How to Save a Hex String as a PNG File in SPIFFS Using ESP32

Описание: Learn how to convert a hex string into a PNG image file using SPIFFS on an ESP32 development board. This guide provides a step-by-step explanation of the process, ensuring you get the desired image output with ease.
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How to Save a Hex String as a PNG File in SPIFFS Using ESP32

Sending images across a TCP/IP connection and reconstructing them on the client side can be challenging, especially when dealing with hexadecimal strings. If you're working with Node.js on the server side and an ESP32 on the client side, this guide is for you.

In this guide, we will walk through the process of saving a hex string as a PNG image file in SPIFFS on an ESP32 development board. We’ll break it down into manageable sections, making it easy for you to follow along.

Understanding the Problem

You have a Node.js server that successfully sends an image file as a hexadecimal string. However, on the client side, when you attempt to save this hex string to create an image, you find that the PNG file generated doesn’t display as expected. Instead of showing the image, it appears as binary data or produces an error.

Here’s a brief overview of the code you already have:

Node.js Server Code

[[See Video to Reveal this Text or Code Snippet]]

This code reads the image file and converts it to a hex string before sending it to the client.

ESP32 Client Code

Your initial attempt to save the received hex data involved using fprintf() to write to the file, which is not suitable for binary data.

The Solution

Why fprintf() Doesn’t Work

You might wonder, “Why can’t I use fprintf()?” The answer lies in how C handles strings. fprintf() is meant for C-strings, which are sequences of characters ending with a null character ('\0'). When you try to write binary data (like PNG files), the presence of null characters will lead to premature termination of the string, resulting in incomplete data being saved.

Using fwrite() instead

To successfully save binary data, you should use fwrite() instead of fprintf(). The fwrite() function allows you to specify the exact number of bytes to write, ensuring all data gets saved correctly.

Updated create_file_app Function

Here’s how to update your create_file_app function to use fwrite():

[[See Video to Reveal this Text or Code Snippet]]

Explanation:

Open in Write-binary Mode ("wb"): Using this mode ensures no unintended character adjustments occur.

fwrite(buffer, 1, length, f): Writes length bytes from buffer to the file.

Reading the Response Correctly

When you read the response on the ESP32 using esp_http_client_read_response(), make sure to store the returned length of data. This length is crucial for accurately writing to the file.

Complete ESP32 Implementation Overview

Here’s a summary of the steps you'll follow:

Send an image from your Node.js server as a hexadecimal string.

On the ESP32 client, set up an HTTP client to make a GET request for the image data.

Read the received data using esp_http_client_read_response(), ensuring to capture the total bytes read.

Use the modified create_file_app() function to save the binary data to a PNG file using fwrite().

Conclusion

By replacing fprintf() with fwrite() and ensuring you capture the correct length of data, you can successfully save hex string data as a PNG image file in SPIFFS on the ESP32. This approach allows you to reconstruct the original image correctly for display.

Make sure to test your implementation to confirm that the image files are being created as expected. Once you have everything in place, you'll be on your way to effectively sending and saving images over TCP/IP connections.

Feel free to reach out if you need further assistance with your implementation, and happy coding!