#6206
At present, our image file does not have file header/footer. When we need to change the image format (such as adding different journal versions to the image), there is no way to distinguish different image formats.
Therefore, we suggest adding file header and footer to the image. By the new image format, we can freely distinguish and define different image reading ways.
The format of the image is as follows:
```
/**
* Image Format:
* |- Image --------------------------------------|
* | - Magic String (4 bytes) |
* | - Header Length (4 bytes) |
* | |- Header -----------------------------| |
* | | |- Json Header ---------------| | |
* | | | - version | | |
* | | | - other key/value(undecided)| | |
* | | |-----------------------------| | |
* | |--------------------------------------| |
* | |
* | |- Image Body -------------------------| |
* | | Object a | |
* | | Object b | |
* | | ... | |
* | |--------------------------------------| |
* | |
* | |- Footer -----------------------------| |
* | | | - Checksum (8 bytes) | |
* | | |- object index --------------| | |
* | | | - index a | | |
* | | | - index b | | |
* | | | ... | | |
* | | |-----------------------------| | |
* | | - other value(undecided) | |
* | |--------------------------------------| |
* | - Footer Length (8 bytes) |
* | - Magic String (4 bytes) |
* |----------------------------------------------|
*/
```
1. Magic Number
One image format is identified by one magic string and one version field. The magic string is save in the first 4 bytes and last 4 bytes in the images.
2. Image Header:
The version is save in the header with json format now.
3. Image Body:
Equal to the original image.
4.Image Footer:
Image footer stores the file offset(index) of many image objects. If necessary, we can read some objects in the image by the footer.
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