An Opacity Mask "D'oh!" Moment

[RichH]

The Sketchucation.com site has a SketchUp forum with a long-running ”D’oh!” thread where posters write about program features that, when they encounter them, cause them to say to themselves, “Of course. That was obvious. Why didn’t I see it before?”

I just had one of those moments with Xara. I had thought opacity masks were just done with various vector shapes. I didn’t realize they could be done with imported bitmap images.

I had created two PNG images with a rendering extension in SketchUp. The first was a rendering of an architectural model. The second was an alpha mask that screened out the sky and anything else that wasn’t part of the model. Previously I have loaded the two into Paint Shop Pro and applied the second image as a layer mask to the first.

As I had XDP open, I thought I’d try to do something similar with it. I imported both of the images, selected and copied the mask image to the clipboard, selected the full rendering image, right-clicked on it, and picked “Paste as opacity map” from the context menu. The result was just like it had been in the image editing program (Paint Shop Pro).

In retrospect, I guess it is sort of obvious. Though the “Opacity Mask” information in Xara Help does focus on vector shapes created with the various tools and the black/white (saturation) levels of their fills, an image is just a rectangular shape with a fill. So “D’oh!”

Maybe everybody knows this, but maybe someone doesn’t.

[gwpriester]

Can you create a mini tutorial on how you did this? Or post a series of images that show the steps?

[RichH]

OK. See the attached image.

[gwpriester]

That's awesome. This old dog (me) might just have learned a new trick.

[Egg Bramhill]

Don't use SketchUp much Rich but why not just export from SU as 2d image with "Transparent Background" ?

[RichH]

Most often I use the standard Sketchup texture, material and shadow rendering just as you're suggesting. In this case I was using the Twilight Render plugin (just one of a long list of 'em) which allows you to do photorealistic rendering within Sketchup. With it you have to do the two step process to mask out the model. A renderer I used to use with AutoCAD got rid of the background by just putting a check in a box in a dialog before saving just as you're showing there.

This just reminded me of the versatility and "swiss army knife" nature of Xara.

[handrawn]

you have always been able to use greyscale true colour images as transparency masks in xara

select what you want to mask

select the masking image in the bitmap gallery, and click the gallery transparency [transp] button

learnt that from another rich [molucca] years ago

[RichH]

As I said, it was somewhat of a "D'oh!" moment.

However, I tried your steps with the two images I used and saw that I needed an inverse or negative version of the black and white one to mask out the background.

[handrawn]

that would be so - solid black is kept and solid white is not, and points in between graduated, with that method

I've never actually used opacity masks in xara much at all, so if it works the other way round that had escaped me