WebGL SkyGen
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Firefox or Chrome Required

SkyGen is a WebGL tool that lets you create skyboxes for your games. The tool uses an approximate atmospheric scattering algorithm to render a realistic atmosphere in real-time. The tool also lets you add and configure simple clouds in your sky. Once you’re satisfied with your result, you can export the images to a separate browser window where you can save each file individually and use them in your skybox. You can also export a single image that will show you a panoramic view of your sky.

This tool uses GLSL noise code from Ashima Arts.