php gd图像类型

Question

I'm currently teaching myself PHP's GD library, and I have a question about image types. Do certain GD functions only apply to certain image types, like jpegs for example? I'm not very knowledgeable about image types. I am wanting to learn their differences and if I should be a little more careful about the functions I'm using or am I just overthinking this stuff too much?

Answer 1

In memory, an image is just plain bitmap data. The only time an image type becomes involved is when loading an image from a file (it has to know what type so that it can decode it into an in-memory bitmap) and when writing the file to disk (same thing, in reverse).

Answer 2

Only the imagecreatefrom...() and the converse "save" functions are format-specific. Once loaded, they're in a gd-specific internal format.

That being said, some GD functions don't make sense in certain contexts. eg JPEG images don't support transparency, so specifying an alpha channel on an image that'll be saved to .jpg isn't very useful. But internally within GD, alpha-involved operations will still function as normal.