新：求The ERDAS Field Guide第四,五章内容

新：求The ERDAS Field Guide第四,五章内容

Chapter 4 Image Display
本章教学要求：1、重点：RGB & Displaying Raster Layers 部分
2 、Using the Viewer 部分,结合实习1
Introduction
This section defines some important terms that are relevant to image display. This may differ from other systems, such as Microsoft Windows NT.
The display hardware contains the memory that is used to produce the image. This hardware determines which types of displays are available (e.g., true color or pseudo color) and the pixel depth (e.g., 8-bit or 24-bit).
Display Memory Size
• display resolution—the number of pixels that can be viewed on the display screen.
• the number of bits for each pixel or pixel depth.
Pixel（file pixel & display pixel）
• the data file value(s) for one data unit in an image
• one grid location on a display or printout
To display an image, a file pixel that consists of one or more numbers must be transformed into a display pixel with properties that can be seen, such as brightness and color.
Colors（RGB）
Red, green, and blue can be added together to produce a wide variety of colors, are therefore the additive primary colors.
（三原色和三补色,及其它颜色特性,推荐阅读彭书 58-66页）
color guns
On a display, color guns direct electron beams that fall on red, green, and blue phosphors. The phosphors glow at certain frequencies to produce different colors.
The combination of the three color guns, each with 28 possible brightness values, yields 224 or 16,777,216 possible colors for each pixel on a 24-bit display. (line 7-9, page 110)
Colormap and Colorcells
A colormap is an ordered set of colorcells, which is used to perform a function on a set of input values. To display or print an image, the colormap translates data file values in memory into brightness values for each color gun.
（SEE Table 4-1）
Colorcells
There is a colorcell in the colormap for each data file value. he red, green, and blue values assigned to the colorcell control the brightness of the color guns for the displayed pixel.
Colormap vs. Lookup Table
The colormap is a function of the display hardware, whereas a lookup table is a function of ERDAS IMAGINE.
Display Types
• 8-bit PseudoColor
• 24-bit DirectColor
• 24-bit TrueColor
32-bit Displays
A 32-bit display is a combination of an 8-bit PseudoColor and 24-bit DirectColor, or TrueColor display.
8-bit PseudoColor： a colormap with 256 colorcells.
This display grants a small number of colors to ERDAS IMAGINE. It works well with thematic raster layers containing less than 200 colors and with gray scale continuous raster layers. For image files with three continuous raster layers (bands), the colors are severely limited.
24-bit DirectColor or 24-bit TrueColor
两种方式（Colorcell or color gun）达到24位彩色实时显示的目的：
enables you to view up to three continuous raster layers (bands) of data at one time, creating displayed pixels that represent the relationships between the bands by their colors.
PC Displays
ERDAS IMAGINE for Microsoft Windows NT supports the following visual type and pixeldepths:
• 8-bit PseudoColor
• 24-bit TrueColor
8-bit PseudoColor
An 8-bit PseudoColor display for the PC uses the same type of colormap as the X Windows 8-bit PseudoColor display.
24-bit TrueColor
A 24-bit TrueColor display for the PC assigns colors the same way as the X Windows 24-bit TrueColor display.
Displaying Raster Layers
Continuous Raster Layers
An image file (.img) can contain >3 continuous raster layers; Therefore, when displaying an image file with continuous raster layers, it is possible to assign which layers (bands) are to be displayed with each of the three color guns.
Band assignments are often expressed in R,G,B order. E.g.
• Landsat TM—natural color: 3, 2, 1
• Landsat TM—color-infrared: 4, 3, 2
• SPOT Multispectral—color-infrared: 3, 2, 1
Contrast Table
When an image is displayed, ERDAS IMAGINE automatically creates a contrast table for continuous raster layers. The red, green, and blue brightness values for each band are stored in this table.
The brightness values often differ from the data file values, but they usually remain in the same order of lowest to highest. Some meaningful relationships between the values are usually maintained.
Contrast Stretch
Since the data file values in a continuous raster layer often represent raw data, the range of data file values is often small. Therefore, a contrast stretch is usually performed, which stretches the range of the values to fit the range of the display.
See figure 4-5.
statistics Files
To perform a contrast stretch, certain statistics are necessary, such as the mean and the standard deviation of the data file values in each layer.
（这些statistics变换下章将完整分析,这里只涉及有关图像显示的部分.这里仅要求观察图像的statistics,e.g. the mean, standard deviation, middle, maximum, minimum）
Figure 4-7 illustrates the general process of displaying continuous raster layers on a 24-bit TureColor display.
Thematic Raster Layers
A thematic raster layer generally contains pixels that have been classified. It is stored in an image (.img) file. Only one data file value (the class value) is stored for each pixel. The class system gives the thematic layer a discrete look, in which each class can have its own color.
即：为每种类别指定一种RGB的组合（颜色）,指定表称为Color scheme.
Color Table
When a thematic raster layer is displayed, ERDAS IMAGINE automatically creates a color table. The red, green, and blue brightness values for each class are stored in this table.
RGB Colors
Using the Viewer (未纳入教材,但对实习有指导意义)
意义：基于内存快速显示和进行某些图像处理（不保存）.
The Viewer not only makes digital images visible quickly, but it can also be used as a tool for image processing and raster GIS modeling.
Chapter 5 Mosaic
Introduction
The Mosaic process offers you the capability to stitch images together so one large, cohesive image of an area can be created. Because of the different features of the Mosaic Tool, you can smooth these images before mosaicking them together as well as color balance them, or adjust the histograms of each image in order to present a better large picture.
某些Mosaic Tool涉及到下章Enhancement的知识.