Vertex Shaders Version 3

As mentioned in how it works WebGL requires 2 shaders every time youdraw something. A vertex shader and a fragment shader. Each shader is a function. A vertexshader and fragment shader are linked together into a shader program (or just program). A typicalWebGL app will have many shader programs.

To use a varying we need to declare matching varyings in both a vertex and fragment shader.We set the out varying in the vertex shader with some value per vertex. When WebGL draws pixelsit will optionally interpolate between those values and pass them to the corresponding in varying inthe fragment shader

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GLSL stands for Graphics Library Shader Language.It's the language shaders are writtenin. It has some special semi unique features that are certainly not common in JavaScript.It's designed to do the math that is commonly needed to compute things for rasterizinggraphics. So for example it has built in types like vec2, vec3, and vec4 whichrepresent 2 values, 3 values, and 4 values respectively. Similarly it has mat2, mat3and mat4 which represent 2x2, 3x3, and 4x4 matrices. You can do things like multiplya vec by a scalar.

That's the point of this entire series of posts. WebGL is all about creating various shaders, supplyingthe data to those shaders and then calling gl.drawArrays, gl.drawElements, etc to have WebGL processthe vertices by calling the current vertex shader for each vertex and then render pixels by calling the current fragment shader for each pixel.

Actually creating the shaders requires several lines of code. Since those lines are the same inmost WebGL programs and since once written you can pretty much ignore them how to compile GLSL shadersand link them into a shader program is covered here.

The component qualifier can be used for any shader stage input/output declaration. This includes interfaces between shaders, fragment shader outputs, tessellation patch variables, and so forth. However, it may not be used on:

Any output variable or output interface block declared with the xfb_offset layout qualifier will be part of the transform feedback output. This qualifier must be specified with an integer byte offset. The offset is the number of bytes from the beginning of a vertex to be written to the current buffer to this particular output variable.

Each buffer has the concept of a stride. This represents the byte count from the beginning of one captured vertex to the beginning of the next. It is computed by taking the output with the highest xfb_offset value, adding its size to that offset, and then aligning the computed value to the base alignment of the buffer. The buffer's alignment is 4, unless it captures any double-precision values in which case it is 8. This means you do not need to manually pad structures for alignment, as you did with outside shader setting.

Geometry Shaders take a particular primitive type as input and return a particular primitive type as outputs. Also, geometry shaders have a defined maximum number of vertices that they can output. These specifications cannot be used on a variable definition; it can only be used on the qualifiers in and out as a whole.

Vertex shaders and pixel shaders are simplified considerably from earlier shader versions. If you are implementing shaders in hardware, you may not use vs_3_0 or ps_3_0 with any other shader versions, and you may not use either shader type with the fixed function pipeline. These changes make it possible to simplify drivers and the runtime. The only exception is that software-only vs_3_0 shaders may be used with any pixel shader version. In addition, if you are using a software-only vs_3_0 shader with a previous pixel shader version, the vertex shader can only use output semantics that are compatible with flexible vertex format (FVF) codes.

The semantics used on vertex shader outputs must be used on pixel shader inputs. The semantics are used to map the vertex shader outputs to the pixel shader inputs, similar to the way the vertex declaration is mapped to the vertex shader input registers and previous shader models. See Match Semantics on vs 3.0 and ps 3.0 Shaders.

The vertex shader output register types have been collapsed into twelve registers (see Output Registers). Each register that is used needs to be declared using the dcl instruction and a semantic (for example, dcl_color0 o0.xyzw).

The 3_0 vertex shader model (vs_3_0) expands on the features of vs_2_0 with more powerful register indexing, a set of simplified output registers, the ability to sample a texture in a vertex shader, and the ability to control the rate at which shader inputs are initialized.

Similarly, a semantic name declared on different input registers in the pixel shader (v0 and v1 in the pixel shader) cannot be used in a single output register in this vertex shader. For instance, this vertex shader cannot be paired with the pixel shader because D3DDECLUSAGE_TEXCOORD1 is used for both pixel shader input registers (v0, v1) and the vertex shader output register o3.

On the other hand, this vertex shader cannot be paired with the pixel shader because the output mask for a parameter with a given semantic does not provide the data that is requested by the pixel shader:

If fog rendering is desired, vs_3_0 and ps_3_0 shaders must implement fog. No fog calculations are done outside of the shaders. There is no fog register in vs_3_0, and additional semantics D3DDECLUSAGE_FOG (for fog blend factor computed per vertex) and D3DDECLUSAGE_DEPTH (for passing in a depth value to the pixel shader to compute the fog blend factor) have been added.

Software implementations (run-time and reference for vertex shaders and reference for pixel shaders) of version 2_0 shaders and above have some validation relaxed. This is useful for debugging and prototyping purposes. The application indicates to the runtime/assembler that it needs some of the validation relaxed using the _sw flag in the assembler (for example, vs_2_sw). A software shader will not work with hardware.

The Vertex Shader is the programmable Shader stage in the rendering pipeline that handles the processing of individual vertices. Vertex shaders are fed Vertex Attribute data, as specified from a vertex array object by a drawing command. A vertex shader receives a single vertex from the vertex stream and generates a single vertex to the output vertex stream. There must be a 1:1 mapping from input vertices to output vertices.

Vertex shaders typically perform transformations to post-projection space, for consumption by the Vertex Post-Processing stage. They can also be used to do per-vertex lighting, or to perform setup work for later shader stages.

The OpenGL specification is fairly lenient on the number of times a vertex shader is invoked by the rendering system. Vertex Specification and Vertex Rendering define a vertex stream: an ordered sequence of vertices to be consumed. The vertex shader will be executed roughly once for every vertex in the stream.

A vertex shader is (usually) invariant with its input. That is, within a single Drawing Command, two vertex shader invocations that get the exact same input attributes will return binary identical results. Because of this, if OpenGL can detect that a vertex shader invocation is being given the same inputs as a previous invocation, it is allowed to reuse the results of the previous invocation, instead of wasting valuable time executing something that it already knows the answer to.

OpenGL implementations generally do not do this by actually comparing the input values (that would take far too long). Instead, this optimization typically only happens when using indexed rendering functions. If a particular index is specified more than once (within the same Instanced Rendering), then this vertex is guaranteed to result in the exact same input data.

Therefore, implementations employ a cache on the results of vertex shaders. If an index/instance pair comes up again, and the result is still in the cache, then the vertex shader is not executed again. Thus, there can be fewer vertex shader invocations than there are vertices specified.

Each user-defined input variable is assigned one or more vertex attribute indices. These can be explicitly assigned in one of three ways. The methods for assigning these are listed in priority order, with the highest priority first. The higher priority methods take precedence over the later ones.

Note that like uniforms, vertex attributes can be "active" and non-active. Active inputs are those that the compiler/linker detects are actually in use. The vertex shader and GLSL program linking process can decide that some input are not in use and therefore they are not active. This is done even if an explicit attribute index is assigned in the vertex shader.

Attributes may be arrays, matrices, and double-precision types (if OpenGL 4.1 or ARB_vertex_attrib_64bit is available). Or combinations of any of these. Some of these types are large enough to require that the input variable be assigned to multiple attribute indices.

There is a case which makes this more complex: double-precision attributes (if OpenGL 4.1 or ARB_vertex_attrib_64bit is available). dvec3 and dvec4 only take up one attribute index. But implementations are allowed to count them twice when determining the limits on the number of attributes. Thus, while a dmat2x3[4] will only take up 8 attribute indices (4 array elements of 2 columns of dvec3s), the implementation is allowed to consider this as taking up 16 indices when determining if a shader is using up too many attribute indices. As such, a dmat2x3[5] may fail to link even though it only uses 10 attribute indices.

Output variables from the vertex shader are passed to the next section of the pipeline. Many of the next stages are optional, so if they are not present, then the outputs are passed to the next one that is. They are in this order:

HLSL is analogous to the GLSL shading language used with the OpenGL standard. It is very similar to the Nvidia Cg shading language, as it was developed alongside it. Early versions of the two languages were considered identical, only marketed differently.[3] HLSL shaders can enable profound speed and detail increases as well as many special effects in both 2D and 3D computer graphics.[citation needed]

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