ArrayBuffer to blob conversion

Question

I have a project where I need to display djvu schemas in browser.

I found this old library on Github which, as far as I understood, converts djvu files to bmp and then puts them into canvas element.

As I said, library is old(last commit was 5 years ago), so I need to make some corrections. Main problem is that lib uses obsolete BlobBuilder.

Steps I made to solve this problem:

1. Decompress this library via Chrome DevTools
2. Initial error is at line 3774 var c = "undefined" != typeof MozBlobBuilder ? MozBlobBuilder : "undefined" != typeof WebKitBlobBuilder ? WebKitBlobBuilder : console.log("warning: cannot build blobs")
3. I commented out this line
4. Next, I commented out line c = new c; and some of following lines too.

So, now it looks this way(variable I is array buffer, and ololo1 and ololo2 are some kind of offset and limit)

var c = new Blob(new Uint8Array(new Uint8Array(I,ololo1,ololo2)))
              , b = b.createObjectURL(c)
              , c = document.getElementById(kb)
              , f = c.getContext("2d")
              , h = new Image
              , g = a[Ea >> 2]
              , i = a[Fa >> 2]
              , j = c.width
              , k = Math.round(i * j / g);

            h.onload = function()
            {
                var a = g / j;
                4 < a && (a = 4);
                1 > a && (a = 1);
                f.globalAlpha = 1;

                for (N = 0; N < a; N++)
                    f.drawImage(h, N, N, g - a + N, i - a + N, 0, 0, j, k),
                    f.globalAlpha *= 1 - 1 / a;
                R(h.complete, "Image /bmp.bmp could not be decoded")
            }
            ;
            h.onerror = function(errorMsg, url, lineNumber, column, errorObj) {
                console.log(errorMsg, url, lineNumber, column, errorObj);
                console.log("Image /bmp.bmp could not be decoded!")
            }           
            ;

And now I stuck at error "Image /bmp.bmp could not be decoded!"(throwed in h.onerror handler).

So, my question is: what I am doing wrong?

Answer 1

I don't know why the author did wrap his Uint8Array in a new one... note that I don't really know either the deprecated BlobBuilder API, but one typo I can see in your code is that you need to wrap your TypedArray in a normal Array:

new Blob([new Uint8Array(buffer, byteOffset, length)]);

The Blob() constructor takes a blobParts sequence as first parameter, and then searches for BufferSource, USVStrings and Blob elements in this sequence. So when you pass a TypedArray, it will actually iterate over all the entries of this TypedArray and treat these as USVString (and thus convert their numerical value to UTF-8 strings in the Blob). That's rarely what you want, so better always pass an Array in this constructor.

Note that if you don't need to slice the buffer, it's probably better to directly pass it instead of using an intermediary Uint8Array (but still in a normal Array):

new Blob([buffer]);

Answer 2

var buffer = new ArrayBuffer(32);

new Blob([buffer]);

so the Uint8Array should be

new Blob([new Uint8Array([1, 2, 3, 4]).buffer]);