OpenGL ES 3.0 & 3.1 Shading Language

OpenGL ES 3.0 & 3.1 Shading Language

Presenters: Ron Fosner & Rudy Cazabón

An Overview of the OpenGL Shading Language (GLSL)

OpenGL-ES 3.0 And Beyond Boston 2014OpenGL-ES 3.0 And Beyond Boston 201422

Agenda

Difference between “classic” and “modern” OpenGLOpenGL Shading Language (GLSL)• Review of syntax and semanticsViews of the Processing PipelineReview of Shader Types Within Processing PipelineVertex Shaders In 3.0 and 3.1Tessellation Shaders No – only in 3.1 via HW vendor EXTsGeometry Shaders No – only in 3.1 via HW vendor EXTsFragment/Pixel Shaders In 3.0 and 3.1Compute Shaders In 3.1


Difference between “classic” and “modern” OpenGL

“Classic” OpenGL defines fixed-function operations – e.g. transform and lighting –

“Modern” OpenGL supersedes ALL stages of the graphics pipeline with programmable shaders

From OGL ES 2.0 onwards ALL applications must use shaders for their graphics processing• At minimum a vertex shader for T&L• a fragment shader to paint to framebuffer


GLSL – Variable Types

Strongly typed language; strict rules on type conversions

Variable Class Types Description

Scalars float, int, uint, boolScalar-based data types for floating-point, integer, unsigned integer, and boolean values

Floating-point vectors float, vec2, vec3,vec4

Floating-point–based vector types of one, two, three, or four components

Integer vector int, ivec2, ivec3, ivec4 Integer-based vector types of one, two, three, or four components

Unsigned integer vector uint, uvec2, uvec3, uvec4

Unsigned integer-based vector types of one, two, three, or four components

Boolean vector bool, bvec2, bvec3, bvec4 Boolean-based vector types of one, two, three, or four components

Matricesmat2 (or mat2x2),mat2x3, mat2x4,mat3x2, mat3 (or mat3x3), mat3x4, mat4x2, mat4x3, mat4 (or mat4x4)

Floating-point based matrices of size 2 × 2, 2 × 3, 2 × 4, 3 × 2, 3 × 3, 3 × 4, 4 × 2, 4 × 3, or 4 × 4


mat2, mat3, mat4→ 2x2, 3x3, 4x4

mat

vec3 → 3-element struct

Example: Manipulation of Vector and Matrix Fields

vec4 → 4-element struct


#version 300 esin vec4 v_Position;in vec4 v_Color;out vec4 o_Color;uniform mat4 ModelViewProjectionMatrix;void main() {

o_Color = v_Color;gl_Position =

ModelViewProjectionMatrix * v_Position;}

Variables Declaration Syntax and KeywordsEvery shader has its execution entry-point in main()

Keywords in/out def’ns data that OpenGL passes into/out-of shaders from/to the pipelineKeyword uniform does not change within the pipeline – it is updated by application

Into shader

Out of shader

Rudy Cazabon

VARYINGS!!!


10,000ft Simplified Model

GPU Data FlowApplication

Framebuffer

VertexProcessing Rasterizer Fragment

Processing

VertexShader

FragmentShader

vertices

vertices

fragments

MinimumRequired

Rudy Cazabon

Do


FragmentGeometry

Tess

ella

tion

Vert

ex

View of All Stages of the Processing Pipeline

DrawProcessing

Draw

VertexProcessing

Vertex

Tesselator

Primitive

PrimitiveProcessing

Stream

Xform FeedbackProcessing

FragmentProcessing

Fragment PixelProcessing

Pixel

Image

Rasterization

Skip Draw

No streamgenerated

Rasterizationdiscard

Fail testdiscard

NoWrite

Tessellation Control

Tessellation Evaluation

Compute (OGLES 3.1)

1

23 4 5

No primitivegenerated


OpenGL ES 3.0 and 3.1 Shading Language “Cookbook Approach”

1. Create Shader Programs

2. Create buffer objects

3. Load data into the buffers

4. “Connect” data locations with shader variables

5. Make shader program current

6. Update shader uniforms (application/host-side data)

7. Set attribute inputs

8. Render


OpenGL ES 3.0 and 3.1 Shading Language Applying the Shaders

• Shaders need to be compiled

• Linked to form an executable shader program

• OpenGL provides the compiler and linker

Create Program

Create Shader

Load Shader Source

Compile Shader

Attach shader to program

Link shader

program

Use shader

program

glCreateProgram()

glCreateShader()

glShaderSource()

glCompileShader()

glAttachShader()

glUseProgram()

glLinkProgram()

These steps are repeated for each shader program type

Compile ALL shaders THEN link them into one program


Vertex Shader

Basic uses of vertex shaders • Transformations – e.g. translate, rotate, scale• Lighting – e.g. per-vertex lighting, cartoon shading• Moving vertex positions – e.g. skinning, height

fieldsIts output must be a position in clip coordinates to the rasterizer


Tessellation ShadersTwo (2) conjoined processing steps in a tessellation shader1. Tessellation control shader (TCS)• Controls how much tessellation a particular control shape surface

(patch) is to receive and defines a size

N.b. Only available in OpenGL ES 3.1 by way of hardware vendor EXTs

2. Tessellation evaluation shader (TES)• Consumes the results of the TCS and computes interpolated per-

vertex data from them, e.g. positions, texture coords, and normals


Geometry Shaders

Geometry Shaders precede the primitive assembly and fragment/pixel shadersGeometry shader invocations take a single Primitive as input and may output zero or more primitives.Usages for geometry shaders• Layered rendering: taking one primitive and rendering it to multiple images

without having to change bound rendertargets and so forth• Transform feedback: employed for doing computational tasks on the GPUInstancing - Major win of geometry shaders

• allows multiple invocations to operate over the same input primitive• Enables layered rendering easier to implement and possibly faster performing,

as each layer's primitive(s) can be computed by a separate GS instanceN.b. Only available in OpenGL ES 3.1 by way of hardware vendor EXTs


Fragment Shaders

Fragment shader is the pipeline stage after the primitive rasterizer

A fragment shader is executed for each “potential” pixel • fragments still need to pass several tests before making it to the

framebuffer Common outputs of fragment shaders• RGB colors, depth values, stencil values TESTsEffects that can be done with fragment shaders • Per-fragment lighting• Bump Mapping• Use a “sampler” inside to enable environment (reflection) maps


Compute ShadersCommon Usages


Use-Case – Image Processing

Gaussian blurs with large kernels• Common in photo processing apps such as Photoshop• Expensive, particular for large neighborhoodsCompute Shaders ideal for such image processing operations• Fast shared memory and hundreds executing threadsNo way to perform these in OpenGL ES prior at these processing rates• Results immediately available in OpenGL for display


Compute ShadersWorkgroups – local, globalWork item fundamental unit of work inside a work-group; executes once per invocation

Workgroup collection of work-items launched simultaneouslyLocal workgroup – collection of workgroups that have topological computational neighbors Global workgroup – full aggregate of (local) workgroups

5 Work Groups

4 work-items

20 total items to compute

Computationalneighbors


Compute Topology

The invocation space for compute shaders can be 1-D (previous slide), 2-D, or 3-DThe size and dimensionality of a workgroup is defined in compute shader source-code via input layout qualifierWhenever an input layout qualifier is omitted it automatically defaults to one (1)

#version 300 es// local input layout qualifierlayout (local_size_x = 4) in;

#version 300 es// local input layout qualifierLayout (local_size_x = 4, local_size_y = 4) in;

#version 300 es// local input layout qualifierLayout (local_size_x = 4, local_size_y = 4,Local_size_z = 4) in;


Shared State Variables and Parallel Compute Model

Communication Achieved via GPU-Internal Shared Memory

Use of the “shared” keyword signals the driver to place the annotated data in special storage that is visible to all compute shader invocations within the same local workgroup

Parallelism Achieved via GPU-Internal HW Threading

• Each thread can share data with other members of the workgroup via shared memory (variables)

• Each thread can issue memory and control barriers to synchronise with other members of the workgroup

• Data cannot be shared between workgroups, unless via images, buffer objects or atomic counters

• Each thread can uniquely identify itself within a workgroup and globally with builtin variables. This is the only method for a thread to determine where to get its input and where to write its output

ComputeThread N

...?...

ComputeThread 1

Shared Variables

Workgroup1

ComputeThread N

...?...

ComputeThread 1

Shared Variables

Workgroup M

AnDevCon Boston 201421 AnDevCon Boston 2014

Back up slides


OpenGL ES 3.0 and 3.1 Shading10K-Foot View of the Processing Pipeline

Primitive Assembly?

Rudy Cazabon

Do


Vert

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tage

Documents

OpenGL ES 3.0 & 3.1 Shading Language