I've problems decoding/unzipping a string in python which was orginially compressed using C# with "GZipStream".

The "opencv mat" was converted to json with:

public override void WriteJson(JsonWriter writer, Mat value, JsonSerializer serializer)
{
    byte[] matBytes = null;
    Cv2.ImEncode(".png", value, out matBytes);

    // Convert byte[] to Base64 String
    string matBase64 = Convert.ToBase64String(matBytes);

    var resCompressed = Utils.StringCompressor.CompressString(matBase64);
    writer.WriteValue(resCompressed);
}

An example string "jsontoken" to uncompress is:

uAoAAB LCAAAAAAABADtls3SsrgWhS IASogOOiB/GgS/gkqMlMUgkFEEGK8 o7v1931DU7VqTqzU9UMfDS1srPY2dmx3pthwmbutnqsxRPgHXF2lfj28xNU1vrniceHr34HPrPGifeJur4q5nqHdocErlnx0YH8eA2fZwhrsz4q2epTEWRSyjESqDjO0VogzpFPK/yDWgD69I5jJLDFKYLOL1ABGyFKLYGfIOa/sf6XWJGbAaDW9kzmUryc6FDcpQMGmbYGaKIGka8J4ZmGYFnC9co1ahpOs JKpXRhE6O6zQg/qtdIzLuNp9YGmgWgfr8wTcImDzVaBAF3wkTxU5KP2MSRflTPZ1rsbtInBAttu4HRqLPTNX5jW78bfkudHZEWEphoOKYyOtsKTzKk3GU2N9o5z7Xe8OGYeih8L 4c7JEy7hi2NM8gS3DIUELbQv0gv6f9x1ZjHZq7RtKzdakBSCflpG4U KaNsVTWkrYZ7hMIl24GW7phmpLUmmP0Ys0c9NT7vAogU79OpvDFbmPco4PQ4KdmGo vBnlCQwpPplaNp3DF5oudzMchWFF5J5bvi3DVhMazF9qoQa6heZal4732OTjmguqfeoBgQG27OcZRnpYuZ2zJ72PSkPecBkXr2XSCeQczcDmJBMVT GSknjXXS0DsVfcTEOrWXmMHJ13sJl4N/nPoztQ8Xva5pSCLLXW OeH2vTjQ04xGn2GA8xcq2/Mx6U5YDjG71ayRioaAVYOM15vAy3/gILj6RbQXfEZ22rtYW19Bd4gbHhvhwWi2dDQu884NUGBourU74/P3XdmBTp92QHRAURsd0yDHkRcc0/1fLPNEdiemuPwmJSWR9hQYTUZkweJvUsH5b2xlcNLWrw4YqrJGw0OhQL14dDujKdMzGLxgS9gYd6ckspuCXFl9zVzVejUbY0zY5ux6mt8S9x5nPI/svKiezo0UZMnqqgjm3HrgcMPh2XJNpmYQESg yQAfwCZFdXeqZQYfLyg0rmd5pLDuvIap/Sqqq1OJ9dsXVMG6Gx6FY3dFfZ3dqq GsDicc9Tj0GTe2d2aa6G9//KaiD3 eq3FHn 9bjaN/fXqCt3X6 4eBzw1wv13fqS9R9POY9mNvv5A58kiMNAtoE2G8G8usceEx wFp 97jYxIV8S2Zxdo7mTvhU/6/Mba6NhThTAe4NQ C3PL6xD3bJZB2CEZYM0MRI6XPzluafbNsTOjylvkdnNKSiC2Uzklcnhl7chqqViRcN5sjU62L73ra/YIlEN85lkRjgZdiPoo9l0wF/Whn61zjz1NOTifBS0/1YDKAcqtecRzUReuzeZPTsMEkOWeQuMeibOAkg6WIDvhBqcRsEVd/07zhLsvy1Pyw8sp6f9h/FPqp1kjXV5kI2rUUD0L/X526OBfh 5G0bGY53aEaqad Vas9Z4vaLATR30aoPtCWQuOMTilU iy6sru12JO1pcmMPrenn6dc12xX3k6uUumLVIdi/4RMGWx0zn59gZpRs ftvDnTWQ8z7aceyu0FXMsJU9ld8H0hWhntsFGR LJHpbjhrVS7CJvNRvtFbdzVIp VI7xGfVvZbw13NbMWXiZHcd4giNRhlrsq9YZ0Yq6YigoSTaQBgf6qzBlGtZYVlMz6mnKD0xPNedBJ0n00KW2MfohLKSkRp788p3DmEzQJO8ND3MYheV7Xoh75PZpjTxDG1oZumzv S53o1BmTdEccKR4ag5QR2 rmWQn/JpD TJVvlFVNFAs1c9oTLtBVSAzHP8egK24MB493VdiCK42xS0EEY jREEeYVKacKz1hQ 4G2Il6ElgtDf6 lzVtIRQC4x68HXuh2kp7okFT6DceXYgChwBOVGP7bcd/H/ey//G q x5KpcbVeiPaTR Gbfv6AmSnaa01NUVdUff/wJwhGNxbgKAAA=

The according C# code to load this data is:

private static void CreateDataFromJsonZip(ref ThresholdMask mask, JObject jsonObj, string maskName)
{
    // get JSON result objects into a list
    JToken jsonToken = jsonObj["Selection"]; # jsonToken

    var res = Utils.StringCompressor.DecompressString(jsonToken.ToString());
    byte[] data = Convert.FromBase64String(res);
    mask.Selection = Cv2.ImDecode(data, ImreadModes.Unchanged);

My attempt to read the data in python is:

jsonToken = r'uAoAAB LCAAAAAAABADtls3SsrgWhS IASogOOiB/GgS/gkqMlMUgkFEEGK8 o7v1931DU7VqTqzU9UMfDS1srPY2dmx3pthwmbutnqsxRPgHXF2lfj28xNU1vrniceHr34HPrPGifeJur4q5nqHdocErlnx0YH8eA2fZwhrsz4q2epTEWRSyjESqDjO0VogzpFPK/yDWgD69I5jJLDFKYLOL1ABGyFKLYGfIOa/sf6XWJGbAaDW9kzmUryc6FDcpQMGmbYGaKIGka8J4ZmGYFnC9co1ahpOs JKpXRhE6O6zQg/qtdIzLuNp9YGmgWgfr8wTcImDzVaBAF3wkTxU5KP2MSRflTPZ1rsbtInBAttu4HRqLPTNX5jW78bfkudHZEWEphoOKYyOtsKTzKk3GU2N9o5z7Xe8OGYeih8L 4c7JEy7hi2NM8gS3DIUELbQv0gv6f9x1ZjHZq7RtKzdakBSCflpG4U KaNsVTWkrYZ7hMIl24GW7phmpLUmmP0Ys0c9NT7vAogU79OpvDFbmPco4PQ4KdmGo vBnlCQwpPplaNp3DF5oudzMchWFF5J5bvi3DVhMazF9qoQa6heZal4732OTjmguqfeoBgQG27OcZRnpYuZ2zJ72PSkPecBkXr2XSCeQczcDmJBMVT GSknjXXS0DsVfcTEOrWXmMHJ13sJl4N/nPoztQ8Xva5pSCLLXW OeH2vTjQ04xGn2GA8xcq2/Mx6U5YDjG71ayRioaAVYOM15vAy3/gILj6RbQXfEZ22rtYW19Bd4gbHhvhwWi2dDQu884NUGBourU74/P3XdmBTp92QHRAURsd0yDHkRcc0/1fLPNEdiemuPwmJSWR9hQYTUZkweJvUsH5b2xlcNLWrw4YqrJGw0OhQL14dDujKdMzGLxgS9gYd6ckspuCXFl9zVzVejUbY0zY5ux6mt8S9x5nPI/svKiezo0UZMnqqgjm3HrgcMPh2XJNpmYQESg yQAfwCZFdXeqZQYfLyg0rmd5pLDuvIap/Sqqq1OJ9dsXVMG6Gx6FY3dFfZ3dqq GsDicc9Tj0GTe2d2aa6G9//KaiD3 eq3FHn 9bjaN/fXqCt3X6 4eBzw1wv13fqS9R9POY9mNvv5A58kiMNAtoE2G8G8usceEx wFp 97jYxIV8S2Zxdo7mTvhU/6/Mba6NhThTAe4NQ C3PL6xD3bJZB2CEZYM0MRI6XPzluafbNsTOjylvkdnNKSiC2Uzklcnhl7chqqViRcN5sjU62L73ra/YIlEN85lkRjgZdiPoo9l0wF/Whn61zjz1NOTifBS0/1YDKAcqtecRzUReuzeZPTsMEkOWeQuMeibOAkg6WIDvhBqcRsEVd/07zhLsvy1Pyw8sp6f9h/FPqp1kjXV5kI2rUUD0L/X526OBfh 5G0bGY53aEaqad Vas9Z4vaLATR30aoPtCWQuOMTilU iy6sru12JO1pcmMPrenn6dc12xX3k6uUumLVIdi/4RMGWx0zn59gZpRs ftvDnTWQ8z7aceyu0FXMsJU9ld8H0hWhntsFGR LJHpbjhrVS7CJvNRvtFbdzVIp VI7xGfVvZbw13NbMWXiZHcd4giNRhlrsq9YZ0Yq6YigoSTaQBgf6qzBlGtZYVlMz6mnKD0xPNedBJ0n00KW2MfohLKSkRp788p3DmEzQJO8ND3MYheV7Xoh75PZpjTxDG1oZumzv S53o1BmTdEccKR4ag5QR2 rmWQn/JpD TJVvlFVNFAs1c9oTLtBVSAzHP8egK24MB493VdiCK42xS0EEY jREEeYVKacKz1hQ 4G2Il6ElgtDf6 lzVtIRQC4x68HXuh2kp7okFT6DceXYgChwBOVGP7bcd/H/ey//G q x5KpcbVeiPaTR Gbfv6AmSnaa01NUVdUff/wJwhGNxbgKAAA='

string = jsonToken
original = base64.b64decode(string)
convert_to_np = np.frombuffer(original, dtype=np.uint8)
print(convert_to_np)
len(convert_to_np)
img = cv2.imdecode(convert_to_np, cv2.IMREAD_UNCHANGED)

Unfortunately, the last step with imdecode does not work, the result is None. Any hints?

CodePudding user response：

This will do exactly what you asked for, following the StringCompressor.DecompressString code from this SO answer (the decompressed length is stored in the first 4 bytes):

original = base64.b64decode(gzip.decompress(base64.b64decode(jsonToken)[4:]))

However, I'd recommend simplifying your C# implementation. Starting from the raw OpenCV mat, you:

1. encode it as a PNG image
2. convert that binary data to base64 string
3. compress the text string with gzip
4. convert to base64 again
5. store it to the json file

Step 2 is completely unnecessary, and step 3 is redundant, since the PNG encoding process already compresses the image data. Adding another layer of compression won't help much, and may even make it worse.

I would just convert to a PNG, encode with base64, and write that to the json file. Then your original Python code would work as expected.