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The header is a section of binary- or ASCII-format data normally found at the beginning of the file, containing information about the bitmap data found elsewhere in the file. All bitmap files have some sort of header, although the format of the header and the information stored in it varies considerably from format to format. Typically, a bitmap header is composed of fixed fields. None of these fields is absolutely necessary, nor are they found in all formats, but this list is typical of those formats in widespread use today. The following information is commonly found in a bitmap header:



Bitmap Index

Palette 1

File Identifier

File Version

Number of Lines per Image

Number of Pixels per Line

Number of Bits per Pixel

Number of Color Planes

Compression Type

X Origin of Image

Y Origin of Image

Text Description

Unused Space

Later in this chapter we will present examples of headers from several actual formats, containing fields similar to those presented above.

File Identifier

A header usually starts with some sort of unique identification value called a file identifier, file ID, or ID value. Its purpose is to allow a software application to determine the format of the particular graphics file being accessed.

ID values are often magic values in the sense that they are assigned arbitrarily by the creator of the file format. They can be a series of ASCII characters, such as BM or GIF, or a 2- or 4-byte word value, such as 4242h or 596aa695h, or any other pattern that made sense to the format creator. The pattern is usually assumed to be unique, even across platforms, but this is not always the case, as we describe in the next few paragraphs. Usually, if a value in the right place in a file matches the expected identification value, the application reading the file header can assume that the format of the image file is known.

Three circumstances arise, however, which make this less than a hard and fast rule. Some formats omit the image file identifier, starting off with data that can change from file to file. In this case, there is a small probability that the data will accidentally duplicate one of the magic values of another file format known to the application. Fortunately, the chance of this occurring is remote.

The second circumstance can come about when a new format is created and the format creator inadvertently duplicates, in whole or in part, the magic values of another format. In case this seems even more unlikely than accidental duplication, rest assured that it has already happened several times. Probably the chief cause is that, historically, programmers have borrowed ideas from other platforms, secure in the belief that their efforts would be isolated behind the "Chinese Wall" of binary incompatibility. In the past, confusion of formats with similar ID fields seldom came about and was often resolved by context when it did happen. Obviously this naive approach by format creators is no longer a survival skill. In the future, we can expect more problems of this sort as users, through local area networking and through advances in regional and global interconnectivity, gain access to data created on other platforms.

This third circumstance comes about when a vendor--either the format creator or format caretaker or a third party--changes, intentionally or unintentionally, the specification of the format, while keeping the ID value specified in the format documentation. In this case, an application can recognize the format, but be unable to read some or all of the data. If the idea of a vendor creating intentional, undocumented changes seems unlikely, rest assured that this, too, has already happened many times. Examples are the GIF, TIFF, and TGA file formats. In the case of the GIF and TGA formats, vendors (not necessarily the format creators) have extended or altered the formats to include new data types. In the case of TIFF, vendors have created and promulgated what only can be described as convenience revisions, apparently designed to accommodate coding errors or application program quirks.

File Version

Following the identification value in the header is usually a field containing the file version. Naturally enough, successive versions of bitmap formats may differ in characteristics such as header size, bitmap data supported, and color capability. Once having verified the file format through the ID value, an application will typically examine the version value to determine if it can handle the image data contained in the file.

Image Description Information

Next comes a series of fields that describe the image itself. As we will see, bitmaps are usually organized, either physically or logically, into lines of pixels. The field designated number of lines per image, also called the image length, image height, or number of scan lines, holds a value corresponding to the number of lines making up the actual bitmap data. The number of pixels per line, also called the image width or scan-line width, indicates the number of pixels stored in each line.

The number of bits per pixel indicates the size of the data needed to describe each pixel per color plane. This may also be stored as the number of bytes per pixel, and is more properly called pixel depth. Forgetting the exact interpretation of this field when coding format readers is a common source of error. If the bitmap data is stored in a series of planes, the number of color planes indicates the number of planes used. Often the value defaults to one. There is an increasing tendency to store bitmaps in single-plane format, but multi-plane formats continue to be used in support of special hardware and alternate color models.

The number of bits in a line of the image can be calculated by multiplying the values of number of bits per pixel, number of pixels per line, and number of color planes together. We can determine the number of bytes per scan line by then dividing the resulting product by eight. Note that there is nothing requiring number of bits per pixel to be an integral number of 8-bit bytes.

Compression Type

If the format supports some sort of encoding designed to reduce the size of the bitmap data, then a compression type field will be found in the header. Some formats support multiple compression types, including raw or uncompressed data. Some format revisions consist mainly of additions or changes to the compression scheme used. Data compression is an active field, and new types of compression accommodating advances in technology appear with some regularity. TIFF is one of the common formats which has exhibited this pattern in the past.

For more information about compression, see Chapter 9, Data Compression.

x and y Origins

x origin of image and y origin of image specify a coordinate pair that indicates where the image starts on the output device. The most common origin pair is 0,0, which puts one corner of the image at the origin point of the device. Changing these values normally causes the image to be displayed at a different location when it is rendered.

Most bitmap formats were designed with certain assumptions about the output device in mind, and thus can be said to model either an actual or virtual device having a feature called the drawing surface. The drawing surface has an implied origin, which defines the starting point of the image, and an implied orientation, which defines the direction in which successive lines are drawn as the output image is rendered. Various formats and display devices vary in the positioning of the origin point and orientation direction. Many place the origin in the upper-left corner of the display surface, although it can also appear in the center, or in the lower-left corner. Others, although this is far less common, put it in the upper- or lower-right corner.

Orientation models with the origin in the upper-left corner are often said to have been created in support of hardware, and there may be some historical and real-world justification for this. People with backgrounds in mathematics and the natural sciences, however, are used to having the origin in the lower-left corner or in the center of the drawing surface. You might find yourself guessing at the background of the format creator based on the implied origin and orientation found in the format. Some formats include provisions for the specification of the origin and orientation.

An image displayed by an application incorporating an incorrect assumption about the origin point or orientation may appear upside down or backwards, or may be shifted horizontally some fraction of the width of the drawing surface on the output device.

Sometimes the header will contain a text description field, which is a comment section consisting of ASCII data describing the name of the image, the name of the image file, the name of the person who created the image, or the software application used to create it. This field may contain 7-bit ASCII data, for portability of the header information across platforms.

Unused Space

At the end of the header may be an unused field, sometimes referred to as padding, filler, reserved space, or reserved fields. Reserved fields contain no data, are undocumented and unstructured and essentially act as placeholders. All we know about them are their sizes and positions in the header. Thus, if the format is altered at some future date to incorporate new data, the reserved space can be used to describe the format or location of this data while still maintaining backward compatibility with programs supporting older versions of the format. This is a common method used to minimize version problems--creating an initial version based on a fixed header substantially larger than necessary. New fields can then be added to reserved areas of the header in subsequent revisions of the format without altering the size of the header.

Often format headers are intentionally padded using this method to 128, 256, or 512 bytes. This has some implications for performance, particularly on older systems, and is designed to accommodate common read and write buffer sizes. Padding may appear after the documented fields at the end of the header, and this is sometimes an indication that the format creator had performance and caching issues in mind when the format was created.

Reserved fields are sometimes only features left over from early working versions of the format, unintentionally frozen into place when the format was released. A vendor will normally change or extend a file format only under duress, or as a rational response to market pressure typically caused by an unanticipated advance in technology. In any case, the upgrade is almost always unplanned. This usually means that a minimal amount of effort goes into shoehorning new data into old formats. Often the first element sacrificed in the process is complete backward compatibility with prior format versions.

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This page is taken from the Encyclopedia of Graphics File Formats and is licensed by O'Reilly under the Creative Common/Attribution license.