Go base64 image decode - image

Question

Im currently getting a base64 image data url from a canvas something like this (not the dataurl im getting just to show how the string looks like)
data:image/png;base64,iVkhdfjdAjdfirtn=
I need to decode that image to check the width and the height of the image
dataurl := strings.Replace(req.PostFormValue("dataurl"), "data:image/png;base64,", "", 1)
reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(dataurl))
c, _, err := image.DecodeConfig(reader)
if err != nil {
log.Fatal(err)
}
log.Println(c.Width)
But Im getting an error while trying to decode the config
Unknown image format
So yeah the way Im making the dataurl must be wrong but cant figure what to do. I also tried passing the full dataurl (with data:image...) still no success

Answer 1

What you have is a Data URI scheme, info on how to decode it and more on this is in this question and answer:
Illegal base64 data at input byte 4 when using base64.StdEncoding.DecodeString(str)
But note that image.Decodeconfig() will only decode image formats that are registered prior to calling this function, so you need the image format handlers to be registered in advance. This can be done with imports like
import _ "image/png"
More on this is in the package doc of image. Or if you know the exact format (e.g. in your example it's PNG), you can directly use png.DecodeConfig().
So it doesn't work for you because your actual encoded image is of PNG format, but you didn't register the the PNG format handler and so image.DecodeConfig() won't use the PNG handler (and so it will not be able to decode it => "Unknown image format").
Also note that replacing the prefix that is not part of the Base64 encoded image is a poor solution to get rid of it. Instead simply slice the input string:
input := "data:image/png;base64,iVkhdfjdAjdfirtn="
b64data := input[strings.IndexByte(input, ',')+1:]
Slicing a string will not even copy the string in memory, it will just create a new (two-word) string header.