Question

Background

I’m working on an internal project that basically can generate a video on the client side, but since there are no JavaScript video encoders I’m aware of, I’m just exporting each frame individually. I need to avoid uploading to the server; this is all happening on the client side.

Implementation

I’m using this FileSaver.js (more specifically, Chrome’s webkit FileSystem API) to save a large number of PNGs generated by an HTML5 canvas. I set Chrome to automatically download to a specific folder, so when I hit ‘Save’ it just takes off and saves something like 20 images per second. This works perfectly for my purposes.

If I could use JSZip to compress all these frames into one file before offering it to the client to save, I would, but I haven’t even tried because there’s just no way the browser will have enough memory to generate ~8000 640×480 PNGs and then compress them.

Problem

The problem is that after a certain number of images, every file downloaded is empty. Chrome even starts telling me in the download bar that the file is 0 bytes. Repeated on the same project with the same export settings, the empty saves start at exactly the same time. For example, with one project, I can save the first 5494 frames before it chokes. (I know this is an insanely large number, but I can’t help that.) I tried setting a 10ms delay between saves, but that didn’t have any effect. I haven’t tried a larger delay because exporting takes a very long time as it is.

I checked the blob.size and it’s never zero. I suspect it’s exceeding some quota, but there are no errors thrown; it just silently fails to either write to the sandbox or copy the file to the user-specified location.

Questions

How can I detect these empty saves? Prevent them? Is there a better way to do this? Am I just screwed?

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EDIT: Actually, after debugging FileSaver.js, I realized that it’s not even using webkitRequestFileSystem; it returns when it gets here:

if (can_use_save_link) {
    object_url = get_object_url(blob);
    save_link.href = object_url;
    save_link.download = name;
    if (click(save_link)) {
        filesaver.readyState = filesaver.DONE;
        dispatch_all();
        return;
    }
}

So, it looks like it’s not even using the FileSystem API, and therefore I have no idea how to empty the storage before it’s full.

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EDIT 2: I tried moving the “if (can_use_save_link)” block to inside the “writer.onwriteend” function, and changing it to this:

if (can_use_save_link) {
    save_link.href = file.toURL();
    save_link.download = name;
    click(save_link);
}else{
    target_view.location.href = file.toURL();
}

The result is I’m able to save all 8260 files (about 1.5GB total) since it’s now using storage with a quota. Before, the files didn’t show up in the HTML5 FileSystem because I assume you didn’t need to put them there if the anchor element supported the ‘download’ attribute.

I was also able to comment out the code that appends “.download” to the filename, and I had to provide an empty anonymous function as an argument to both instances of “file.remove()”.

Answer 1

I ended up modifying FileSaver.js (see “EDIT 2” in the original post).