Partial URL-Encoding

Use Data URL in the Metadata of your NFTs

Fully on-chain CNFTs are typically implemented by assigning appropriate Base64-encoded dataURIs to the token metadata's src and image fields. DataURIs can encode a variety of data types like HTML, audio, image etc. As a result, a fully on-chain NFT can virtually encode any type of media file as long as its metadata is no larger than 16Kb. This is a standard and general approach, with the main disadvantage being the same Base64 encoding, which may make the original data 30% larger (on average). This is a difficult limitation because the 16Kb limit is a hard constraint that must always be met.

To overcome this problem, a possible, non-standard solution that works well with textual data (e.g., html and svg) is to replace base64 encoding with a variant of the original plain text in which some special characters have been substituted by their url-encoded version.

In specific, there are the following replacements:

% is replaced by %25
is replaced by %23
< is replaced by %3C
> is replaced by %3E

Actually, to display the partially url-encoded, on-chain NFTs only characters % and # must be replaced. The substitution of characters < and > is only required by NMKR for minting.

Use case for URL encoding

This method is easy to implement and saves a lot of space, which allows to mint way more complex On-Chain NFTs to be encoded and minted. A common use case are interactive NFTs like games.

Example Metadata

{
  "721": {
    "4864e4cf3f7e7fd2318b3bb4a7060665278a47fc4a6216c55dcbbeb0": {
      "chrz2": {
        "Color": "Arctic white",
        "Enemy (Ambushman)": "Face with symbols on mouth",
        "Enemy (Happy Stoner)": "Zany face",
        "Enemy (Professor)": "Exploding head",
        "Enemy (Shadowman)": "Cowboy hat face",
        "Hero": "Face without mouth",
        "Keyboard controls": "Press any key to start, arrow keys to move hero",
        "Power-up": "Nazar amulet",
        "Special": "False",
        "Touch controls": "Tap to start, swipe up/down/left/right to move hero",
        "Twitter": "twitter.com/gamezonchain",
        "Variant": "Classic",
        "files": [
          {
            "mediaType": "text/html",
            "src": [
              "data:text/html;utf8,%3C!DOCTYPE html%3E%3Cmeta charset='utf-8'/%",
              "3E%3Cbody style='margin:0;touch-action:none;overscroll-behavior:",
              "contain;background:%23000;'%3E%3Ccanvas id='a' style='margin:aut",
              "o;display:block;'%3E%3C/canvas%3E%3Cscript%3Efunction aa(a){var ",
              "d=0;return function(){return d%3Ca.length?{done:!1,value:a[d++]}",
              ":{done:!0}}}function b(a){var d='undefined'!=typeof Symbol&&Symb",
              "ol.iterator&&a[Symbol.iterator];return d?d.call(a):{next:aa(a)}}",
              " var e,f,g,ba='ontouchstart'in window,h,l,n,p,q,r,t,u,v,w,x,y,ca",
              ",z,da=String.fromCodePoint(0x1F44E),ea=[String.fromCodePoint(129",
              "311),String.fromCodePoint(129304),String.fromCodePoint(128077),S",
              "tring.fromCodePoint(128074)],fa=String.fromCodePoint(0x1F9FF),B=",
              "[String.fromCodePoint(0x1F920),String.fromCodePoint(0x1F92C),Str",
              "ing.fromCodePoint(0x1F92F),String.fromCodePoint(0x1F92A),String.",
              "fromCodePoint(128064)],C=[String.fromCodePoint(127817),String.fr",
              "omCodePoint(127818),String.fromCodePoint(127826),String.fromCode",
              "Point(127827), String.fromCodePoint(128142)],D,ha=String.fromCod",
              "ePoint(128561),ia=String.fromCodePoint(0x1F636),E,ja,F,G,H,ka,I,",
              "la,ma,J,na,oa,pa,K,L,M,qa,ra,N=new AudioContext,O,P,Q,sa=[[1,1,1",
              ",1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1],[1",
              ",2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,1",
              "],[1,4,1,1,1,1,2,1,1,1,1,1,1,1,1,2,1,2,1,1,1,1,1,1,1,1,2,1,1,1,1",
              ",4,1],[1,2,1,1,1,1,2,1,1,1,1,1,1,1,1,2,1,2,1,1,1,1,1,1,1,1,2,1,1",
              ",1,1,2,1],[1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2",
              ",2,2, 2,2,2,1],[1,2,1,1,1,1,2,1,2,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,",
              "2,1,2,1,1,1,1,2,1],[1,2,2,2,2,2,2,1,2,2,2,2,2,2,2,2,1,2,2,2,2,2,",
              "2,2,2,1,2,2,2,2,2,2,1],[1,1,1,1,1,1,2,1,1,1,1,1,1,1,1,2,1,2,1,1,",
              "1,1,1,1,1,1,2,1,1,1,1,1,1],[0,0,0,0,0,1,2,1,2,2,2,2,2,2,2,2,2,2,",
              "2,2,2,2,2,2,2,1,2,1,0,0,0,0,0],[1,1,1,1,1,1,2,1,2,1,1,1,1,1,1,1,",
              "0,1,1,1,1,1,1,1,2,1,2,1,1,1,1,1,1],[2,2,2,2,2,2,2,2,2,1,0,5,5,0,",
              "0,0,0,0,0,0,5,5,0,1,2,2,2,2,2,2,2,2,2],[1,1,1,1,1,1,2,1,2,1,1,1,",
              "1,1,1,1,1,1,1,1,1,1,1,1,2,1,2,1,1,1,1,1,1],[0,0,0,0,0,1,2,1, 2,2",
              ",2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,1,2,1,0,0,0,0,0],[1,1,1,1,1,1,2,1",
              ",2,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,2,1,2,1,1,1,1,1,1],[1,2,2,2,2,2",
              ",2,2,2,2,2,2,2,2,2,2,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,1],[1,4,1,1",
              ",1,1,2,1,1,1,1,1,1,1,1,2,1,2,1,1,1,1,1,1,1,1,2,1,1,1,1,2,1],[1,2",
              ",2,2,2,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,1,2,2,4,2,1],",
              "[1,1,1,1,2,1,2,1,2,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,2,1,2,1,2,1,1,1",
              ",1],[1,2,2,2,2,2,2,1,2,2,2,2,2,2,2,2,1,2,2,2,2,2,2,2,2,1,2,2,2,2",
              ",2,2,1],[1,2,1,1,1,1,1,1,1,1,1,1,1,1,1,2,1,2,1,1,1, 1,1,1,1,1,1,",
              "1,1,1,1,2,1],[1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,",
              "2,2,2,2,2,2,2,1],[1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,",
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  }
}

The check metadata option in NMKR is not able to verify whether the metadata are syntactically correct. Please, if in doubt, mint a test NFT.

Example NFT

Special credits to Gamez On Chain for providing a lot of valuable information to this topic.