Transparency is no more transparent if the opacity is less than 1

Question

I have a 32 bit PNG representing a blue square in the middle of a transparent square. Below I displayed this blue square with three different opacity values:

From left to right, the opacity is 1, 0.5, and 0.1, and as you can see (if you can't, just trust the Photoshop eyedropper), the area around the blue square is transparent only in the left image.

Here is the same image with color values:

I know that this is barely visible, but I would like to know what came of it. Plus with some images and background, contrast is more noticeable.

Thanks!

EDIT: Here is my VS solution for those who want to try at home: http://www.mediafire.com/?gt704e62d53z7ae

Answer 1

This is not smoothing, and most likely not Premumbiplied Alpha Blending , as its goal is to do what you do.

This is more like a rendering problem.

Have you tried this in multiple browsers? On another machine with a different GPU? On the same machine with updated drivers?

Answer 2

It can be anti-aliasing, creating a mixture between a transparent “frame” and a translucent “blue square”, causing the strange intermediate color halo you see.