A raster image in CAD (Computer-Aided Design) refers to a type of digital image that is composed of a grid of individual pixels, each with its own color value.

Here are some key points about raster images in the context of CAD:

Characteristics of Raster Images

Pixel-Based: Raster images are made up of tiny squares called pixels. Each pixel represents a specific color, and together they form the complete image.

Resolution Dependent: The quality of a raster image is determined by its resolution, which is measured in pixels per inch (PPI). Higher resolution means more detail but also larger file sizes.

File Formats: Common raster image formats include JPEG, PNG, BMP, and TIFF. These formats are typically used for photographs and complex images.

Usage in CAD

Backgrounds and Textures: Raster images can be used as backgrounds or textures in CAD designs, providing visual context or enhancing the presentation of a design.

Importing Scans: CAD software often allows users to import scanned images (like blueprints or sketches) as raster images, which can then be traced or referenced in the design process.

Limitations: Unlike vector images (which are based on mathematical equations and can be scaled without loss of quality), raster images become pixelated when enlarged. This makes them less suitable for detailed drawings that need to be resized frequently.

How to attach raster images?

Use IMAGEATTACH command to select and attach raster images, or bitonal, 8-bit gray, 8-bit color, or 24-bit color image files to a drawing. The image file can be inserted as blocks as many times as you like once attached to the current drawing, you can clip the attached raster image and setup its lightness, contrast, fading and transparency.

To attach a raster image:

1. Choose Insert > Raster Image reference from the main menu.
2. Specify a file to attach, and then click Open.
3. In the Image dialog box, in the Insertion point and Scale, click Specify on-screen. Specify an angle value in Rotation, and then click OK.
4. Specify a insertion point.
5. Specify a scale.

How to scale raster images?

You can specify scale factor in the Image dialog box, otherwise to attach it by its original size. The raster image will be scaled by the specified factor, the scale factor is used without unit by default.

How to detach raster images?

Raster images can be detached if it no longer needs to use in the drawing, a specified image detached from the drawing together with its multiple copies, links and definitions, but the original image file will not be influenced.

How to modify and manage raster images?

Controls the properties such as displaying and clipping boundary of raster images. You can view and manipulate attached raster image and change its saving path in Image Manager. Users can turn on/off image boundary in current view using IMAGEFRAME command and setting up the values 0 (off) 1(on).

To turn image frames on or off for all images:

1.Choose Modify>Object> Image> Frame

2.Do one of the following to toggle frames off and on:

-Type value 1 to display and print frames for all images in a drawing.

-Type value 0 to hide all frames on the screen and during printing.

To clip an image in the shape of a rectangle and polygon:

1.Choose Modify>Clip> Image

2.Select the edge of the image you want to clip.

3.Type N (New boundary) to create a new clipping boundary.

4.If you choose Rectangular:

-Define the first corner of the clipping rectangle.

-Define the second corner of the clipping rectangle. The selected image is clipped so that only the interior of the rectangle is visible.

5.If you choose Polygon:

-Select the points for the polygon, and then press Enter when the polygon is complete. The selected image is clipped so only the interior of the polygon is visible.

How to change raster image brightness, contrast, and fade

Use IMAGEADJUST to adjust displaying result and lightness, contrast and fade that are related to the display and plot effect when plotting drawings. IMAGEADJUST does not affect the original raster image as well as other instances of the image.

How to improve display speed of raster images?

You can adjust the display speed by setting the raster image quality. The image quality is sorted into high and draft levels, If the quality is set to draft, the image will be displayed with some granular materials, however with faster display speed.