AllyCAD: CAD Design Software - Tech Tip: TECH TIP NAME

To increase the redraw speed of large bitmaps, AllyCAD creates a tiled bitmap file, this file has the same name as the original bitmap, but with a BBF file extension. The creation and location of the tiled file can be modified.

Use Tile Compression	This compresses each tile during preprocessing. This makes for a smaller BBF file, but slower redraws, as each tile has to be decompressed again for display.
Bitmap threshold size for tiling	Any bitmap smaller than this file size (in kb) will not be tiled, and will just be handled by AllyCAD as a single entity.
Path for tiled file	This specifies the directory where the BBF file will be placed. If you give a . (period) as the path, then the BBF file will be placed in the same directory as the original bitmap.

If you are working with bitmaps from an aerial survey company, they may supply you with a log file giving the coordinates of the bitmap. If AllyCAD finds such a log file, it will scale and position the bitmap without any further intervention from you. If the log file is not found, then AllyCAD will display the Set Bitmap Parameters dialog box.

Each bitmap is made up of little squares called pixels. You need to type the width and height each pixel should be into the dialog box. Width and height are measured in the units you have set up in the Settings Menu › Drawing Settings.

For example, if you have scanned in a map that represents an area measuring 1200m x 1200m on the ground, and in its bitmap form this map now measures 600 x 600 pixels, then the real world pixel size is 2m in both width and height.

If you need to change the Real World Pixel Size of a bitmap once you have loaded it into your drawing, double click on the bitmap. This brings up the Set Bitmap Parameters dialog box again, which can then be edited.

If you check this option the bitmap will appear on the screen and print as a placeholder. This is a purple cross-hatched rectangle with the name of the bitmap written across it. Displaying bitmaps as placeholders speeds up redraws.

You can change a bitmap from a placeholder to full detail or vice versa at any stage after loading by double clicking on it, which brings up the Set Bitmap Parameters dialog box for editing.

You can also view all the bitmaps on your drawing as placeholders, or all of them in full detail. See Settings › View Settings.

The resample mode dictates how your bitmap will be displayed on the screen when you demagnify the screen or scale the bitmap to make it smaller. When you do either of these things, AllyCAD needs to change the way it displays your bitmap.

Preserve Black	AllyCAD will display fewer pixels than previously. The black pixels will be displayed at the expense of other pixels.
Preserve White	AllyCAD will display fewer pixels than previously. The white pixels will be displayed at the expense of other pixels.
Thin Pixels	All the pixels in your bitmap will be displayed, but they will be smaller than previously.

The default setting, Preserve Black, seems to work best for most images. However, for scanned maps and photographs you might want to try the Thin Pixels setting. You should experiment with the settings to see how they affect your bitmaps as you zoom in and out.