I created a number of gradient masks applied to images with a basic 100 to 0 black gradient, on my Mac at home. They look smooth in Illustrator, and rendered smooth both as a PDF and exported to a PNG.
Gradient Mask from my Mac at home:
But when I open the same illustrator document on my Mac at work the gradation of the mask is clipping. So instead of a smooth transition there's now a hard line.
Gradient Mask on my Mac at work:
This is the same file, opened on different Macs. Both identical versions of Illustrator CS6, identical color settings, identical color profiles.
The main differences are my Mac at home is a 2009 MacPro running OS 10.9, and my Mac at work is a 2008 MacPro running OS 10.8.
I've created and recreated gradient masks over and over again, and I cannot get a smooth transparency gradient mask at all on my Mac at work.
This presented a problem, as the client had to have last minute revisions when I was at work, and could not produce a smooth image, I was forced to deliver the final graphic with clipped gradient masks.
If you export a .png from both computers and post them here, i see no reason why they would be different. Does the file look bad at work when you open, or after you create the .jpg (what you posted), or .png.
Are you using save for web, sounds like you have a setting different there. Compression level or image size tab/optimized is set to none
The thing is, this is also how it renders in Illustrator. In Illustrator at home a smooth gradient mask, in Illustrator at work the mask is clipped. Both my home and work machines are configured with the same settings for exporting, and color profiles. It's the same file yeilding different results. My next step will be to trash the preferences on both machines, and see if they still render differently.
So deleting preferences didn't fix it. I even rasterized the gradient to see if an image mask would work better, that didn't work. Then I toggled "invert mask." I was suprised to see that once I inverted the mask (which reversed the gradient), the transparency was smooth from 0 to 100.
So I've figured out a work around for the problem, but it's still not resolved.
Here's the result:
Now if someone can explain to me why a gradient mask doesn't work in one direction, but it does when it's inverted.