[This would be easier with pictures, but I don't believe we can embed images in our posts — can we?}
I've been experimenting with blur to simulate the “Glamor Glow” filter in Nik Software's Color Efex Pro. I applied blur to entire image, then used the Eraser — full strength set on Highlights only — over the entire image. Accidentally, I later used the Eraser again (full strength, HIghlights only) and discovered (using Color Overlay) that additional areas where affected. Using Eraser (same settings) again and again removed more and more of the blur to higher toned areas of the image. I thought no more would be removed because I had already removed it from the image's highlights.
This lead me to set up a test image (using another app) with vertical strips 240px wide each across the image from pure black on left to pure white on right. I imported to Aperture and applied blur at full strength to entire image. Then I started playing with the Eraser (full strength) set on Highlights. Going over and over the same areas with multiple applications I finally realized. Tried the same with midtones and then Shadows.
Finally is dawned on me. Aperture was not applying the changes to the Highlights of the “image” but rather the highlights of the “MASK!”
Imagine a mask as PhotoShop would display it. Aperture can do this when you select “Brush Strokes” under the gear wheel in the blur brush dialog box. When first applied the entire brush strokes image displays white, that is, applied to entire image. But if you Erase using only “Midtones” for example, you will see the middle “strips” (in my black to white image) have the blur removed the most and less on black and white areas of the image. However, additional applications of the Eraser (set to Midtones) will affect more and more “strips” with each application. The reason is that with each application the gray tones of the “MASK” (not the midtones of the image) are getting darker and darker. With enough applications of the Midtone Eraser the blur is removed from nearly all of the middle strips leaving only the pure black and pure white strips unaffected.
Wow — I just read this post and I'm sure I've confused most of you. If anyone is interested I can try to show examples on my SmugMug site.
The point is: when Aperture says shadow, mid tone, or highlights in the brush tool — at least when erasing an effect (like blur) it means the relative tone of the “mask” at any point in the process.
I'd be thankful for feedback on this, even if its to say “what in the world are you talking about?” Again, I'll post sample images if this is not clear.