Working with raster/attached images in CAD

A few thoughts/suggestions: When doing trace/reference work from sketches where I want a high level of detail, I tend to use the .tif rather than .bmp format. .tif offers the option of compression schemes that CAD can usually also read. if I’m going to be plotting something I’ll create a .jpg at the specific output size/resolution of my final output to make sure I have the smallest file possible with only as much detail as needed. This sometimes means having multiple versions of the same image for plotting at different scales or for different purposes.

It’s also important to understand some of the basic differences in image types/formats when considering resolution and file size. If there are no shading values in the sketch (black and white only), a bitmapped image will be smaller though more jaggy than a grayscale image. If there is shading but no color, a grayscale will again be smaller than a full color file. Recognize that image format or extension doesn’t necessarily govern image type: .tif can be any of those things; a .jpg can only be grayscale or color.

Plotting referenced images is a whole other layer of complication. Memory issues and raster plotting language conflicts (particularly with layering) are typically the causes behind referenced images missing parts or not appearing at all on a plot. There are also relevant image/plotting settings in CAD, such as attachment scale, whether it was attached in paper or model space, and plot scaling.

It looks like you’re taking pdfs of a plan set to use as a base. Are those pdfs the same size as your sheet? Using an 11×17 pdf as a base on a 24×36 sheet is going to be a challenge, even at higher resolutions. What are the file sizes of your attached images? Is there any way, as Nick suggested, to get CAD files to use as xrefs rather than images? Are you attaching in paper or model space? What program and resolution do you use when converting the pdf to a bmp? Do you scale the image in CAD? Are the printing errors random or fairly consistent? What kind of CAD program are you using? The example appears to be irrigation design, and I know from talking with some lighting and irrigation consultants that specialized CAD programs for those applications can be really finicky when it comes to images, other CAD formats, and plotting.

I think Chris Whitted has done a great job providing some tips as well as asking some great questions. Addressing some of his questions may allow us to offer better insight.

As to the use of raster images in CAD, we only use two graphic formats, PCX and JPG.

– For reference / trace purposes, we scan items as a B&W PCX file. It is jaggy like a BMP tends to be, but the benefit is that you can drop the white space from the image in CAD and just see the linework. PCX also allows for your layer color / associated pen widths to affect the image when printed. This has proven handy when using scanned images for graphic elevations. Items to the rear are put on a layer that prints grayscale, while other foreground images are put on a layer that prints black

– JPG is used for all other graphic purpose solely for file size management.

– We hardly scan as B&W due to the jaggy nature of the image, unless it is in PCX (see above). All other scans are grayscale (for file size management) or color; depending on need.

– Scan resolution never tops 250 dpi; above that is pretty much overkill.

Hope some of this is of use to you. Good luck.

Thank you.

The transparency issue is a function of the image type. Black and white bitmapped images have only two possible colors, and ACAD tends to automatically interpret white as ‘should be transparent’. I believe there’s also a setting in the attachment/image options to turn this on and off, or to specify a color for transparency. With grayscale images, ACAD treats white as just another color so you get a solid background.

Your case exemplifies what I was saying about the difference between file types and file formats, and that .tif is a very flexible format (or container if you will) that can hold a lot of different image types/options. Your scanner produces a bitmapped image under the black and white (often also called line art) setting. If you open the .tif from your .pdf in Photoshop and go to Image > Mode, you’ll likely see it set to Grayscale or possibly Indexed Color. If you change it to Bitmapped (and enter a target conversion resolution), save it and then bring it into CAD, you’ll get rid of the background – at the cost of more jaggy lines, depending on how closely you’ve zoomed in.

The memory issues can sometimes be resolved by setting it to ‘process in computer’ instead of relying on the plotter’s memory, especially since current machines generally far outstrip plotters in memory (but it depends on your setup, and if it runs through a print server first and not straight to the plotter there are other factors to consider). Keep in mind that complex layers and transparency, for example from an Illustrator file with lots of layers and masks, can have something of an exponential impact on required memory in addition to frequently causing raster language conversion problems.

this is truth.

A couple of the other comments have made me remember one other thing that can cause distortion, particularly in bitmapped images. There’s a setting I only know how to access through Photoshop that lets you see the Pixel Aspect Ratio. 9 times out of 10, it’s set to square and everything displays right. Once in a while I’ll find a file that either doesn’t specify or specifies the wrong aspect ratio, so the image appears ‘squished’ in some applications – presumably those that actually pay attention to the setting, while it looks normal in those that don’t.

I use Photoshop, which happens to be capable of opening .pdfs directly. It’s overkill for a lot of people, but then again so is AutoCAD. An Elements version will probably get you all the tools you would need for a cheaper price than the full version, but it’s been a while since I looked into what they strip out. Acrobat should be able to save .pdfs to an image format as well, but I don’t know if that funcationality is limited to the full version or if you can do with with the free viewer version too.