You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: captcha.go
    path of file: ./repos/quickshare/src/handlers/multiusers
    
    the code of the file until where you have to start completion: package multiusers

import (
	"bytes"
	"errors"

	"github.com/dchest/captcha"
	"github.com/gin-gonic/gin"

	q "github.com/ihexxa/quickshare/src/handlers"
)

type GetCaptchaIDResp struct {
	C
    
func (h *MultiUsersSvc) GetCaptchaImg(c *gin.Context) {
	captchaID := c.Query(q.CaptchaIDParam)
	if captchaID == "" {
		c.JSON(q.ErrResp(c, 400, errors.New("empty captcha ID")))
		return
	}

	capWidth := h.cfg.IntOr("Users.CaptchaWidth", 256)
	capHeight := h.cfg.IntOr("Users.CaptchaHeight", 64)

	// TODO: improve performance
	buf := new(bytes.Buffer)
	err := captcha.WriteImage(buf, captchaID, capWidth, capHeight)
	if err != nil {
		c.JSON(q.ErrResp(c, 500, err))
		return
	}

	c.Data(200, "image/png", buf.Bytes())
}