When you start out selling QR Codes, there are a variety of uses for them. More likely than not your client is going to eventually use their QR Code in printed materials, whether it's in a Yellow Pages ad, newspaper advertisement, or even possibly to print on t-shirts or other promotional materials.
While there are a lot of free QR Code creators out there, none of them provide a high quality, print ready image. In order for the code to be scannable by all phones, you will need to have it be at least 300 dpi. Fortunately, due to the "pixelated" nature of QR Codes, they are easy to enlarge, but you need to do it correctly so that it doesn't turn out blurry and unreadable.
Here is how to take your free image and convert it to a high quality, print ready image:
First, create your free QR Code at Kaywa (QR-Code Generator)... put in the URL Of your QR Code, make sure the size is L (large) and then hit generate. Save the image to your computer.
Next, you will need to download the free photo editor, GIMP (GIMP - The GNU Image Manipulation Program). GIMP is a free alternative to Photoshop and has quite a bit of the same capabilities, including allowing you to open up PSD files.
Once you have downloaded and installed GIMP, open up the QR Code image that you created. If you selected the large size from Kaywa, it should be 248 pixels by 248 pixels.
Now, we are going to increase the size of the image by scaling it up. To do this, select Scale Image which you'll find in the drop down menu from the Image header bar. (To find this look for File, Edit, Select, View, Image up top in the header of GIMP. Scale Image will be under Image.)
When you select Scale Image a box will pop up that gives both the Image Size (248 x 248) and the Resolution (72 x 72). What you want to do is increase the size of everything by the same factor.
To be on the safe side, I always increase everything by a factor of 10. So you will increase the Width and Height from 248 x 248 to 2480 x 2480. You will also want to increase the X & Y Resolution from 72 x 72 to 720 x 720. It is very important that you increase both the size and resolution by the same factor, or you will get distortions in your image.
Once you have increased the scale of the image, you'll notice that it is HUGE. This is good, this is what we want. You can play around with it and shrink it small or blow it up even bigger, but it is now a high quality image that is suitable for printing. I'm not exactly sure of the specific dpi this produces, but it is larger than the minimum of 300. Now just go to File and Save a Copy and save the size of the image to send to your clients.
This is the technique that I use to give my clients a high quality printable image and I have never had a problem with it scanning in any yellow pages, newspapers, or magazines.
One thing to let your client know is to make sure the final printed size of the image is no smaller than an inch by an inch. Technically, codes that are a little smaller can still scan, but if it's less than an inch, it makes it difficult for cameras with lower resolutions to scan.
I hope this helps answer some of the questions that I've seen in other threads on this forum. If anybody has any other suggestions, it would be great to hear them as well!