CBDF Specification: Resources Section

1. Overview

The Resources section stores binary data (images, custom fonts, audio, video, embedded sub-documents) that are referenced by the Text section. It is the LAST data section before Logic, enabling abortable downloads: clients can close the network stream after receiving the Text section.

It sits between the third FS (end of Text) and the fourth FS (start of Logic). It begins with a 4-byte LE length prefix.

Structure:

[Length: 4 bytes LE] [Resource Count: 2 bytes LE] RS [Resource 0] RS [Resource 1] ...

2. Resource Record Format

Each resource record is separated by RS (0x1E) and has this structure:

Total overhead per resource: 6 bytes + RS separator = 7 bytes.

Resource IDs

• Must be unique within the document.
• Range 0-255 (1 byte).
• Referenced by IMAGE (0x16) commands in the Text section and by Image Definition records in the Styles section.
• ID assignment is at the document author's discretion (no required ordering).

Resource Types

3. Resource Referencing

Resources are referenced in two places:

A. Image Definitions (Styles section, sub-table 6J): The Image Definition record includes a Resource ID field (byte 1) that points to the resource containing the image data.

B. Text section (IMAGE command 0x16): [IMAGE image_def_index] inserts the image at the current text position. The index references an Image Definition in the Styles section, which in turn contains the Resource ID for the binary data.

The indirection chain is:

IMAGE [image_def_index] -> Image Definition -> Resource ID -> binary data

4. Missing Resource Behavior

When a resource is unavailable (section not downloaded, or resource stripped by a server/filter), the renderer must handle it gracefully:

• A. Image Definitions include Width and Height fields: the renderer can display a placeholder box of the correct size.
• B. Client choice:
  • Show a placeholder box (outlined rectangle, broken-image icon)
  • Reflow text as if the image doesn't exist
  • Show a "download" button for the user to request the resource
• C. Server stripping: A server (spam filter, bandwidth limiter) may remove specific resources by deleting their records. Because resources are referenced by ID (not positional index), removing a resource does NOT invalidate references to other resources.

5. Resource Section Layout

Complete section:

[Length: 4 bytes LE]        -- total bytes of section content
[Count: 2 bytes LE]         -- number of resource records
1E                          -- RS (before first resource)
[ResID:1][Type:1][Len:4][Data:N]   -- Resource 0
1E                          -- RS
[ResID:1][Type:1][Len:4][Data:N]   -- Resource 1
...

Empty section (no resources):

[0x00 0x00 0x00 0x02]      -- Length: 2 bytes
[0x00 0x00]                 -- Count: 0 resources

Single image example (a 1024-byte JPEG with ID=1):

[0x00 0x04 0x00 0x09]      -- Length: 1033 bytes
[0x01 0x00]                 -- Count: 1 resource
1E                          -- RS
[0x01]                      -- Resource ID: 1
[0x01]                      -- Type: image/jpeg
[0x00 0x04 0x00 0x00]      -- Data length: 1024 bytes
[... 1024 bytes of JPEG data ...]

6. Size Limits

• No overall size limit on the Resources section.
• Individual resource data length: up to 4 GB (4-byte LE uint32).
• Maximum 65535 resources per document (2-byte count field), though the 1-byte Resource ID limits unique references to 256. Documents needing more than 256 resources can use the ESC extended command mechanism (Phase III) for 2-byte resource IDs.

7. Abortable Download Behavior

The Resources section is deliberately placed LAST (before Logic) so that clients can implement "abortable downloads":

1. Client begins downloading the CBDF document.
2. Meta section arrives first (small, fast).
3. Styles + Text arrive next (may be compressed).
4. Client renders the text with image placeholders.
5. Client displays the rendered content to the user.
6. If the user doesn't need images (e.g., text-only mode, low bandwidth): client closes the connection. Resources never download.
7. If images are desired: client continues reading the Resources section. Images fill in progressively as each resource record is parsed.