CODEWITHSUNDEEP
firefoxmemory-managementopenlayers
Andrew Christensen
Andrew Christensen

Reputation: 1

avoid massive memory usage in openlayers with image overlay

I am building a map system that requires a large image (native 13K pixels wide by 20K pixels tall) to be overlayed onto an area of the US covering about 20 kilometers or so. I have the file size of the image in jpg format down to 23 MB and it loads onto the map fairly quickly. I can zoom in and out and it looks great. It's even located exactly where I need it to be (geographically). However, that 25 MB file is causing Firefox to consume an additional 1GB of memory!!! I am using Memory Restart extension on Firefox and without the image overlay, the memory usage is about 360 MB to 400 MB, which seems to be about the norm for regular usage, browsing other websites etc. But when I add the image layer, the memory usage jumps to 1.4 GB. I'm at a complete loss to explain WHY that is and how to fix it. Any ideas would be greatly appreciated.

Andrew

Upvotes: 0

Views: 988

Answers (1)

Tom Zych
Tom Zych

Reputation: 13576

The file only takes up 23 MB as a JPEG. However, the JPEG format is compressed, and any program (such as FireFox) that wants to actually render the image has to uncompress it and store every pixel in memory. You have 13k by 20k pixels, which makes 260M pixels. Figure at least 3 bytes of color info per pixel, that's 780 MB. It might be using 4 bytes, to have each pixel aligned at a word boundary, which would be 1040 MB.

As for how to fix it, well, I don't know if you can, except by reducing the image size. If the image contains only a small number of colors (for instance, a simple diagram drawn in a few primary colors), you might be able to save it in some format that uses indexed colors, and then FireFox might be able to render it using less memory per pixel. It all depends on the rendering code.

Depending on what you're doing, perhaps you could set things up so that the whole image is at lower resolution, then when the user zooms in they get a higher-resolution image that covers less area.

Edit: to clarify that last bit: right now you have the entire photograph at full resolution, which is simple but needs a lot of memory. An alternative would be to have the entire photograph at reduced resolution (maximum expected screen resolution), which would take less memory; then when the user zooms in, you have the image at full resolution, but not the entire image - just the part that's been zoomed in (which likewise needs less memory).

I can think of two approaches: break up the big image into "tiles" and load the ones you need (not sure how well that would work), or use something like ImageMagick to construct the smaller image on-the-fly. You'd probably want to use caching if you do it that way, and you might need to code up a little "please wait" message to show while it's being constructed, since it could take several seconds to process such a large image.

Upvotes: 1

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