differences between -resize and -size, and between -repage and -page for convert?

Question

For Imagick's convert, what are the differences

• between -resize and -size, and
• between -repage and -page?

If I want to convert some png files to a pdf file, and change the size of images (as number of pixels, or as some standard page size such as A4), which options shall I use? (It seems like -page a4 works as it intends to, while -repage a4 doesn't make any change.)

From its manpage

-size geometry       width and height of image

-resize geometry     resize the image

-page  geometry      size  and  location of an image canvas (setting)

-repage geometry     size  and  location  of  an image canvas

Answer 1

-size: Input Settings are specifically restricted to controlling the creation of images that are created or read in. Typically they are used to assign or override specific meta-data that is to be associated with the image(s) created after that setting was defined. They are created or read in from an external file.

-label  -delay  -dispose  -page  -comment  -size

Remember, they are ONLY applied when an image is created or read in and are otherwise completely ignored.

-resize: Just resize an image.

-page and -repage: With IM version 6, command line restructure, the -page option became purely a image read/create setting for use in creating GIF animations and Layers of Images. As such separate -repage and -set page operators were added to allow users to set or change the virtual canvas information.

Answer 2

Looking at the ImageMagick site the answer is different for both.

The size and resize options both do the same thing however can accept different inputs. The size option allows you to give the desired resolution, for example 640x480. The resize option allows you to provide the desired resolution as well, however the resize option also allows you to provide a % value of the current image, such as 75%.

The repage option is modifying the entire image canvas as opposed to just the image, which the page option is setting, this one is a little less clear to me but the links below should help clear things up.