上篇文章讲过NFT的元数据。每一个NFT都有一个json格式的metadata与之对应，metadata有一个参数指定了NFT的图案。所以只要访问这个NFT的metadata，然后就能得到图像地址，就可以显示出NFT的图案了。
一般的NFT,metadata与图片都是保存到非区块链的。例如中心化服务器，或者去中心化的IPFS。这样的话就有可能导致资料遗失。如果所有数据都保存到区块链上，则能够完美解决这个问题。但是，在区块链上保存大量数据非常昂贵，所以这么做，必须尽量减少存储的数据量。
查看 opensea metadata标准 发现还有一个image_data可供使用。
我们可以使用SVG图案档。SVG的本质其实就是一段文字，因此我们可以根据一定的要求随时“拼凑”出来。
tokenURI()我们需要做如下处理
function tokenURI(uint256 tokenId)
public
view
override(ERC721)
returns (string memory)
{
require(
_exists(tokenId),
"ERC721Metadata: URI query for nonexistent token"
);
string memory json = Base64.encode(
bytes(
string(
abi.encodePacked(
'{"name": "FDF NFT #',
uint2str(tokenId),
'",',
'"image_data": "',
fdfSVG.getSvgImage(tokenId),
'"',
"}"
)
)
)
);
return string(abi.encodePacked("data:application/json;base64,", json));
}
上面拼接json时有一句 fdfSVG.getSvgImage(tokenId) 这个方法作用是根据传入的tokenId返回一个svg数据。
下面贴出完整代码
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
library Base64 {
string internal constant TABLE_ENCODE =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
bytes internal constant TABLE_DECODE =
hex"0000000000000000000000000000000000000000000000000000000000000000"
hex"00000000000000000000003e0000003f3435363738393a3b3c3d000000000000"
hex"00000102030405060708090a0b0c0d0e0f101112131415161718190000000000"
hex"001a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132330000000000";
function encode(bytes memory data) internal pure returns (string memory) {
if (data.length == 0) return "";
// load the table into memory
string memory table = TABLE_ENCODE;
// multiply by 4/3 rounded up
uint256 encodedLen = 4 * ((data.length + 2) / 3);
// add some extra buffer at the end required for the writing
string memory result = new string(encodedLen + 32);
assembly {
// set the actual output length
mstore(result, encodedLen)
// prepare the lookup table
let tablePtr := add(table, 1)
// input ptr
let dataPtr := data
let endPtr := add(dataPtr, mload(data))
// result ptr, jump over length
let resultPtr := add(result, 32)
// run over the input, 3 bytes at a time
for {
} lt(dataPtr, endPtr) {
} {
// read 3 bytes
dataPtr := add(dataPtr, 3)
let input := mload(dataPtr)
// write 4 characters
mstore8(
resultPtr,
mload(add(tablePtr, and(shr(18, input), 0x3F)))
)
resultPtr := add(resultPtr, 1)
mstore8(
resultPtr,
mload(add(tablePtr, and(shr(12, input), 0x3F)))
)
resultPtr := add(resultPtr, 1)
mstore8(
resultPtr,
mload(add(tablePtr, and(shr(6, input), 0x3F)))
)
resultPtr := add(resultPtr, 1)
mstore8(resultPtr, mload(add(tablePtr, and(input, 0x3F))))
resultPtr := add(resultPtr, 1)
}
// padding with '='
switch mod(mload(data), 3)
case 1 {
mstore(sub(resultPtr, 2), shl(240, 0x3d3d))
}
case 2 {
mstore(sub(resultPtr, 1), shl(248, 0x3d))
}
}
return result;
}
function decode(string memory _data) internal pure returns (bytes memory) {
bytes memory data = bytes(_data);
if (data.length == 0) return new bytes(0);
require(data.length % 4 == 0, "invalid base64 decoder input");
// load the table into memory
bytes memory table = TABLE_DECODE;
// every 4 characters represent 3 bytes
uint256 decodedLen = (data.length / 4) * 3;
// add some extra buffer at the end required for the writing
bytes memory result = new bytes(decodedLen + 32);
assembly {
// padding with '='
let lastBytes := mload(add(data, mload(data)))
if eq(and(lastBytes, 0xFF), 0x3d) {
decodedLen := sub(decodedLen, 1)
if eq(and(lastBytes, 0xFFFF), 0x3d3d) {
decodedLen := sub(decodedLen, 1)
}
}
// set the actual output length
mstore(result, decodedLen)
// prepare the lookup table
let tablePtr := add(table, 1)
// input ptr
let dataPtr := data
let endPtr := add(dataPtr, mload(data))
// result ptr, jump over length
let resultPtr := add(result, 32)
// run over the input, 4 characters at a time
for {
} lt(dataPtr, endPtr) {
} {
// read 4 characters
dataPtr := add(dataPtr, 4)
let input := mload(dataPtr)
// write 3 bytes
let output := add(
add(
shl(
18,
and(
mload(add(tablePtr, and(shr(24, input), 0xFF))),
0xFF
)
),
shl(
12,
and(
mload(add(tablePtr, and(shr(16, input), 0xFF))),
0xFF
)
)
),
add(
shl(
6,
and(
mload(add(tablePtr, and(shr(8, input), 0xFF))),
0xFF
)
),
and(mload(add(tablePtr, and(input, 0xFF))), 0xFF)
)
)
mstore(resultPtr, shl(232, output))
resultPtr := add(resultPtr, 3)
}
}
return result;
}
}
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
import {Base64} from "./Base64.sol";
interface FdfSVG {
function getSvgImage(uint256 tokenId) external pure returns (string memory) ;
}
contract FdfMetadataNFT is ERC721, ERC721Enumerable {
FdfSVG public fdfSVG;
constructor() ERC721("FDFOnChainNFT", "FDFOCNFT") {
fdfSVG = FdfSVG(0x23D6D1d1cd44CCD4fF6A3b786DebDEF273195f8A);
for (uint i=1000; i < 1100; i++) {
_safeMint( 0x3dc0252648774a3280ECe2Ff4AeD70A370f535D8, i);
}
}
function mint(address to, uint256 tokenId) public {
_safeMint(to, tokenId);
}
function tokenURI(uint256 tokenId)
public
view
override(ERC721)
returns (string memory)
{
require(
_exists(tokenId),
"ERC721Metadata: URI query for nonexistent token"
);
string memory json = Base64.encode(
bytes(
string(
abi.encodePacked(
'{"name": "FDF NFT #',
uint2str(tokenId),
'",',
'"image_data": "',
fdfSVG.getSvgImage(tokenId),
'"',
"}"
)
)
)
);
return string(abi.encodePacked("data:application/json;base64,", json));
}
function _beforeTokenTransfer(
address from,
address to,
uint256 tokenId
) internal override(ERC721, ERC721Enumerable) {
super._beforeTokenTransfer(from, to, tokenId);
}
function supportsInterface(bytes4 interfaceId)
public
view
override(ERC721, ERC721Enumerable)
returns (bool)
{
return super.supportsInterface(interfaceId);
}
function uint2str(uint256 _i)
internal
pure
returns (string memory _uintAsString)
{
if (_i == 0) {
return "0";
}
uint256 j = _i;
uint256 len;
while (j != 0) {
len++;
j /= 10;
}
bytes memory bstr = new bytes(len);
uint256 k = len;
while (_i != 0) {
k = k - 1;
uint8 temp = (48 + uint8(_i - (_i / 10) * 10));
bytes1 b1 = bytes1(temp);
bstr[k] = b1;
_i /= 10;
}
return string(bstr);
}
}
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
import {Base64} from "./Base64.sol";
contract FdfSVG {
constructor(){}
function getSvgImage(uint256 tokenId) external pure returns (string memory) {
string memory svgA = "<svg xmlns='http://www.w3.org/2000/svg' xmlns:xlink='http://www.w3.org/1999/xlink' x='0px' y='0px' viewBox='0 0 1200 1200' xml:space='preserve'><style type='text/css'> </style> <g><path class='st0' d='M425.59,943.4c-2.39-78.3-4.68-153.65-7.01-230.19c8.42-1.82,13.15-3.71,17.89-3.75c204.05-1.68,408.1-3.22,612.15-4.62c6.92-0.05,13.86,1.43,22.93,2.43c2.71,80.68,5.38,159.91,8.14,242.13c-29.1,3.15-55.04,5.95-83.8,9.06c-9.06-51.39-39.45-83.05-88.58-84.93c-49.42-1.89-81.18,28.62-96.29,78.6C683.73,949.25,556.97,946.38,425.59,943.4z'/><path class='st0' d='M1070.89,648.47c-142.88,0-277.62,0-414.63,0c0-54.48,0-107,0-161c18.42,0,35.34,0.17,52.25-0.03c64.54-0.77,128.96,2.51,192.32,14.49c56.55,10.69,107.36,34.87,141.52,83.6C1054.19,602.44,1059.9,623.64,1070.89,648.47z'/><path class='st0' d='M406.73,759.3c0,60.1,0,120.11,0,183.44c-11.3,0.51-21.89,1-31.79,1.45c-22.15-64.81-55.61-93.22-106.25-91.95c-47.5,1.19-79.45,32.08-100.44,98.34c-29.19-0.06-59.26,14.13-91.73-5.5c1.77-10.55,3.71-22.04,5.48-32.59c57.46-25.81,63.26-54.03,17.55-90.66c8.48-31.25,17.13-63.69,26.12-96.03c3.6-12.96,10.72-21.02,26.08-20.88c82.04,0.76,164.08,0.88,246.13,1.35c1.83,0.01,3.65,1.54,8.12,3.57c0.48,8.3,1.03,17.95,1.78,31.02c-29.19,3.21-56.2,6.19-86.16,9.48c13.28,14.39,28.13,7.74,40.98,8.73C376.64,760.13,390.83,759.3,406.73,759.3z'/> <path class='st0' 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class='st0' d='M1136.29,1056.82H139.63c-1.01,0-1.83-0.82-1.83-1.83v-19.73c0-1.01,0.82-1.83,1.83-1.83h996.66c1.01,0,1.83,0.83,1.83,1.83v19.73C1138.13,1056,1137.3,1056.82,1136.29,1056.82z'/><text style='white-space:pre' x='128.356' y='262.274' fill='#1e00e7' font-family='"Kelly Slab"' font-size='55'>#";
string memory svgB = "</text><path d='M60.146 520.686l349.431 4.914 65.648 81.33c0 3.26-351.281-6.234-351.281-6.234-3.26 0-67.058-80.01-63.798-80.01z' transform='matrix(.589 -.808497 .762745 .651 -339.374725 97.441727)' stroke-opacity='.55' paint-order='fill' fill='#eef0ea' fill-opacity='.39' stroke='rgba(255, 255, 255, 0)'/></svg>";
return string(abi.encodePacked(svgA, uint2str(tokenId), svgB));
}
function uint2str(uint256 _i)
internal
pure
returns (string memory _uintAsString)
{
if (_i == 0) {
return "0";
}
uint256 j = _i;
uint256 len;
while (j != 0) {
len++;
j /= 10;
}
bytes memory bstr = new bytes(len);
uint256 k = len;
while (_i != 0) {
k = k - 1;
uint8 temp = (48 + uint8(_i - (_i / 10) * 10));
bytes1 b1 = bytes1(temp);
bstr[k] = b1;
_i /= 10;
}
return string(bstr);
}
}
接下来在rinkeby部署合约
合约地址为 0x23D6D1d1cd44CCD4fF6A3b786DebDEF273195f8A
接下来部署NFT合约
合约地址 0xB81A99A0179e9CbC96a5289584001bD9c393E1AA
在testopensea上查看效果
