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Indexed GIFs Cannot Be Displayed by Browsers

To save everyone else from wasting a day of their life on this, if you're getting "the image cannot be displayed because it contains errors" messages in Firefox, don't spend hours of your life trying to figure out what's wrong with your code. Check the images first.

I'm working on an application that displays images (GIFs) from a document scanning system, and due to where the images are located in relation to the webapp, I'm using CFCONTENT to display them. No matter what I tried, I could not get the images to display properly.

After much head bashing (my poor wall) I decided to grab some of the images directly. To my surprise, they couldn't even be displayed in the default image viewer in Ubuntu.

The GIMP to the rescue. I opened one of the GIFs in GIMP and they're all indexed GIFs. I changed the mode from indexed to grayscale and saved the GIF, and voila, all is right with the world. So my very hard learned lesson of today is that indexed GIFs cannot be displayed by browsers.

Good thing it's a long weekend and Open Source Bridge is next week, because clearly I need a break.


WebVeteran said…
That's sounds REALLY weird. I don't understand . Aren't all GIFs indexed? Can you pack a few of the offending GIFs in a ZIP for us to check out?
Matthew Woodward said…
Unfortunately I can't share the GIFs, and bear in mind I'm a complete moron when it comes to image formats, etc., but when I open the GIFs that don't work in GIMP, the mode says it's "indexed," and at the top of the image window it says "Indexed, one layer." The other mode choices in GIMP are grayscale and RGB. When I change from indexed to grayscale, everything works fine.Doing a bit more research you're right--it looks like technically all GIFs are indexed, so my terminology is probably a bit off here. But whatever the difference between "indexed" and "grayscale" modes are in GIMP, that's what's causing the browser to not be able to display the image. The default image viewer in Ubuntu can't display this image either--when I try to open it, it says "does not appear to be a GIF file." So something's awry.
WebVeteran said…
When you change the format from indexed to greyscale or rgb, it's not really a gif anymore.I bet the problem is with your image scanning. It's probably outputting bmp or tiff data with a .gif extension.
Matthew Woodward said…
You're probably right--wouldn't be surprised if it's actually a tiff. I'll try to get to the bottom of how things are being scanned. Thanks!
WebVeteran said…
Been there, done that. I dealt with an automated (receipt) scanning solution before. It output 24bit BMPs which had to be beamed from consumers' home across the net for OCR. What a nightmare.So when you said 'scanning' a red flag went up.I'm interested to see the outcome of this. Keep up posted!
Matthew Woodward said…
Got to the bottom of this, kind of. There's a .NET app involved that programmatically prints the image via MS Word at which point it's a TIFF. The application then saves the images using a .NET image class (Image.Save('myfile.gif')) and that supposedly converts the TIFF image to a GIF based on the file extension of the file name to which you're saving.The person who wrote the .NET app changed Image.Save('myfile.gif') to Image.Save('myfile.gif', System.Drawing.Imaging.ImageFormat.Gif) and that creates a "real" GIF that behaves normally.

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