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I am reading a PNG file from a website:

    String zoom = "5";   // Test Values
    String xmap = "30";  // Need to be adjusted
    String ymap = "30";
    String serverPathMap = "http://tile.openweathermap.org/map/precipitation_new/" + 
    zoom + "/" + xmap + "/" + ymap + ".png?" + "APPID=" + openWeatherMapApiKey;

I'm using this to get the PNG file into the String JSONBuffer.

    jsonBuffer = httpGETRequest(serverPathMap.c_str());

I verified the PNG file is correct by the correct received header. However, I need it in HEX formatted data.
This is an Example of the format of a file created directly by the compiler.

    const LV_ATTRIBUTE_MEM_ALIGN uint8_t ui_img_clock_face_150x150_fixed_png_data = {
0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF, ...

I have the code set to have the website read every hour, or at the press of a button. Currently the PNG file is stored in a String "JSONBuffer". I can get the size easily by JSONBuffer.length();

I'm trying to display the image in a Squareline Studio Image placeholder:

    //lv_image_set_src(ui_OpenWeatherMap, "Image from the Buffer");

    ui_OpenWeatherMap //is the name of the image placeholder.

My question is, how to convert to my JSonBuffer to HEX. I have seen it done in python, but I'm not good enough in python to convert it into C++.

Any help would be greatly appreciated.

I have tried several things.

Displayes the entire buffer. Not in HEX

    Serial.println("jsonBuffer: ");
    Serial.println(jsonBuffer);

Only displays the 1st 10 or so HEX numbers.

    snprintf(tempMap, bufferMapStringSize, "%s", jsonBuffer.c_str(), HEX);
    Serial.println("jsonBuffer: ");
    Serial.println(jsonBuffer);

Trying to insert the buffer directly doesn't work either.

   lv_image_set_src(ui_OpenWeatherMap, jsonBuffer.c_str());

I have tried many other things as well.

Improve this question
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  • 3
    It sounds like you only want the image data (and not the meta data) from the PNG file converted, correct? And please clarify what you mean by HEX? You mean a byte array of colour values? I think that is what you want, but no C++ programmer would call that HEX. Hex implies text data, and I think you want binary data. Commented Dec 26, 2024 at 14:59
  • 2
    PNG is a binary image format, why are you downloading it as a String (let alone a JSON string)? And you can't treat a binary image as a null-terminated string as you are, as there are likely to be 0x00 bytes in the image data. I don't think you understand what lv_image_set_src() is actually expecting from you. Commented Dec 26, 2024 at 16:02
  • 1
    "This is an Example of the format of a file created directly by the compiler." -- That example is expressed in hex, but the data is binary, just like all data processed by a computer. (As an example, you could change each 0xFF to 255 -- that is, change the hexadecimal representation to the equivalent decimal representation -- without changing the compiled result.) Commented Dec 26, 2024 at 17:29
  • "but the data is binary, just like all data processed by a computer". The Russians built a ternary computer in the late 50's. Obviously never caught on though. Commented Dec 26, 2024 at 18:03
  • @RemyLebeau It could be base-64 encoded, I guess. Of course then it also needs decoding into bytes. Commented Dec 27, 2024 at 0:23
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