Loading Bitmaps

This Tech Tip explains how to load bitmaps.

Types of Bitmaps Supported

AllyCAD can load many different types of bitmaps, these include:

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.

Setting Menu › System Settings

Loading the Bitmap

File Menu › Load › Load Bitmap

Select the file and click on the Open button.

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.

Real World Pixel Size
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.

Placeholder for Bitmap
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.

If the option is not checked the bitmap will be displayed in full detail.

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.

Resample Mode
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.

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.

AllyCAD will prompt:

Once the coords have been entered the bitmap will be loaded.

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