This program is specifically designed for enhancing astrophotographs. It enables you to take of one of your images, loaded from a file, and process it to neutralise background colour cast. This is all done in your browser; no data are sent to the server.
When you first display the page for the program you will see a dialogue for controlling it. If it obscures an image this dialogue may be moved by dragging its title bar.
Your first step should be to browse for an image file and load it. Some browsers show "Browse..." on the button but others use different words.
If your image is larger than the canvas size it will be scaled down to fit. Do not be surprised to see a black area either to the right of your image or below it, depending on the aspect ratio of your image.
Once your image has loaded the program analyses its histogram - the frequencies of each of the brightness levels in the 3 bands (red, green blue). It finds the mode of each band, the level which occurs most frequently. In astrophotos this will usually determine the colour of the background.
Then click the PROCESS button and see what happens. The program stretches the levels of two of the bands so that their modal values are the same as the darkest one. That makes the background a neutral dark grey. The histogram is redrawn and you should see that 3 peaks are at the same level.
When processing is complete the resulting image can be saved as a PNG file in your downloads folder. This process is cumbersome because of the important security arrangements in all browsers. You can name the file and it will end up in your default downloads directory but you will see that it is a multi-step process. Browsers require definite confirmation from the user when downloading to your file system.
The saved image will be the full size of the original, not just as scaled to fit the screen. It will be in PNG format. That is better than JPEG because it does not lose details by compression. From here you can use a photo program to convert to other formats.
There is a limit on how large an image can be saved. The limit is very dependent on which browser you are using. Opera allows only very small images. Chrome is probably the best in this regard. This program should detect the problem and offer the alternative of saving the scaled-down screen. Key X to hide the dialogue. Use the PrtScn key (in Windows) or a screen saving utility to put the image in the clipboard. Then in Affinity Photo (or the Pixel mode of Affinity 3) use the file menu option "New from clipboard".