Unhelpful exception on exceeding buffer bounds about omggif HOT 5 CLOSED

Discussion at #27 (comment)

from omggif.

Yeah, my claim is a lie. Sorry about that, misremembered the problem. Looks like I'll be ripping this mod out of the PR.

For the record, I was doing buffer management improperly. The proper technique is here: #27 (comment)

Also for the record, @deanm has superhuman patience for devs who are lost in the woods and don't know it.

from omggif.

@deanm, I think there is a problem writing with a global color table using the new buffer/position setter/getters.

I modified gifwrap and ran my tests. I have tests for writing and reading both local and global color table versions of GIFs.

When I use your latest omggif.js and provide a huge buffer, all of my tests run fine. But when I provide a buffer that has to scale in size, only the local color table tests work, even though they both run the same frame-writing and buffer-scaling code.

When trying to scale the buffer using a global color table, the encoding completes without error. However, when I try to read it back, I get GifError: Unknown gif block: 0x0. Also, my Mac is unable to show it.

I don't have a stripped-down test of encoding/decoding to send you. I'm hoping you have a personal test for this. Mine is intimately tied to gifwrap. However, you can see the algorithm here in my revision of a _writeFrame() function that I use for writing all frames:

        let buffer = gifWriter.getOutputBuffer();
        let startOfFrame = gifWriter.getOutputBufferPosition();
        let endOfFrame;
        let tryAgain = true;

        while (tryAgain) {
            endOfFrame = gifWriter.addFrame(frame.xOffset, frame.yOffset,
                    frame.bitmap.width, frame.bitmap.height, frameInfo.buffer, options);
            tryAgain = false;
            if (endOfFrame >= buffer.length - 1) {
                const biggerBuffer = new Buffer(buffer.length * 1.5);
                buffer.copy(biggerBuffer);
                gifWriter.setOutputBuffer(biggerBuffer);
                gifWriter.setOutputBufferPosition(startOfFrame);
                buffer = biggerBuffer;
                tryAgain = true;
            }
        }

The two test cases that fail are this one and the next, which use this GIF and this GIF, respectively. Both of these GIFs are officially in the public domain.

I'm still keening for an omggif test suite. ;-)

from omggif.

Okay, figured it out. The initial buffer has to be large enough for the header, including a global color table. My test started at a buffer size of 500 bytes to make sure everything was forced to scale, but that broke the global lookup table tests.

from omggif.

Okay, I posted a new version of gifwrap that depends on an external omggif. Thank you for so quickly taking care of all of this.

Also, I included tests for scaling the buffer size, which exercises both my code and yours.

from omggif.