I am trying to crop an image captured by espcam the image is in a jpg format I would like to crop it. As the image is stored as a single-dimensional array I tried to rearrange the elements in the array but no changes occurred

CodePudding user response：

JPG is a compressed format, meaning that your rows and columns are not corresponding to what you would see by displaying a 1:1 grid on the screen. You need to convert it to the plain RGB (or equivalents) format and then copy it.

JPG achieves compression by splitting the image into YCbCR components, using a mathematical transformation and then filtering. For additional information I refer to this page.

Luckily you can follow this tutorial to do the inverse JPEG transformation on an Arduino (tip: forget to do this in real time, unless your time constraints are very relaxed).

The idea is to use a library that converts the JPEG image into an array of data:

Using the library is fairly simple: we give it the JPEG file, and the library will start generating arrays of pixels – so called Minimum Coded Units, or MCUs for short. The MCU is a block of 16 by 8 pixels. The functions in the library will return the color value for each pixel as 16-bit color value. The upper 5 bits are the red value, the middle 6 are green and the lower 5 are blue. Now we can send these values by any sort of communication channel we like.

For your use case you won't send the data through the communication channel, but rather store it in a local array by pushing the blocks into adjacent tiles, then do the crop.

CodePudding user response：

Looking at the output format table for the ESP32 Camera Driver one can see that most output formats are non-jpeg. If you can handle a RAW format instead (it will be slower to save/transfer, and be MUCH larger) then that would allow you to more easily crop the image by make a copy with a couple of loops. JPEG is compressed and not easily cropped. The page linked also mentions this:

• Using YUV or RGB puts a lot of strain on the chip because writing to PSRAM is not particularly fast. The result is that image data might be missing. This is particularly true if WiFi is enabled. If you need RGB data, it is recommended that JPEG is captured and then turned into RGB using fmt2rgb888 or fmt2bmp/frame2bmp