On Mon, Oct 13, 2003 at 10:32:13AM -0500, Albert Wagner wrote:

I have several layered images developed for an animation. I built them using a white background for ease in drawing. All other layers were transparent. For each I then deleted the background layer, flattened the image, and saved as *.png. However, the flattened image still had a white background, when I intended that it be transparent. What did I do wrong?

That's exactly what flatten is intended to do. If you want an alpha channel in your png, just use save as a png without flattening. It'll ask you to merge the visible layers for the export because png can't handle them, and save a png just like you want it to: with alpha channel.

Marco Wessel

Hi Marco -

I've been working with some facets of digital images for a long time, but I still don't completely understand everything about transparency. In particular, you mention one of the things here that I am confused about. Could you please explain further exactly what transparency and alpha channels have to do with each other? Does the value of the alpha channel provide the transparency level for each pixel, and if so, then what does that have to do with "alpha"? Also, how is level of transparency actually applied in order compute the final display values for a pixel when a semi-transparent pixel is overlaid onto an underlying non-transparent pixel? Although the original question in this thread involved png files, I am more interested in tiff files, but I suspect that essentially the same answer applies to both. Thanks!

s/KAM

----- Original Message ---

Hi,

"Kevin Myers" writes:

Does the value of the alpha channel provide the transparency level for each pixel, and if so, then what does that have to do with "alpha"?

Your guess is right, or rather, almost right. The alpha channel actually provides the opacity levels for each pixel since usually a fully transparent pixel has an alpha value of 0 while a fully opaque pixel has an alpha value of 1.0 (or 100% or 255 or 65535 or ..., depending on how the data is actually stored).

Also, how is level of transparency actually applied in order compute the final display values for a pixel when a semi-transparent pixel is overlaid onto an underlying non-transparent pixel?

The formula is basically

dest = (1 - alpha) * dest + alpha * src

We assume that the alpha value runs from 0.0 to 1.0 here.

Sven

On Mon, Oct 13, 2003 at 11:06:09AM -0500, Kevin Myers wrote:

Hi Marco -

I've been working with some facets of digital images for a long time, but I still don't completely understand everything about transparency. In particular, you mention one of the things here that I am confused about. Could you please explain further exactly what transparency and alpha channels have to do with each other?

First off -- unless you mean you want to see through parts of an image entirely, you mean translucency. Transparent -> invisible; translucent -> see-through.

The alpha channel is basically a channel just like red, green, and blue, except that it determines the translucency of the pixel, instead of the colour.

what does that have to do with "alpha"?

I have no clue why they called it an alpha channel, if that's what you mean.

Also, how is level of transparency actually applied in order compute the final display values for a pixel when a semi-transparent pixel is overlaid onto an underlying non-transparent pixel?

While typing this email, I see sven has answered this.

Although the original question in this thread involved png files, I am more interested in tiff files, but I suspect that essentially the same answer applies to both. Thanks!

Both support alpha channels, yes.

Marco