Extra file format information
Image file formats often specify more information than is represented by CgDeviceIndependentImage. For example, image data in BMP format can either be uncompressed or compressed using run-length encoding. The application might need to know the data compression technique used by a loaded image's file so that the image can later be unloaded in the same way. The image file format objects preserve this extra information as part of their state when an image is loaded, and use this information when an image is unloaded.
Each of the CgFileFormat classes provides format-specific methods to access and change the extra information. These methods are outlined in the following sections.
Following these sections, an example illustrates how to manipulate extra information.
GIF
aspect
Aspect ratio of the creating device
background
Background pixel value of the image
extensions
GIF89a control extensions of the image file
screenExtent
Point specifying the extent of the screen on which the image was created
signature
signature of the GIF file. Possible values are: 'GIF87a' and 'GIF89a
transparentPixel
Color index of the transparent pixel or nil if none
JPEG
acHuffmanTables
AC-component Huffman tables of the receiver
app0
Application defined segment of the receiver
arithmeticCoding
Arithmetic coding state (true or false)
arithmeticTables
Arithmetic conditioning tables
bufferCurrentPosition
Current position of the local data buffer
byteStream
Byte stream
comments
Comments
dataBuffer
Local data buffer
dcHuffmanTables
DC-component Huffman tables
errorHandler
Error handler
frameHeader
Frame header
imageComponents
Image components
nextRestartNumber
Next restart number
quantizationTables
Quantization tables
restartInterval
Restart interval
restartsToGo
Restarts-to-go counter
scanHeader
Scan header
PCX
The following methods provide extra information preserved and used by the CgPCXFileFormat class:
hRes
Horizontal resolution of device
paletteInfo
How to interpret palette (color or gray scale)
version
Format version number
vRes
Vertical resolution of device
PM BMP
The following methods provide extra information preserved and used by the CgPMBMPFileFormat class:
colorEncode
Color encoding scheme
colorsUsed
Number of colors used by the image
compression
Data compression algorithm used
ident
Application specific identifier
importantColors
Number of colors important for displaying image
recording
Recording algorithm used
rendering
Halftoning algorithm used
resUnits
Units of measure for xRes and yRes
size1
Halftoning correction factor
size2
Halftoning correction factor
xRes
Horizontal resolution of device
yRes
Vertical resolution of device
TIFF
The following methods provide extra information preserved and used by the CgTIFFFileFormat class:
colorScheme
Color scheme (gray scale, palette, RGB)
compression
Data compression algorithm used
predictor
Prediction method, used with some types of compression
Windows BMP
The following methods provide extra information preserved and used by the CgWinBMPFileFormat class:
compression
Data compression algorithm used
importantColors
Number of colors important for displaying image
pelsPerMeter
Horizontal and vertical resolution of device
Example
The following code illustrates how to unload an image in Windows BMP format, using run-length encoding of the data if the image has a depth of 4 or 8. The device resolution and number of important colors are also specified.
| image format |
image := self imageToUnload.
format := CgWinBMPFileFormat new.
format compression: 0. "No compression by default."
image depth = 8
ifTrue: [format compression: 1]. "RLE8 compression."
image depth = 4
ifTrue: [format compression: 2]. "RLE4 compression."
format importantColors: (1 bitShift: image depth) - 3.
"All but 3 colors are important. Approx. 75 dots per inch."
format pelsPerMeter: 3000 @ 3000.
format unload: image intoFile: 'img-out.bmp'