. "Program" is also a better name than "shader" because there are things you can do per-pixel other than change the color. For example you could change the depth written to the Z-buffer, or you could cause the pixel to be skipped based on some criteria, like whether the texture color is pink. Anyway, it's just a tiny program that executes either per-vertex or per-pixel. For example you could write a vertex program which moves each vertex in a sinewave pattern based on time. Or you could write a fragment program to change the color of each pixel from red to green and back based on time. Then there are more advanced/recent concepts like a "geometry program," which lets you generate triangles based on vertices or edges. Sometimes I wonder if it's overly complicated, or if the problem domain is just complicated. It took me years as a kid to finally grok this, but once I learned it, it turned out to be very simple. Honestly it wasn't until I got up enough courage to sit down with the OpenGL specs and read through them that everything clicked. They're dry reading but not difficult.

wcarss 13y ago
"Anyway, it's just a tiny program that executes either per-vertex or per-pixel." Thank you. I have never understood the concept of a <thing>-shader; I just knew they were useful and somehow caused neat effects. Even having pasted some shader code into a program once (for a barrel distortion for my rift), I never thought much about them, likely because of the opacity of the name. Just ... magic. Your description has just made the entire concept click. I'm now actually interested in learning about them, because it's such a simple, sensible idea. Thanks. side-note: I wish the world had more one-sentence intuitive sum-ups of jargon-laden concepts, even if they are a little 'leaky' as abstractions. Just to give people disconnected from the topic a place to start thinking about it.
- sillysaurus3 13y ago
  You're welcome! Please let me know if you have any other questions. I'll do my best to help. :)
pyalot2 13y ago
The reason it's not called vertex program is because the pipeline is defined in terms of stages (vertex -> tessellation control -> tessellation evaluation -> geometry -> rasterizer -> fragment). Together they are a program. When talking about this structure the choice that OpenGL made is: "A shader program consists of a vertex shader (VS), tessellation control shader (TS control), tessellation evaluation shader (TS eval), geometry shader (GS) and fragment shader (FS)." You could instead say this, but it would be confusing: A shader program consists of a vertex program, tessellation control program, tessellation evaluation program, geometry program and fragment program. And it would even get more confusing if you drop the first shader: "A program consists of a vertex program, tessellation control program, tessellation evaluation program, geometry program and fragment program." So for the sake of it being easy to talk about, a (shader) program is the whole thing, whenever somebody talks about a "program" it's the whole assemblage. And when somebody talks about a shader, it means one of the programs tied to a stage.
- sillysaurus3 13y ago
  Yeah, this has the side effect of making it almost impossible for newcomers to have any clue what the heck is going on until they have several months of (painful) experience trying to figure out each piece. It doesn't have to be this complicated. Humans just made it that way. I'm just trying to make sure everyone understands that there's nothing mysterious or even especially interesting about these terms. It's complicated like an internal combustion engine is complicated, not like math.
  - nightski 13y ago
    So you are proposing that the difference in calling it a "program" vs. a "shader" increases the complexity of learning the concept by several months?
    - sillysaurus3 13y ago
      Sure, it did for me. I think mental models are important, and that it's kind of dopey to call something by a confusing name. (I'm looking at you, physics!) But on the other hand, I'm not very smart, and I had to spend most of my time on gradeschool instead of learning shaders. It was also before wonderful resources like stackoverflow/HN/reddit/etc, so there weren't a lot of people to help clarify my mental model. As I've gotten older it's become easier to think abstractly and accept that names sometimes have nothing to do with what things are. But when you're first starting out, it's natural to want to visualize everything you learn as what it sounds like.
jakobe 13y ago
Shader is a good name, because it implies what one of the main purposes of the code is: to apply lighting (shading) to the stuff. How do you shade a vertex? Read about Gouraud shading. The idea is to do the expensive lighting calculation only at the vertices, and interpolate in-between. The "vertex shader" would calculate the shading at the vertices. Obviously, you can use a vertex shader to do things other than shading, but "shader" is just a name, not a definition of all things this program can do.
- 0x0 13y ago
  I found the name confusing the first time I encountered it. At first, I assumed it only had something to do with applying fancy lighting effects. Only after studying things further did it become obvious that a shader is responsible for doing 3d->2d projection and pixel filling - and you need them even if you only want to draw solid color shapes!
ANTSANTS 13y ago
Traditionally, lighting was calculated per vertex, not per pixel, and the results of those calculations were simply interpolated across the surface of the triangle. A "shader" is not the most general term they could have used for a GPU program, but you very well can "shade" a vertex. Edit: Is this seriously being downvoted? "How do you "shade" a vertex? That implies color, when in fact vertex "shading" is really about deforming the position of vertices. It has nothing to do with color!" is factually inaccurate because you can, and many games do, calculate color and lighting information per-vertex.
- sillysaurus3 13y ago
  Actually, the per-vertex color is passed into the pixel shader, which then decides what to do with it. The vertex shader itself has nothing to do with color except calculating an initial value and passing it to the pixel shader. The pixel shader can be set up to interpret that value as anything. It could make it a grayscale color, it could invert it, it could use only one of the four channels, etc. There used to be no pixel shaders, which is why vertex shaders had to be used to do shading. That was circa 2004 though. EDIT: This paragraph is incorrect, see comment below. Also, the whole idea of "color" in a vertex shader is mistaken. There is only one color: the RGB that ultimately shows up on the screen. Until then, there are values which are passed from the main application source code to the vertex / pixel shaders, which then decide what to do with those values. Sometimes the pixel shader interprets them as a color, but they're really just floating point numbers. That may seem like a silly distinction, but again, if your mental model is incorrect as a newcomer then you're going to have a hard time.
  - reitzensteinm 13y ago
    According to this, it's the other way around. "The first shader-capable GPUs only supported pixel shading, but vertex shaders were quickly introduced once developers realized the power of shaders" I actually very much disagree with your point about names. I write shaders when I have to, I'm not very good at it, but it feels fundamentally very broken, much as I picture the days of 16 bit x86 assembly (memory segmentation?). There are arbitrary length, register etc limits per DX version, there's no clean cross platform method of writing shaders, the documentation is fragmented, vague, for the wrong platform or non existent. Sure, the naming could be better, but page one of a decent textbook should set you straight. The other issues, not so much. Of course, that's more or less the price you pay for being on the bleeding edge of performance. http://en.wikipedia.org/wiki/Shader#History http://en.wikipedia.org/wiki/Shader#History
lingoberry 13y ago
I just want to say that you're correct, I've had this problem as well. As a non-native speaker for some reason it also didn't occur to me that the "shade" in shader has anything to do with color. It's just one of those things you can misunderstand. The way you explain it is much easier to understand. I think all the people defending the term are all quite invested in the area already, so it's obvious to them.

wcarss 13y ago

"Anyway, it's just a tiny program that executes either per-vertex or per-pixel." Thank you. I have never understood the concept of a <thing>-shader; I just knew they were useful and somehow caused neat effects. Even having pasted some shader code into a program once (for a barrel distortion for my rift), I never thought much about them, likely because of the opacity of the name. Just ... magic. Your description has just made the entire concept click. I'm now actually interested in learning about them, because it's such a simple, sensible idea. Thanks. side-note: I wish the world had more one-sentence intuitive sum-ups of jargon-laden concepts, even if they are a little 'leaky' as abstractions. Just to give people disconnected from the topic a place to start thinking about it.

sillysaurus3 13y ago
You're welcome! Please let me know if you have any other questions. I'll do my best to help. :)

pyalot2 13y ago

The reason it's not called vertex program is because the pipeline is defined in terms of stages (vertex -> tessellation control -> tessellation evaluation -> geometry -> rasterizer -> fragment). Together they are a program. When talking about this structure the choice that OpenGL made is: "A shader program consists of a vertex shader (VS), tessellation control shader (TS control), tessellation evaluation shader (TS eval), geometry shader (GS) and fragment shader (FS)." You could instead say this, but it would be confusing: A shader program consists of a vertex program, tessellation control program, tessellation evaluation program, geometry program and fragment program. And it would even get more confusing if you drop the first shader: "A program consists of a vertex program, tessellation control program, tessellation evaluation program, geometry program and fragment program." So for the sake of it being easy to talk about, a (shader) program is the whole thing, whenever somebody talks about a "program" it's the whole assemblage. And when somebody talks about a shader, it means one of the programs tied to a stage.

sillysaurus3 13y ago
Yeah, this has the side effect of making it almost impossible for newcomers to have any clue what the heck is going on until they have several months of (painful) experience trying to figure out each piece. It doesn't have to be this complicated. Humans just made it that way. I'm just trying to make sure everyone understands that there's nothing mysterious or even especially interesting about these terms. It's complicated like an internal combustion engine is complicated, not like math.
- nightski 13y ago
  So you are proposing that the difference in calling it a "program" vs. a "shader" increases the complexity of learning the concept by several months?
  - sillysaurus3 13y ago
    Sure, it did for me. I think mental models are important, and that it's kind of dopey to call something by a confusing name. (I'm looking at you, physics!) But on the other hand, I'm not very smart, and I had to spend most of my time on gradeschool instead of learning shaders. It was also before wonderful resources like stackoverflow/HN/reddit/etc, so there weren't a lot of people to help clarify my mental model. As I've gotten older it's become easier to think abstractly and accept that names sometimes have nothing to do with what things are. But when you're first starting out, it's natural to want to visualize everything you learn as what it sounds like.

jakobe 13y ago

Shader is a good name, because it implies what one of the main purposes of the code is: to apply lighting (shading) to the stuff. How do you shade a vertex? Read about Gouraud shading. The idea is to do the expensive lighting calculation only at the vertices, and interpolate in-between. The "vertex shader" would calculate the shading at the vertices. Obviously, you can use a vertex shader to do things other than shading, but "shader" is just a name, not a definition of all things this program can do.

0x0 13y ago
I found the name confusing the first time I encountered it. At first, I assumed it only had something to do with applying fancy lighting effects. Only after studying things further did it become obvious that a shader is responsible for doing 3d->2d projection and pixel filling - and you need them even if you only want to draw solid color shapes!

ANTSANTS 13y ago

Traditionally, lighting was calculated per vertex, not per pixel, and the results of those calculations were simply interpolated across the surface of the triangle. A "shader" is not the most general term they could have used for a GPU program, but you very well can "shade" a vertex. Edit: Is this seriously being downvoted? "How do you "shade" a vertex? That implies color, when in fact vertex "shading" is really about deforming the position of vertices. It has nothing to do with color!" is factually inaccurate because you can, and many games do, calculate color and lighting information per-vertex.

sillysaurus3 13y ago
Actually, the per-vertex color is passed into the pixel shader, which then decides what to do with it. The vertex shader itself has nothing to do with color except calculating an initial value and passing it to the pixel shader. The pixel shader can be set up to interpret that value as anything. It could make it a grayscale color, it could invert it, it could use only one of the four channels, etc. There used to be no pixel shaders, which is why vertex shaders had to be used to do shading. That was circa 2004 though. EDIT: This paragraph is incorrect, see comment below. Also, the whole idea of "color" in a vertex shader is mistaken. There is only one color: the RGB that ultimately shows up on the screen. Until then, there are values which are passed from the main application source code to the vertex / pixel shaders, which then decide what to do with those values. Sometimes the pixel shader interprets them as a color, but they're really just floating point numbers. That may seem like a silly distinction, but again, if your mental model is incorrect as a newcomer then you're going to have a hard time.
- reitzensteinm 13y ago
  According to this, it's the other way around. "The first shader-capable GPUs only supported pixel shading, but vertex shaders were quickly introduced once developers realized the power of shaders" I actually very much disagree with your point about names. I write shaders when I have to, I'm not very good at it, but it feels fundamentally very broken, much as I picture the days of 16 bit x86 assembly (memory segmentation?). There are arbitrary length, register etc limits per DX version, there's no clean cross platform method of writing shaders, the documentation is fragmented, vague, for the wrong platform or non existent. Sure, the naming could be better, but page one of a decent textbook should set you straight. The other issues, not so much. Of course, that's more or less the price you pay for being on the bleeding edge of performance. http://en.wikipedia.org/wiki/Shader#History http://en.wikipedia.org/wiki/Shader#History

lingoberry 13y ago

I just want to say that you're correct, I've had this problem as well. As a non-native speaker for some reason it also didn't occur to me that the "shade" in shader has anything to do with color. It's just one of those things you can misunderstand. The way you explain it is much easier to understand. I think all the people defending the term are all quite invested in the area already, so it's obvious to them.