I understand why blending alpha from the alpha mask pixels could cause this, but couldn't you just ignore those pixels or weight that alpha in the interpolation?
imm
I'm still confused by where does the black in the composite comes from and how it is fixed by premultiplying. By 50% blue pixels with 50% alpha does the slide mean 50% blue, 50% black with 50% alpha?
kencheng
If you don't premultiply, and instead do straight color interpolation, you'll pick up unwanted colors in the color channel.
In this case, the blue edge picks up black via interpolation and becomes dark blue. When we apply this to the yellow background, we'll apply dark blue (with some alpha) on top of yellow.
If instead, we had premultiplied colors, the blue edge doesn't pick up any of the black (it only looks darker because of lack of light). When we apply this on to the yellow background, we'll apply blue (with some alpha) on top of yellow.
coco
Thanks, that makes a lot of sense!
kbeache
I'm confused about why we are representing alpha as white over black . Doesn't alpha suggest transparency
username
Alpha means opacity, from slide 37 we can see that if alpha is 1, it represents a fully opaque surface. For gray level image, 1 means white, so I guess that's why we use white to represent that alpha equals 1 here.
csciutto
This helped a ton!
http://www.adriancourreges.com/blog/2017/05/09/beware-of-transparent-pixels/
I understand why blending alpha from the alpha mask pixels could cause this, but couldn't you just ignore those pixels or weight that alpha in the interpolation?
I'm still confused by where does the black in the composite comes from and how it is fixed by premultiplying. By 50% blue pixels with 50% alpha does the slide mean 50% blue, 50% black with 50% alpha?
If you don't premultiply, and instead do straight color interpolation, you'll pick up unwanted colors in the color channel.
In this case, the blue edge picks up black via interpolation and becomes dark blue. When we apply this to the yellow background, we'll apply dark blue (with some alpha) on top of yellow.
If instead, we had premultiplied colors, the blue edge doesn't pick up any of the black (it only looks darker because of lack of light). When we apply this on to the yellow background, we'll apply blue (with some alpha) on top of yellow.
Thanks, that makes a lot of sense!
I'm confused about why we are representing alpha as white over black . Doesn't alpha suggest transparency
Alpha means opacity, from slide 37 we can see that if alpha is 1, it represents a fully opaque surface. For gray level image, 1 means white, so I guess that's why we use white to represent that alpha equals 1 here.
This helped a ton! http://www.adriancourreges.com/blog/2017/05/09/beware-of-transparent-pixels/