TweetFollow Us on Twitter

March 95 - SOMEWHERE IN QUICKTIME

SOMEWHERE IN QUICKTIME

Choosing the Right Codec

JOHN WANG

[IMAGE Wang.GIF]

As described in Chapter 3 of Inside Macintosh: QuickTime , the Image Compression Manager performs compression and decompression by invoking image compressor and decompressor components. These components, calledcodecs , present a standard interface to the Image Compression Manager using the Component Manager. But each codec is unique because each implements a different compression and decompression algorithm. Codecs can vary greatly in the three major characteristics that are used to judge image compression algorithms: compression ratio, compression speed, and image quality. QuickTime 2.0 ships with eight codecs that can be selected for use by your QuickTime application under various conditions. In addition, users or third-party products can install custom codecs into the system by simply placing them in the Extensions folder.

With all the choices for image compression and decompression, the only way to choose the best codec for a particular purpose is to have some understanding of all the codecs available on your system. Inside Macintosh provides general descriptions of the standard QuickTime codecs, but detailed information is important in making an intelligent codec selection. The way to find this detailed information is to communicate programmatically with the codec and request its capabilities through the codec call CDGetCodecInfo. As described inInside Macintosh: QuickTime Components, this call returns a compressor information structure.

For example, information about what pixel depths a compressor supports for storing image data is important to choosing the right codec. If your application creates and compresses a picture, the decision of whether to create the picture in 8-bit, 16-bit, or 32-bit color can be based partially on what pixel depths the compressor supports. If the compressed data can store only 16 bits of color information, it would be inefficient to create a picture with 32 bits of color.

Accompanying this column on this issue's CD is the sample application GetCodecInfoApp, which (by calling CDGetCodecInfo) allows you to easily obtain detailed information about codecs installed in your system. I'll discuss GetCodecInfoApp and point out some characteristics that should be considered in choosing a codec.

USING CODECS
There are actually two separate parts to a codec: one for the compression and one for the decompression. Not all codecs provide both; nevertheless, all compressor and decompressor combinations are referred to as codecs. One example of a decompression-only codec is the codec that comes with the QuickTake 100 digital camera from Apple. The hardware in the QuickTake 100 camera performs the compression and downloads compressed data to the Macintosh. The Macintosh only needs to perform decompression.

A compressor is a component of type compressor-ComponentType ('imco') and a decompressor is a component of type decompressorComponentType ('imdc'). Detailed information on writing a codec is provided in Chapter 4 ofInside Macintosh: QuickTime Components . But to select the appropriate codec to use, you don't need to do any programming; you can simply use GetCodecInfoApp, without any need to understand how it was written. This application creates a text file containing a report of all the codec components installed in your system. For example, the output for the Cinepak codec looks like this:

Compressor Name: Cinepak
------------------------------------
- version = 1
- revisionLevel = 1
- vendor = appl
- compressionAccuracy = 128
- compressionLevel = 128
- minimum height = 1
- minimum width = 1
- compress pipeline latency = 0
- compression capabilities:
	directly compresses 32-bit pixel maps
	supports temporal compression
	can recompress images without accumulating errors
	can rate constrain to caller defined limit
- compression format:
	can store images in 24-bit color
	can store images in 8-bit grayscale
	can store custom color table
	compressed data requires non-key frames to be
		decompressed in same order as compressed
- estimated compression speed:
640x480 32-bit RGB = 11485 milliseconds

Decompressor Name: Cinepak
------------------------------------
- version = 1
- revisionLevel = 1
- vendor = appl
- decompressionAccuracy = 128
- minimum height = 1
- minimum width = 1
- decompress pipeline latency = 0
- decompression capabilities:
	directly decompresses into 32-bit pixel maps
	supports temporal compression
	can recompress images without accumulating errors
	can rate constrain to caller defined limit
- decompression format:
	can decompress images from 24-bit color
		compressed format
	can decompress images from 8-bit grayscale
		compressed format
	can store custom color table
	compressed data requires non-key frames to be
		decompressed in same order as compressed
- estimated decompression speed:
640x480 32-bit RGB = 56 milliseconds

GetCodecInfoApp gets information about codecs by calling the codec's CDGetCodecInfo function, which all codecs must support; if you're writing a codec, it's important to report your capabilities with this function. To measure the codec's speed, the application actually passes it an image to compress or decompress, and reports the result.

The Image Compression Manager function GetCodecInfo can also be used to obtain information about codecs, but only for compressor codecs; you won't be able to get information about decompression-only codecs with GetCodecInfo. *

An example of a characteristic you can determine with GetCodecInfoApp is what pixel depths the decompressor can decompress directly into. This is important because it affects the speed of the image decompression. If the codec can't decompress directly into the destination pixel map, the Image Compression Manager will have to decompress into an offscreen buffer and move the image data into the destination after converting the pixel depth. This results in additional memory and processor bandwidth requirements. If you know exactly what pixel depths a decompressor supports, you can set up the destination for the best performance.

Most codecs support only a limited number of pixel depths for the compressed data storage format. For example, the Video Compressor will store image data only in 16-bit color. If you compress a 32- bit color image, you'll lose information, since the compressed format will store the equivalent of 16 bits of data. The pixel depth for the compressed data storage format also determines which of the different compression settings are available -- for example, the pixel depth pop-up menu for Compression Settings displayed by the standard image-compression dialog component (used, for example, by Picture Compressor, an application that's part of the QuickTime Starter Kit) will only allow you to choose Color for the Video Compressor. The Animation Compressor is one of the few compressors that will store compressed data in nearly all pixel depth formats: Black and White, 4 Grays, 4 Colors, 16 Grays, 16 Colors, and so on.

When compressing movies, you'll often want to select a codec that supports temporal compression; not all codecs do. Temporal compression is the use of frame differencing to compress consecutive image frames by skipping data that doesn't change from frame to frame. Temporal compression is useful only for sequences of images stored as QuickTime movies. Knowing which codecs support temporal compression will allow you to choose the best codec for compressing sequences.

If you're compressing pictures with scientific data, it may be extremely important that there be no image quality loss. In this case, you'll want to look for a codec that supportslossless compression. For example, the Photo Compressor (JPEG codec) is a lossy codec because even at the highest quality setting, there may still be some loss of image quality. On the other hand, the Animation Compressor is lossless at higher quality settings and will preserve every pixel value.

There are many additional features a codec may support that are important to know. For example, certain codecs will support data spooling so that only portions of the compressed data need to be read into memory at any one time. This can be a requirement when working with very large compressed images that will be displayed in systems with limited memory. Another example is support for stretching to double size during decompression. This is extremely useful, since the performance is much greater if the scaling is performed during decompression rather than as a separate step after decompression.

SOME RECOMMENDATIONS
For most video clips, the Cinepak Compressor is the recommended codec. As you can see from GetCodecInfoApp's report, this codec is very slow in compression. However, its decompression speed and compression level are excellent, making it the best choice for most video data for CD-ROM playback.

An alternative to Cinepak is the Video Compressor. Since its compression speed is fairly quick, it's better for an application that requires fast compression.

If your source material is animation graphics in a movie, there are several compressors that may do the job. The Animation Compressor and Graphics Compressor may be equally suitable. In this case, you may need to experiment to determine which is the best codec to use. Finally, if you're compressing photo images, the Photo Compressor is the best codec to use. It has only moderate compression and decompression speed, but the compression ratio and quality are excellent and the compression ratio scales accordingly with image quality. If you want better image quality at the expense of larger compressed data size, you can easily achieve this with the Photo Compressor.

PRESSING ON
If you're writing a codec, you can see from this column that it's very important to properly report the codec's capabilities; GetCodecInfoApp may be useful for you to verify that your codec is doing this properly. For the rest of you, I hope this column has provided some insight on how to choose the right codec for producing the best movies and compressed images.


JOHN WANG (AppleLink WANG.JY) used to be a proud member of the PIGs (Printing, Imaging, and Graphics group) in Apple's Developer Technical Support group. But he decided that there are other challenges in life and programming. So now John spends his entire day waiting for MPW to compile code that he's writing in his software engineering role in the Image Capture group. Just in case you fail to notice, we're sure he'd like us to point out that he makes a gratuitous plug for his group's product, the QuickTake 100 digital camera, in this column. *

Thanks to Peter Hoddie, Don Johnson, Kent Sandvik, and Nick Thompson for reviewing this column. *

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Shadow Blade: Reload guide - How to hack...
Shadow Blade: Reload is the kind of action-platformer that would have happily sucked up hours of your time on a console a few years back.Now, you can take it with you wherever you go, and its mobile conversion is not too shabby at all. To help you... | Read more »
Tomb of the Mask guide - How to increase...
Tomb of the Mask is a great endless arcade game from Happymagenta in which quick reflexes and a persistent attitude can go a long way toward earning a top score. Check out these tips to see if you can give yourself an edge on the leaderboards. [... | Read more »
Smooth Operator! (Games)
Smooth Operator! 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Smooth Operator is a weird, weird two-player kissing game. Squeeze in for 2 player fun on a single iPad, creating awkward... | Read more »
Sinless: Remastered (Games)
Sinless: Remastered 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: | Read more »
_PRISM Guide - How to solve those puzzle...
_PRISM is a rather delightful puzzle game that’s been tailor made for touch screens. While part of the fun is figuring things out as you go along, we thought we’d offer you a helping hand at getting in the right mindset. Don’t worry about messing... | Read more »
Fractal Space (Games)
Fractal Space 1.3.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.3.1 (iTunes) Description: Live the memorable experience of Fractal Space, a unique first person adventure & puzzle game by Haze Games! Will you... | Read more »
Set off on an adventure through the Cand...
Like match three puzzlers? If so, Jelly Blast, the innovative iOS and Android game which launched last year, is worth a look. Jelly Blast sees you head off on an epic adventure through the Candy Kingdom with your friends Lily, Mr. Hare, and Mr.... | Read more »
Ellipsis - Touch. Explore. Survive. (...
Ellipsis - Touch. Explore. Survive. 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Abzorb (Games)
Abzorb 1 Device: iOS Universal Category: Games Price: $2.99, Version: 1 (iTunes) Description: Abzorb is a tilt game about consuming orbs by getting as close as you can to them without touching. By tilting your mobile device, collect... | Read more »
A Short Tale (Games)
A Short Tale 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: It’s been years since I lost Ben, so long yet it doesn’t seem long enough. I never thought I’d return here, to where everything... | Read more »

Price Scanner via MacPrices.net

cb Hardcase – Handmade and Premium Protective...
Baden-Baden, Germany based company cb innovations has introduced the new cb Hardcase for iPhone. Featuring fine Italian Premium leather that makes for a unique look and feel, the cb Hardcase... Read more
Sale! B&H Photo offers 12-inch Retina Mac...
B&H Photo has 12″ Retina MacBooks on sale for $300 off MSRP for a limited time. Shipping is free, and B&H charges NY tax only: - 12″ 1.1GHz Gray Retina MacBook: $999 $300 off MSRP - 12″ 1.... Read more
App Annie Reveals Future of the App Economy:...
App Annie, a San Francisco based mobile app data and insights platform, has launched its first comprehensive app economy forecast. This new offering will provide brands, agencies, investors and app... Read more
Apple restocks Certified Refurbished Mac mini...
Apple has restocked Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $... Read more
What iPad Pro Still Needs To Make It Truly Pr...
I love my iPad Air 2. So much that I’m grudgingly willing to put up with its compromises and limitations as a production tool in order to take advantage of its virtues. However, since a computer for... Read more
21-inch 3.1GHz 4K on sale for $1399, $100 off...
B&H Photo has the 21″ 3.1GHz 4K iMac on sale $1399 for a limited time. Shipping is free, and B&H charges NY sales tax only. Their price is $100 off MSRP: - 21″ 3.1GHz 4K iMac (MK452LL/A): $... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Save up to $240 with Apple Certified Refurbis...
Apple is now offering Certified Refurbished 12″ Retina MacBooks for up to $240 off the cost of new models. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The... Read more
Apple refurbished 13-inch Retina MacBook Pros...
Apple has Certified Refurbished 13″ Retina MacBook Pros available for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz... Read more
Apple refurbished Time Capsules available for...
Apple has certified refurbished Time Capsules available for $120 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $179, $120 off - 3TB... Read more

Jobs Board

Lead Engineer *Apple* OSX & Hardware -...
Lead Engineer Apple OSX & Hardware **Job ID:** 3125919 **Full/Part\-Time:** Full\-time **Regular/Temporary:** Regular **Listed:** 2016\-02\-10 **Location:** Cary, Read more
*Apple* Mobile Master - Best Buy (United Sta...
Job Title Apple Mobile Master **Brand** Best Buy **Job Description** **What does a Best Buy Apple Mobile Master do?** At Best Buy, our mission is to leverage the Read more
Infrastructure Engineer - *Apple* /Mac - Rem...
…part of a team Requires proven problem solving skills Preferred Additional: Apple Certified System Administrator (ACSA) Apple Certified Technical Coordinator (ACTC) Read more
Lead Engineer - *Apple* OSX & Hardware...
Lead Engineer - Apple OSX & Hardware **Job ID:** 3125919 **Full/Part\-Time:** Full\-time **Regular/Temporary:** Regular **Listed:** 2016\-02\-10 **Location:** Cary, Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.