Image Types

Manifold works with several standard types of images:




Grayscale Images - These are coded using one number per pixel representing one of 256 different gray tones ranging from black to white.




Palette Images - These are images coded using one number per pixel, where the number specifies which color in a palette of up to 256 different colors should be displayed for that pixel. The colors in the palette can be True Color RGB colors. Palette images save space at the cost of a reduced total number of colors available for use in the image. The image shown about uses only 16 colors.




RGB Images - These images use three numbers for each pixel, allowing possible use of millions of colors within the image at the cost of requiring three times as much space as grayscale or palette images. They are often called True Color RGB images in Microsoft applications.




RGBa Images - These images are RGB images with a fourth number added for each pixel that specifies the transparency of that pixel in the range 0 to 255. When seen in an image window, grayscale, palette and RGB images will be shown on a background of solid color (white by default). RGBa images are shown on a background of alternating white and light gray checkerboard pattern so that differences in transparency are more visible. (Use the Layers pane with images to click ON a background color layer or to show a border around an image.)




RGBa images are used when combining multiple images in maps for elaborate graphics composition or creation of special visual effects in maps. For example, the RGBa image illustrated above is shown in a layer above a grid of lines that become visible to an increasing degree as transparency increases towards the bottom of the image.




Compressed Images - Compressed images use sophisticated wavelet compression technology to not only compress the amount of data an image requires but also to reconstitute the image dynamically on demand. At any given zoom level the desired view of the image is reconstituted from the compressed data store. Compressed images can be viewed, but not edited or otherwise manipulated. Compressed images are used to display very large images that would require too much time for display and possibly too much room for storage if they were not compressed.


Compressed images have many of the display characteristics of RGBa images. The Layers pane will show Red, Blue and Green channels, and the View - Display Options command will allow choice of what image channels (in the case of multi-spectral compressed images) will be used for Red, Green, Blue and alpha channels.


Manipulating Images


Not all operations will work with all types of images. For example, compressed images may be viewed but cannot be edited, as can the other image types. For many operations the non-compressed types of images will seem equivalent. We can select a rectangular region of pixels in an image using Select Box, for example, without it mattering what type of image is involved. Some operations will only work for certain types of images. For example, the Hue / Saturation command only works with RGB or RGBa images and so this command will be disabled whenever the focus is on a grayscale image.


We can always convert a grayscale or palette image to RGB in order to use a particular command and then convert it back. To convert an image from one type to another, use the Image - Convert To dialog.


Other Types of Images


In addition Manifold can import data from multispectral raster data images that contain many channels. When importing from formats that support many channels Manifold will import each channel into one of the above image types (most normally, as a grayscale image). Operations on multispectral raster data sets can then proceed by choosing those images/channels to use. For example, three of the imported images can be combined into a single RGB image to create a "false color" image that uses three images/channels as R, G and B channels in a single image.


Saving Drawings, Maps, Surfaces and Labels as Images


Manifold can create images from other components. See the Tools - Make Image topic.


Tech Tip: Image Sizes


RGB and RGBa images require more space for storage than do grayscale or palette images. Each pixel of an RGB or RGBa image requires four bytes for actual color information and one byte to store information about selection, saved selections and invisible pixel status. At five bytes per pixel a 16000 x 16000 RGBa image requires 1.25 gigabytes of memory. Such a large image, when used for display only, should be converted into a compressed image for much faster speed.


At times when we would like to edit an image we cannot use compressed images and must convert such images back into RGB or RGBa or other image types. In such cases the compressed image will expand dramatically in size as it is decompressed and will require massively more processor time for computation. Working with very large, non-compressed images for anything other than display requires exceptionally fast computers and gigabytes of RAM.