Does JIMP (nodeJS) create bmp files from top-to-bottom? Is there a way to reverse this?

Question

I am trying to generate bmp images (graphs) in nodeJS for use in C to display on a low-res display.

Creating the graphs (with d3.js) and converting the svg-code to a bmp works fine (via svg2img and Jimp) and everything appears correctly in the bmp-file.

When I try to display it on the low-res screen, the C code reports that the image height is negative and fails. Now I read that bmp's can be stored top-to-bottom or bottom-to-top (here for example).

In which direction does Jimp work and how could it be reversed?

I have converted the bmp that I generated with Jimp again (using XnConvert) and tried the resulting bmp, which did successfully display on the low-res screen.

In node:

        svg2img(body.select('.container').html(), function(error, buffer) {
            //returns a Buffer
            fs.writeFile('foo1.png', buffer, function(){
              Jimp.read('./foo1.png')
                .then(image => {
                  // Do stuff with the image.
                  image.background(0xffffffff).resize(1200, 500);
                  image.dither565();
                  image.write("./public/test.bmp"); // save
                })
                .catch(err => {
                  // Handle an exception.
                  res.send("error1");
                });
            });
        });

In the C-script logs:

BMP_cfSize:1800054 
BMP_cfoffBits:54 
BMP_ciSize:40 
BMP_ciWidth:1200 
BMP_ciHeight:-500 <---------------------
//etc.
*****************************************

total_length:1800000,1800000
bytesPerLine = 3600
imageSize = -1800000

Is there a way to revert the order in Jimp? Or am I missing something else? Or would it be easier to try to revert the order in the C-library (I'm not very good with C)?

Answer 1

As pointed out in the comments, making the negative value positive in the bmp-js library that Jimp uses in this line flipped the image but also solved the issue with the C library that required that order.

Using image.flip(false, true) in Jimp, I could keep the correct orientation in the final result.

Issue reported in the bmp-js GitHub.