Convert PNG with transparency to JPG - image-processing

Question

Edit: The original title of this question was "Drawbacks of using -flatten option". However, after this question was answered, I decided to change its title, to make it easier found on Google. Also, new title is actually better describes what is told below.
As I discovered today, to convert PNG with transparency into JPG, we need to use -flatten option.
Try it yourself: download Google logo and convert it from PNG to JPG with the following line
convert google.png google.jpg
the picture will be messed:
With -flatten option, it works fine:
convert google.png -flatten google.jpg
I'm wondering are there any drawbacks of using -flatten permanently, for all conversions between PNG, JPG/JPEG and GIF.

Answer 1

The problem with converting PNG to JPG is when the PNG has transparency. JPG does not allow transparency and any transparent areas will show what color is underneath the transparency, which is often black. So you should use -flatten to properly do that conversion. But you should specify -background somecolor before -flatten, if you do not want the default background color. GIF only allows binary transparency -- fully transparent or fully opaque. PNG allows 8-bit transparency (partial transparent). I know of no significant issues using -background xx -flatten when converting PNG or GIF to JPG. However, the background color you use will change the appearance in transparent areas from that of the underneath color. Here is what is happening:
Input:
Turn alpha off:
convert google.png -alpha off google_aoff.jpg
The stripes are from the underneath color below the alpha channel.
Alpha Channel (nicely antialiased):
convert google.png -alpha extract google_alpha.jpg
Simple Flatten (default background is white):
convert google.png -flatten google_flatten.jpg
Flatten with black background:
convert google.png -background black -flatten google_flatten_black.jpg
Often one will reprocess the original transparent PNG image so that it has some constant color underneath the alpha channel so that later one can remove the alpha channel and not have odd colors showing. It will look the very same as the original PNG.
convert google.png -background white -alpha background google_bg_white.png
However, if you simply remove the alpha channel the JPG will show aliasing since only the fully transparent pixels' background colors were changed to white. You have a nice clean background, but the image is still aliased (as it was in the original when the alpha channel was remove).
convert google_bg_white.png google_bg_white.jpg
So one still needs to flatten the result, so that the antialiasing of the alpha channel will smoothly blend the colors near the boundaries.
convert google_bg_white.png -flatten google_bg_white_flatten.jpg
An alternate method to -flatten is to use -alpha remove, which is discussed http://www.imagemagick.org/Usage/masking/#alpha_remove. So starting with the original PNG, we do
convert google.png -background white -alpha remove google_alpharemoveoff.jpg
The result is the same as -background white -flatten. We do not need the -alpha off mentioned in the reference, since JPG does not support any alpha channel. The reference says this is more efficient and is the preferred method.

Answer 2

#John C wrote:
1st approach:
convert google.png -flatten google_flatten.jpg
2nd approach:
convert google.png -background white -alpha background google_bg_white.png
convert google_bg_white.png -flatten google_bg_white_flatten.jpg
3rd approach:
convert google.png -background white -alpha remove google_alpharemoveoff.jpg
More properly, these should be
1st approach
convert google.png -background white -flatten google_flatten.jpg
2nd approach
convert google.png -background white -alpha background -flatten google_bg_white_flatten.jpg
3rd approach
convert google.png -background white -alpha remove -alpha off google_alpharemoveoff.jpg
In case 1: -background white is the default. But if you want some other background color you need to specify it.
In case 2: there is no need to save to an intermediate file
In case 3: you will need -alpha off if you save to PNG. JPG does not support transparency, so turning alpha off is not needed.