Make JPG white background transparent - actionscript-3

Question

I am loading some images and I want to go from PNG to JPG to have a smaller file size. However as JPG there is no transparency information so the background of the image shows up as white.
How can I make this white background be transparent once I load my images?
Thanks in advance

Answer 1

I am assuming that you want to change the white pixels to transparent pixels at runtime. First I'll just say that the BEST answer , is to just use the PNG format. My opinion is that doing something like this at runtime is not a good solution. If you have that many images that the file size matters, then the runtime processing brings along it's own negative of a long pause while processing the images.
I personally would explore what I could do to decrease the size of the PNG files, and would likely NEVER use a solution like this.
However, if you choose to go this route, what you need to do is this :
Convert the image to a BitmapData with an alpha channel.
Loop through the BitmapData and make those pixels transparent.
or
Use the threshold() method of BitmapData.
As someone mentioned in the comments for your question, using white as the background color is not a good idea as ALL white pixels will be made transparent, not just the background ones.
So choosing a color that is not in the palette of any of these images is what you want to use as a background color
As a commenter noted, the threshold() method of BitmapData would probably work best.

Answer 2

JPG (Technically, JPEG File Interchange Format) files, as understood by common browsers and graphics applications, do not support storing an alpha channel with the image like PNG does. Depending on your application, you might want to store a separate black-and-white "image" file which is the transparency mask for the JPG, and use the pair. There are extensions to the format that can be used in custom software, but they won't be understood elsewhere.
But first, make sure that JPG is really what you want to use. JPG is only appropriate for "photographic" images, which don't typically have transparency masks (though there are certainly exceptions). If the image is more Icon/Drawing-like, then it is likely that JPG won't be much smaller than the PNG anyway, and have ugly aliasing artifacts, so just optimizing the PNG will give you better results.

Answer 3

You may want to consider embedding the png's in a Flash library, then export as swf. You can then use Flash to compress your png using the lossy JPEG algorithm, but it will still keep the alpha channel. This could result in much smaller file sizes but with the benefit of keeping the 8-bit alpha channel (each pixel has 256 transparency levels).
By masking out a specific color, you'll only get a 1-bit alpha mask (a pixel is either fully visible or completely hidden.) You can also run into problems like when the color in the image corresponds with the matte color, which deletes pixels from the actual image you want to display.
The other problem with using a matte color in a JPEG image is that the JPEG compression will leave artifacts, especially around the edges of the object and the matte color. This makes it hard to properly mask out a specific color.