Bitmap File Types

The following notes are the result of my personal experience; as ever, with any information you come across on the Internet; where you have a critical technical decision to make, crosscheck and compare information with other sources.

You will probably know already that there is a bafflingly large number of file types for storing images. Unfortunately, many of them exist purely for proprietary reasons and have absolutely no advantages over other file types.

On the PC platform, the part of a name following a dot is its file type. For example, if you receive a file called hlr0001.jpg the “hlr0001” part of the name is just that, a name. The “0001” bit indicates that it is more than likely just one of a whole sequence of picture files. The last part of the name, that is “.jpg” indicates that the file is a bitmap picture using lossy compression (see below) of the “jpeg” type.

To simplify as much as possible: a bitmap graphic is a file which represents picture information. It is built up of coloured dots called "pixels", very much like the pictures used in print media, (so-called "half-tone").

You will find a more detailed description of pixels and bitmaps in the notes about aspect ratios, here.

Vector images are a different way of representing imagery to bitmap images and are stored in their own file types, such as Postscript (.ps or .eps) or Illustrator files. I very rarely use vector image files, so I will concentrate on the type that is my bread and butter, the bitmap files. The widely used Flash program, by the way, works natively in vector image mode. It is very easy, however, to output the final animation as bitmap files which are needed when transferring to video.

The most common bitmap files I use are as follows:

 

file type

extension name

alpha channel
?

options

TIFF

.tif or .tiff

yes

comes in very many varieties. I mostly use RGBA tiff files with LZW compression (lossless)

BMP

.bmp

no

useless for most of our purposes, but included because it is a bog-standard Windows file, hence pretty much universally readable

JPEG

.jp or .jpeg

no
(newer variants may well offer this)

always compressed, to a degree that the user can set. The compression is lossy, which means it loses picture information which cannot be recovered. Used primarily because it does a reasonable job at making small files of acceptable quality

Targa

.tga
.vst

yes

can also have lossless compression, but for some reason this is not so common. Seems to be falling out of favour.

PNG

.png

yes

everyone agrees this is the bitmap file of the future, unfortunately that day never seems to come. Lossless compression. I occasionally use it because it is the only file format output by Flash 5 that can include an alpha channel.

GIF

.gif

1 bit

GIF files are really only suitable for web usage. Their advantage is that they can use small numbers of colours and for simple images with flat colours can produce very small clean looking images. You can have a transparent background in a GIF file, but it can’t be used for clean edges such as produced by an 8 bit alpha channel

PSD

.psd

yes

Native Photoshop files. Many applications can read these files, though you must be aware that newer versions of Photoshop may produce files not readable by older programs.
These files are optimised in size and are especially useful in that they can contain layers which can be individually switched on or off

 

note: most MACs can read PC bitmap files and the reverse is true - but be aware that a bitmap image on a MAC user's screen is probably going to look different (in colour and contrast) to what a PC viewer will see. The three most commonly used platforms, PC, MAC and SGI will all display the same graphic file differently and all three will usually look different again to the same file seen on a video monitor.
Kodak and others do have cross-platform solutions, but colour management is a complex issue and far from universal.
My advice is to look at your work on a good and properly set up video monitor viewing the image as YUV or RGB component or as a digital image via SDI (serial digital interface). Even if the picture is slightly incorrect in terms of overall colour bias and saturation, these deficiencies can be easily adjusted in a post-production environment where expensive grade 1 monitors are used. Grade 1 monitors are the only guaranteed way of ensuring that you are looking at a neutral and correctly colour banced image for broadcast.
Just working on a computer monitor alone can be misleading, though it is worth noting that there are now switchable high brilliance computer monitors coming on to the market which claim to emulate video monitors.

For more information on vector and bitmap graphics, click here

 

back to Pixels and Pegbars  

>home