ALPHA COMPOSTING

TECHNOLOGY

GUIDED BY,MR.S.K .SINGH

Presented by:DIKSHA R. GUPTA

Introduction What is Alpha Composting History Architecture Advantages Disadvantages Applications Conclusion Bibliography

Contents

What Is Alpha Composting Technology?

The process of using the transparency value, alpha, to combine two images together.

•Alpha composting is the process of combining an image with another image as its background.

•This creates the appearance of full or partial transparency of the resulting image. •It is mostly used in 2D graphics. Composting is also used to combine images and live footage.

• This is a very useful process as it is widely used in many fields. It is a simple and vital process.

•To combine these images in an effective manner, it is essential to keep the matte of each element.

INTRODUCTION

• The information is regarding the shape of each element. Alpha channel is the concept designed to store information. 

•Additional information is stored regarding each pixel in the alpha channel with a value between zero and one.

•The technique is used in many applications. It also has support in many graphical user interfaces (GUI) and widgets.

INTRODUCTION CONT…

•The concept of an alpha channel was introduced by Alvy Ray Smith in the late 1970s, and fully developed in a 1984 paper by Thomas Porter and Tom Duff.

•In a 2D image element, which stores a color for each pixel, additional data is stored in the alpha channel with a value between 0 and 1.

• A value of 0 means that the pixel does not have any coverage information and is transparent.

• There was no color contribution from any geometry because the geometry did not overlap this pixel.

• A value of 1 means that the pixel is opaque because the geometry completely overlapped the pixel.

History

Date Notes

2011-10-12 Added to iOS Developer Library and updated for iOS 5.0.

2011-06-06

Added additional constants for OS X v10.7.

New document that describes the programming interface for high-performance alpha compositing operations.

2007-07-12 The content in this document was formerly part of Optimizing Image Processing With vImage.

History table

Architecture

Architecture of Alpha Composting

Transparency• Alpha is a pixel’s transparency– from 0.0 (transparent) to 1.0 (opaque)– 32-bit RGBA pixels: each pixel has red, green, blue, and alpha values

• Uses for alpha– Antialiasing– Nonrectangular images– Translucent components

In many graphics toolkits, the pixel model includesa fourth channel in addition to red, green, and blue:the pixel’s alpha value, which represents its degreeof transparency

• Compositing rules control how source anddestination pixels are combined• Source– Image– Stroke drawing calls• Destination– Drawing surface

When pixels include alpha values, drawing getsmore interesting. When you draw on a drawingsurface – whether it’s using stroke calls such asdrawRect(), or pixel copying like drawImage(),there are several ways that the alpha values of yourdrawing calls can interact with the alpha of thedestination surface. This process is called alphacompositing.

Alpha Compositing

Antialiasing

Source pixel [Rs Gs Bs As ]Destination pixel [ Rd Gd Bd Ad ]

1. Premultiply RGB by A[RGB]s = [RGB]sAs[RGB]d = [RGB]dAd

2. Compute weighted combination of source anddestination pixel[RGBA]d = [RGBA]sfs + [RGBA]dfdfor weights fs, fd determined by the compositing rule

3. Postdivide RGB by A[RGB]d = [RGB]d / Ad unless Ad == 0

Porter-Duff Alpha Compositing Rules

clear fs=0, fd=0[RGBA]d = 0

src fs=1, fd=0[RGBA]d = [RGBA]s

dst fs=0, fd=1[RGBA]d = [RGBA]d

Simple Copying

• Uncompressed 8-, 10-, or 12-Bit SDI Output

• Ancillary Data Support

• 2D Video Pass Through Compositing

• Genlock (House Synchronization)

• 512MB to 1.5GB Ultra-Fast Graphics Frame Buffer Memory

• 8K Texture and Render Processing

• Fast 3D Texture Transfer

Advantages

• If a feature is sensitive to that dilution or interference, it disappears. If that feature would have been relevant to the user’s task, alpha blending becomes unsuitable. Hence it introduces visual ambiguity.

•The relationships between features become obscured: it can be difficult to determine whether two observed features belong to the same window.

Disadvantages

•Alpha value of a pixel governs transparency.

•Instead of overwriting a destination pixel we mix its color with the source pixel.

Alpha Composting : Blitting + Alpha Blending

• To create special effects in visual arts

• To help us connecting real images with artificially created ones

• To correct photographic mismatches (e.g. color)

• To change or substitute backgrounds And foregrounds

• To create illusion of depth.

Application

Film Composting

King Kong (1933) Superman(1978)

Conclusion

Compositing is the combining of visual elements from separate sources into single images, often to create the illusion that all those elements are parts of the same scene.

We have introduced a novel set of composting models and matting methods which expand the working range of matting and composting in two directions: eliminating the need for special setups for acquiring foreground plates in traditional matting, and moving beyond the limitations of the traditional compositing model. We also present a novel application of matting and compositing to animate still pictures.

BIBLIOGRAPHY

1. [Online].[Available]:https://developer.apple.com/.../AlphaCompositingOperations/Alpha Comp..

2. [Online].[Available]:https://developer.apple.com/library/mac/...alpha/RevisionHistory.htm

3. [Online].[Available]:en.wikipedia.org/wiki/Alpha compositing

4. [Online].[Available]:www.nvidia.co.uk/page/qfx_4500_fb.html

5. [Online].[Available]:www.collegelib.com/t-alpha-composting-computer-technology-seminar-

6. [Online].[Available]:sarveshspn.blogspot.com/2012/06/alpha-blending-and-z-buffer.htm

7. [Online].[Available]:www.cs.princeton.edu/courses/archive/spr06/cos426/.../smith95c.pdf

Thank You…..