The download command can be used to download assets from the grid.

llInstantMessage(CORRADE,
    wasKeyValueEncode(
        [
            // Download a texture by UUID
            "command", "download",
            "group", wasURLEscape(GROUP),
            "password", wasURLEscape(PASSWORD),
            // or the path to an item in Corrade's inventory
            "item", "742c887b-a8ed-ba7b-6e7f-8ba8e9d1d76a",
            "type", "Texture",
            "callback", wasURLEscape(URL)
        ]
    )
);

and the data retrieved is compatible with the data supplied to the upload command.

In case the path optional parameter is not specified, then the asset will be downloaded, encoded as a Base64 string, then URI escaped and then placed as the value to the data key from the callback. If the path parameter is specified, then the data will be saved to the indicated path.

Grid clients such as Corrade download and upload textures to the grid in the JPEG2000 image format. The JPEG2000 image format is defined with various profiles, such that if the downloaded image is saved to a file and then inspected by the file or exiftool command line utilities, the detected image description should be:

JPEG2000 codestream

which indicates a Profile 0 JPEG2000 which is what the SecondLife grid is using.

The proper file extension for such images is "j2c" which stands for "JPEG2000 codestream". Some tools may not properly implement nor respect the file extension. One tool that manages to correctly convert images to "JPEG2000 codestream" is ImageMagick with the mogrify tool. For instance, supposing that an image x.png exists in the current directroy, then issuing the command:

mogrify -format "J2K" x.png

will result in a new image created x.j2k that, when inspected with the file tool:

file x.j2k

should output:

x.J2K: JPEG 2000 codestream

Other tools, such as ffmpeg will convert an image that when inspected with the file utility will read x.j2k: JPEG 2000 Part 1 (JP2) which will fail to upload with Corrade to SecondLife.

Furthermore, Corrade follows the SecondLife limits on texture uploads without any processing such that the user must ensure that the texture to be uploaded conforms to the specification.

One condition imposed by SecondLife is that the height and width of the image should be a power of 2 otherwise the image will not be uploaded. When uploading with a regular viewer, the viewer disguises a transformation that changes the image such that the width and height are a power of 2. However, when using Corrade, this transformation has to be performed beforehand in order for the upload to succeed.

By default sounds are stored on the grid in OGG-Vorbis format and Corrade can download the sound file as OGG.

Except for:

• notecards,
• scripts (LSLText)

all the other assets can be grabbed by Corrade's download command in case the UUID of the item is known.

Textures, sounds and animations are well-known formats that, once received, can be easily reused and converted to something else. On the other hand, clothing, landmarks, gestures, notecards and scripts (LSLText) are Linden-based formats that cannot be converted directly.

Some of these formats, for example Clothing, include references to images which will have to be pulled separately by first retrieving the clothing, going through the file and gathering UUIDs and then issuing another download to fetch the textures.