javascript - 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 an 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.