Fixed Function to Shaders

Porting a fixed-function application to “modern” Opengl.

Watch the video here: http://bit.ly/1TA24fU

OutlineThere are many tutorials that introduce you to

“modern” OpenGL: (OpenGL 3.3/OpenGL ES 2.0 or greater which is where the fixed-function APIs were removed from the spec.)

Here we will compare and contrast old fixed-functionality and it’s new modern replacement.

We will cover some basic things you need to get going: Vertex/Attribute data, rendering, and 3-D math.

GeometryLet’s use the famous OpenGL triangle as a

platform to talk about geometry/attributes. It’s the probably the very first OpenGL program

you saw when learning OpenGL.

RenderingWhat is the minimum need to “light up” a pixel?First you need a window on your platform with

an OpenGL context bound to it.You used to use GLUT and GLU “helper”

librariesHere we use Qt to replace both. For window/platform integration we’re using

QOpenGLWindowinitializeGL(), resizeGL(), paintGL(), keyPressEvent()

GLUT OpenGL Triangle (clip-space)

Show code in Qt Creator.

OpenGL 1.1 Vertex Arrays GLUT Triangle

Show code in Qt Creator.

Modern Open Vertex Arrays Qt Triangle

Show code in Qt Creator.

Doesn’t Work ??Previous slide will not render a triangle. Why not? Fixed-function example uses “fixed functionality”

to render. With modern OpenGL, you have to program that

functionality yourself in the form of “shaders”.

ShadersMany kinds of shaders in Modern OpenGL: Vertex ShaderTessellation Control ShaderTessellation Evaluation ShaderGeometry ShaderFragment ShaderCompute Shader

Only two are required.

Vertex ShaderProgram that runs on the GPU.Invoked once for each vertex in primitive shapes

drawn.

Input: Attribute data from vertex arrays

Output: Clip space position of vertex: gl_PositionData to pixel shader: varying variables.

Fragment ShaderProgram run on the GPU once for each

fragment (pixel-candidate) displayed on screen.

Inputs: varying variables from Vertex Shader

Outputs: pixel color: gl_FragColor

Pixels are produced by “Rasterization”

Rasterization

http://www.raywenderlich.com

ProgramAll the shaders are compiled and linked together

similar to C++ program.

QOpenGLShaderProgram makes it easy

Once compiled and linked bind() must be called to make it active.

Hold On ...You may notice the fragment shader is

assigning the output color directly from it’s input varying v_color variable set by the Vertex Shader.

How is it that the colors are “mixed” inside the triangle?

Outputs of the vertex shader (and corresponding inputs to the pixel shader) are interpolated between the vertices.

Equivalent to glShadeModel(GL_SMOOTH);

What about GL_FLAT ?Okay so attribute data output from the vertex

shader is interpolated to the pixel shader inputs.

What about glShadeModel(GL_FLAT)?Use flat attribute on variable declaration in

shader code.flat varying vec3 v_color;Default is smooth. These are equivalent:smooth varying vec3 v_color;varying vec3 v_color;

Review: QGLBufferObjectMemory buffer on graphics card that holds

vertex attribute data. Equivalent to glBegin/glEnd inside a display

listAttributes inside glBegin/glEnd copied to

video card instead of being rendered.Equivalent to alloc() on QGLBufferObject.Vertex Buffer Objects (VBO) don’t save primitive

type.Instead pass as parameter to glDraw()Just like OpenGL 1.1 Vertex Arrays

Review: QGLVertexArrayObjectOpenGL 1.1 Vertex Arrays require setting up

attribute array specifications each time before calling glDraw().

Modern OpenGL captures attribute array specifications once when data is uploaded to card using Vertex Array Objects (VAO).

VAO “remembers” vertex array state and applies this state when .bind() is called.

Modern code only needs a vao.bind() before glDraw()

Another thing to note …Fixed-function primitive types: GL_QUAD, GL_QUAD_STRIP, GL_POLYGON have been removed.

You must change your geometry to GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, or GL_TRIANGLE.

MathFixed-function OpenGL had Matrix Stacks built

into the API.Used to create concept of a “camera”

(GL_MODELVIEW) rendering a world through a window (GL_PROJECTION) that’s painted on your computer screen.

Convenience Functions: glLoadIdentity, glTranslate, glRotate, glScale

Matrix stack: glMatrixMode, glPushMatrix, glPopmatrix

Sorry...Sorry, that’s all gone now.You, the programmer, have to perform all this math.Recall the vertex shader is responsible for outputting

the vertices clip-space position by assigning to gl_Position.

It is this math that you use in the vertex shader to perform this conversion.

On the CPU the typical thing to do is recreate the camera/window idiom with model transform matrices, a view transform matrix, and a projection matrix.

Pass the matrices to the shaders as “uniforms”

AgnosticModern OpenGL is agnostic about these idioms.

But it does help you by providing matrix math operators in the shader language.

You, the programmer, get to decide how to transform your vertex positions to clip space.

If you can code it, you can use it.

Math LibraryIf you want to use the Model-View-Projection

concept in your program you have to perform the math yourself.

Qt has a powerful/concise library built-in which supports vectors, matrices, and quaternions.

Matrix functions to replace GL & GLU gluPerspective, gluOrtho2D, glFrustum, gluLookAt, glTranslate, glRotate, glScale, etc.

Checkout: QVector[2,3,4]D, QQuaterion, QMatrix4x4

Move out of Clip space (fixed-function)

Show code in Qt Creator.

UniformsA uniform is a OpenGL Shading Language

(GLSL) constant parameter.Set in CPU code between glDraw() calls.Constant in the fashion that it has a constant

value for all shader invocations originating from glDraw() calls until the value is changed.

Use QOpenGLProgram.setUniform() to pass Model, View, Projection matrices to shader code before drawing.

Move out of Clip Space (modern GL)

Show code in Qt Creator.

User Clip PlanesAnother thing to note is the glClipPlane()

has been removed.

Perform point/plane equation tests in your pixel shader and use keyword discard (instead of assigning to gl_FragColor) to inform OpenGL that that particular pixel should not be displayed.

Managing OpenGL StateAnother thing to note is that

glPushAttrib(), glPopAttrib(), glPushClientAttrib() and glPopClientAttrib() have been removed.

You have to manually manage your OpenGL state by either keeping track of it in your C++ program (the preferred method) or by using glGet() to read the current state and then restoring it afterwards.

Wrapping UpWe were able to cover transitioning from fixed-

function Vertex/Attribute data and the built-in Matrix stacks (and associated matrix functionality) to Modern OpenGL.

We learned that Modern OpenGL replaced the “fixed” stuff with programmable shaders. We learned about the Vertex and Fragment shaders, what they do and how data flows through them.

We learned that using Qt makes is very easy to create cross-platform Modern OpenGL code.

For More InformationFor more information checkout the four-part

blog series I wrote covering this topic.www.ics.com/blog/fixed-function-modern-opengl-part-1-4

Also, check out our training class coming up in April out in Silicon Valleywww.ics.com/learning/training/state-art-opengl-and-qt-3

OutlineFor the Blog: Journal Entry Style- Introduction:

Spent a lot of time in past life on porting complex, scenegraph based, fixed function OpenGL code to Modern Pipeline Code...

- Three Things that spring out to be addressed- Geometry and Lighting and Texturing- Picking, Text is another one for another day- Explain Geometry and Lighting using a simple

scene example

Simple Scene from <insert link here>ScreenshotCode: window/context, geometry, drawing, resize, camera modelview, projection, viewport,light

Modern Code: QOpenGLWindow....Geometry: VertexBuffer, IndexBuffer, VertexArrayObjects, ...

GLUT OpenGL Triangle (clip-space)void init(void) { glMatrixMode(GL_MODELVIEW); glLoadIdentity();}

void display(void) { glClear(GL_COLOR_BUFFER_BIT); glBegin(GL_TRIANGLES); glColor3f(1.0, 0.0, 0.0); glVertex3f(-1.0, -1.0, 0.0); glColor3f(0.0, 1.0, 0.0); glVertex3f( 0.0, 1.0, 0.0); glColor3f(0.0, 0.0, 1.0); glVertex3f( 1.0, -1.0, 0.0); glEnd(); glutSwapBuffers();}

void reshape(int w, int h) { glViewport(0, 0, (GLsizei) w, (GLsizei) h); glMatrixMode(GL_PROJECTION); glLoadIdentity();}

void keyboard (unsigned char key, int , int ){ if (key == 27) exit(0);}