Alpha Channel Transparency?

[cadet]

When inserting a raster image into a DWG drawing, does AutoCAD support Alpha Channel Transparency? (AutoCAD 2004)

I’m trying to add an effect to a small portion of a drawing using Photoshop. I printed the AutoCAD drawing to a raster file format (png), opened the file in Photoshop, deleted everything except the portion I wanted to modify, saved the work on a transparent background, and then inserted the transparent image back into my AutoCAD drawing on paper space above an underlying model. I kept the pixel size the same as the drawing size to maintain registration (at 600 dpi).

I was expecting the underlying AutoCAD drawing to show through the transparent part of the overlaying raster image in paper space, but the overlying raster image completely blocks everything underneath.

[Neil Hardy]

You can use the draw order command.

Type dr on the command line ( this is shortfor Draw Order)

You are then asked to select a component e.g. line, block, or in this case an image...so...select the raster image you have inserted...press return

It then asks you where in the draw order it should go, in front of another object, behind another object, generally to the front or generally to the back, in this case by hitting return you choose the default which is 'generally to the back' .

So to summarise,

type dr ...hit enter

select image...hit enter

hit enter again

hey presto the CAD line work now shows over the image.

Regards

Neil

[CADTutor]

AutoCAD has very limited transparency support (it does not support alpha transparency). See here for details.

An alternative method would be to plot your drawing (minus images) to EPS (see here) . Import the drawing to Photoshop (see here) and then add the images as layers working with alpha transparenct, opacity etc. as required.

[cadet]

Thanks for the input. Apparently, I need to dig in and learn some channel basics. I just recently discovered the alpha channel (whatever that is) carries the transparency properties. It looks like AutoCAD supports “bi-tonal” transparency. I don’t know what the difference between bi-tonal and alpha channel transparency is, but I can speculate. I was able to set the AutoCAD properties on an imported black & white image to enable transparency, but haven ‘t tried it on a shaded image.

I’m somewhat new to Photoshop. Coming from a Windows background and AutoCAD graphics, learning Photoshop is overwhelming, frustrating and not intuitive. The help sections for Photoshop typically reference numerous items without a method to tunnel into the references, so it’s like reading Greek.

I’ll do some more reading on channels and continue hacking away. I’ll need to abort my current hopes of polishing a particular AutoCAD image because I ran out of time and had to make delivery, but will try again on future projects.

[Paul Sweet]

Try Modify > Object > Image > Adjust. Select the image, and in ACAD 2005 it brings up an Image Adjust dialog box with sliders for Brightness, Contrast, and Fade. I used this to fade a multi-colored screenshot image so my lines (on another layer) would stand out.

[CADTutor]

Just note that "Fade" is not the same as opacity. The images fades to the background colour but underlying linework will not be seen.

The truth is that AutoCAD image support is just not very good - something for the wish list I think.

[f700es]

Just note that "Fade" is not the same as opacity. The images fades to the background colour but underlying linework will not be seen.

The truth is that AutoCAD image support is just not very good - something for the wish list I think.

Maybe on the next release . How long did it take to get gradient fills again...

[CADTutor]

How long did it take to get gradient fills again...

Do you mean before or after solid fills?