Graphics programming in open gl

Graphics Programming in OpenGL

Arvind Devaraj

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OpenGL and GLUT Overview

�� OpenGL is graphics API.OpenGL is graphics API.�� Makes graphics programming Makes graphics programming h/wh/w

independent independent

�� GLUT is a toolkit GLUT is a toolkit library of utilities using OpenGLlibrary of utilities using OpenGLmakes easier to use OpenGL makes easier to use OpenGL

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A

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High Level

� Represent Objects as Primitives� Apply Transformations on objects

� Projection : Visible portions are rendered

� Add realism by adding Light and Texture

� Remember using State information� Customize using Shaders� Framebuffer operations

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OpenGL Geometric Primitives

GL_QUAD_STRIPGL_QUAD_STRIP

GL_POLYGONGL_POLYGON

GL_TRIANGLE_STRIPGL_TRIANGLE_STRIP GL_TRIANGLE_FANGL_TRIANGLE_FAN

GL_POINTSGL_POINTS

GL_LINESGL_LINES

GL_LINE_LOOPGL_LINE_LOOPGL_LINE_STRIPGL_LINE_STRIP

GL_TRIANGLESGL_TRIANGLES

GL_QUADSGL_QUADS

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Shapes Tutorial

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Transformations

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Camera Analogy

�� Graphics Rendering is like taking a Graphics Rendering is like taking a photograph photograph

�� 3D model 3D model �� 2D image2D image

camera

tripod model

viewingvolume

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Transformations

�� Steps in Forming an ImageSteps in Forming an Image� specify geometry (world coordinates)� specify camera (camera coordinates)� project (window coordinates)� map to viewport (screen coordinates)

�� Each step uses transformationsEach step uses transformations�� Transformation = change in coordinate Transformation = change in coordinate

systems systems

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Camera Analogy of Transformations

�� Projection transformationsProjection transformations� adjust the lens of the camera

�� Viewing transformationsViewing transformations� tripod–define position and orientation of

the viewing volume in the world�� Modeling transformationsModeling transformations

� moving the model�� ViewportViewport transformationstransformations

� enlarge or reduce the physical photograph

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Affine Transformations

�� Want transformations which preserve Want transformations which preserve geometrygeometry

� Rotation, translation, scaling� Projection� Concatenation (composition)

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Affine Transformations

� Basic Transformations� Any transformation is Composition of these

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Affine Transformation Matrix

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3D Transformations

�� A vertex is transformed by 4 x 4 matricesA vertex is transformed by 4 x 4 matrices� all affine operations are matrix multiplications� all matrices are stored column-major in

OpenGL� matrices are always post-multiplied� product of matrix and vector is

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Programming Transformations

�� Prior to rendering, view, locate, and Prior to rendering, view, locate, and orient:orient:� eye/camera position� 3D geometry

�� Manage the matricesManage the matrices� including matrix stack

�� Combine (composite) transformationsCombine (composite) transformations

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vertex

ModelviewMatrix

ProjectionMatrix

PerspectiveDivision

ViewportTransform

Modelview

Modelview

Projection

���

object eye clip normalizeddevice

window

TransformationPipeline

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Viewing Transformations

�� Position the camera/eye in the scenePosition the camera/eye in the scene� place the tripod down; aim camera

�� To To ““fly throughfly through”” a scenea scene� change viewing transformation and

redraw scene�� gluLookAtgluLookAt( ( eyeeyexx, , eyeeyeyy, , eyeeyezz,,

aimaimxx, , aimaimyy, , aimaimzz,,upupxx, , upupyy, , upupzz ))

� up vector determines unique orientation

tripod

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Transformations in OpenGL

�� ModelingModeling�� ViewingViewing

� orient camera� projection

�� AnimationAnimation�� Map to screenMap to screen

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Transformation Tutorial

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Projection

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Projection Tutorial

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Connection: Viewing and Modeling

�� Moving camera is equivalent to moving Moving camera is equivalent to moving every object in the world towards a every object in the world towards a stationary camerastationary camera

�� Viewing transformations are equivalent Viewing transformations are equivalent to several modeling transformationsto several modeling transformations

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Double Buffering

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16

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16FrontBuffer

BackBuffer

Display

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Depth Buffering andHidden Surface Removal

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16ColorBuffer

DepthBuffer

Display

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Lighting

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Phong Principles

�� Lighting simulates how objects reflect Lighting simulates how objects reflect lightlight� material composition of object� light’s color and position

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Various Lighting Models

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Simulating Lights

�� PhongPhong lighting modellighting model� Computed at vertices

�� Lighting contributorsLighting contributors� Surface material properties� Light properties� Lighting model properties

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SurfaceNormals

�� NormalsNormals define how a surface reflects define how a surface reflects lightlight

glNormal3f( glNormal3f( x, y, zx, y, z ))

� Current normal is used to compute vertex’s color

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Light Position Tutorial

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Texture Mapping

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TextureMapping�� Apply image to geometry Apply image to geometry

�� UsesUses� Reduce Geometric complexity� Add Realism � Reflections

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Texture Mapping

Reduce Geometric Complexity

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Reflections – Environmental Map

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Texture Mapping

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x

y

z

image

geometry screen

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Texture Example

�� The texture (below) is a The texture (below) is a 256 x 256 image that has been256 x 256 image that has beenmapped to a rectangularmapped to a rectangularpolygonpolygon

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Tutorial: Texture

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OpenGL’s State MachineAll Rendering info are remembered as state info

State InformationLightingShadingTexturingBlending

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OpenGL’s State MachineAll Rendering info are remembered as state info

Manipulating vertex attribute changes state info

glColor()glNormal()glTexCoord()

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Shaders

� Allow these transformations to be user programmable

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Shaders

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Getting to the Framebuffer

BlendingBlendingDepthTest

DepthTest DitheringDithering Logical

Operations

LogicalOperations

ScissorTest

ScissorTest

StencilTest

StencilTest

AlphaTest

AlphaTest

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Blending

Stencil test

Depth test

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Alpha : for blending effect

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Blending (using Alpha)

�� AlphaAlpha--Measure of OpacityMeasure of Opacity� simulate translucent objects

� glass, water, etc.

� composite images� AntialiasingglEnable( GL_BLEND )

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Blending

�� Combine pixels with whatCombine pixels with what’’s in alreadys in alreadyin the in the framebufferframebufferglBlendFuncglBlendFunc( ( srcsrc, , dstdst ))

FramebufferFramebufferPixelPixel((dstdst))

BlendingEquation

BlendingEquation

FragmentFragment((srcsrc))

BlendedBlendedPixelPixel

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Blending- Demo

� Exercises� add multiple shaders� simple blend� Demo (url devmaster)� Advanced (notes)

Other blending modesTexture blending

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Stencil test

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Stencil test

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Accumulation buffer

� The accumulation buffer is simply an extra image buffer that is used to accumulate composite images.

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Accumulation buffer

� Color buffer loses precision, so use float buffer

�� CompositingCompositing�� AntialiasingAntialiasing�� FilteringFiltering�� Motion BlurMotion Blur

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OpenGLES

� There is no support for glBegin or glEnd. Use vertex arrays and vertex buffer objects instead.

� The only supported rasterizationprimitives are points, lines and triangles. Quads are not supported

� There is no support for display lists.

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OpenGLES

� There is no support for the fixed-function graphics pipeline. You must use your own vertex and fragment shader programs.

� There is no support for viewing transforms such as glFrustumf. You must compute your own transformation matrix, pass it to the vertex shader as a uniform variable, and perform the matrix multiplication in the shader.

� There is no support for specialized functions such as glVertexPointer and glNormalPointer. Use glVertexAttribPointer instead.

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OpenGLES- Buffer Objects

� OpenGL cannot directly see any of your memory

� Therefore, the first step is to allocate some memory that OpenGL can see . This is done with something called a buffer object.

Think of a buffer object as an array of GPU memory.

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OpenGLES- Buffer Objects

� Pass arrays of vertices, colors, etc. to OpenGL in a large chunkglVertexPointer( 3, GL_FLOAT, 0, coords )glColorPointer( 4, GL_FLOAT, 0, colors )

glEnableClientState( GL_VERTEX_ARRAY ) glEnableClientState( GL_COLOR_ARRAY )

glDrawArrays( GL_TRIANGLE_STRIP, 0, numVerts );

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OpenGLES - BufferObjects� void InitializeVertexBuffer() {

glGenBuffers(1, &positionBufferObject); glBindBuffer(GL_ARRAY_BUFFER, positionBufferObject);

glBufferData(GL_ARRAY_BUFFER, siz, vertexPositions, GL_STATIC_DRAW);

glBindBuffer(GL_ARRAY_BUFFER, 0);

}

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Exercises

� Moving ball animation� Blending� Texturing

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