RISC OS Graphics Converters

Use this skill when creating, porting, reviewing, or calling RISC OS image conversion code. Use riscos-output as well when the converter emits sprites, palettes, masks, Draw files, plots, fonts, or any other RISC OS graphics output format.

ImageFileConvert Modules

ImageFileConvert converter modules register one or more file type pairs. The pair word has destination file type in bits 0-11 and source file type in bits 16-27. For example Clear (&690) to Sprite (&FF9) is:

#define CONVERT_FILETYPE ((0x690 << 16) | 0xFF9)

Register with ImageFileConvert_Register using a converterdef_t:

converterdef_t def;

def.api = CONVERTER_API_VERSION;     /* 100 */
def.flags = CONVERTER_VALID;         /* 0 */
def.type = CONVERT_FILETYPE;
def.name = "Module\t0.01\tAuthor";
def.private = pw;
def.convert = sprite_convert_Entry;
def.miscop = sprite_miscop_Entry;

Include these headers when available:

#include "ImageFileConvert/ImageFileConvert.h"
#include "ImageFileConvert/ImageFileConvertSWIs.h"
#include "ImageFileConvert/converters.h"

Use the ImageFileConvert include path in the makefile:

INCLUDES = <Lib$Dir>.ImageFileConvert.

Converters do not have to target Sprite. A vector converter can register a pair such as WMF (&B2F) to DrawFile (&AFF). If the destination format is renderable through ImageFileRender, ImageFileConvert may expose useful intermediate routes through that destination. For example, a WMF to DrawFile converter can allow WMF to Sprite, SVG, PNG, BMP, or JPEG conversions through existing DrawFile and Sprite converters.

CMHG And Veneers

Register converter entry points as generic veneers so ImageFileConvert can call C handlers with a register block:

#include "ImageFileConvert.h"

service-call-handler: Mod_Service Service_ImageFileConvert_Started,
                                  Service_ImageFileConvert_ConverterChanged,
                                  Service_ImageFileConvert_Dying,
                                  Service_ModeChange

generic-veneers: sprite_convert_Entry/sprite_convert_Handler,
                 sprite_miscop_Entry/sprite_miscop_Handler

Declare the veneer entry symbols in C:

extern void sprite_convert_Entry(void);
extern void sprite_miscop_Entry(void);

The C handlers usually have this shape:

_kernel_oserror *sprite_convert_Handler(_kernel_swi_regs *r, void *pw);
_kernel_oserror *sprite_miscop_Handler(_kernel_swi_regs *r, void *pw);

Registration Lifecycle

Initialisation should try to register if ImageFileConvert is already present, but failure to register must not prevent the module from loading. Leave the module quiescent and try again when Service_ImageFileConvert_Started is received.

Recommended behaviour:

• Mod_Init: call a helper which attempts registration and returns NULL even if ImageFileConvert is absent.
• Service_ImageFileConvert_Started: attempt registration.
• Service_ImageFileConvert_Dying: mark cached registration state as false; do not call back into the dying module.
• Mod_Final: deregister only if this module believes it is registered.
• Display- or font-dependent converters may also need to listen for Service_ModeChange and discard cached converted output.

Avoid returning the ImageFileConvert_Register SWI error from module initialisation unless the user explicitly wants the module to require ImageFileConvert at load time.

Convert Entry Point

ImageFileConvert calls the converter with registers based on ImageFileConvert_Convert:

For Sprite destinations, bit 0 of R0 is Converter_Convert_Sprite_CreateSprite. If set, return a bare sprite. If clear, return a sprite file with the 12-byte sprite-area header before the sprite.

For non-Sprite destinations, do not apply Sprite destination flags. If the destination defines no flags, reject any set bits:

#define CONVERT_RESERVED (~0u)

Reject unsupported flags with err_ConverterBadFlags. Use a narrow mask such as:

#define CONVERT_RESERVED (~Converter_Convert_Sprite_CreateSprite)

On successful size query (R4 == 0), set r->r[5] to the required byte count. On successful conversion, set r->r[5] to the byte count used.

If the conversion engine streams output and discovers the final size by running once, it is acceptable for the low-level writer to signal err_ConverterOutputBufferTooSmall during a size query, provided the public converter handler converts that into success once R5 contains the required size.

Calling ImageFileConvert

Callers that use ImageFileConvert_Convert directly should use the same pair word layout as converter registration: source file type in bits 16-27 and destination file type in bits 0-11.

When building a caller that offers export choices to the user, enumerate the available converter pairs first rather than hard-coding destination types. ImageFileConvert_EnumerateConverters returns packed type pairs in the same layout used by registration and conversion calls, so the caller can filter for the current source type by comparing the high 16 bits:

unsigned long pair = -1;

do
{
    _kernel_oserror *err;
    err = _swix(ImageFileConvert_EnumerateConverters,
                _INR(0,1) | _OUT(1),
                0, pair, &pair);
    if (err)
        pair = -1;
    else if (pair != (unsigned long)-1 &&
             (pair & 0xFFFF0000u) == (source_type << 16))
    {
        unsigned int destination_type = pair & 0xFFF;
        /* Offer this destination to the user. */
    }
} while (pair != (unsigned long)-1);

Practical points:

• Treat the packed pair as the durable identity of an export choice. If the UI needs to remember which menu entry or button the user picked, store the full pair and derive the destination type from it later.
• Derive the displayed name of the destination from the destination file type at UI-update time, for example through OS_FSControl 18, rather than keeping a private list of names.
• If enumeration fails or no pair matches the source type, present no export choices rather than guessing. An empty export list is safer than offering a conversion route that is not actually registered.

Recommended caller sequence:

• First call ImageFileConvert_Convert with R4 == 0 and R5 == 0 to query the required output size.
• Allocate an output buffer of the returned size and call again with R4 pointing at the buffer and R5 set to the buffer length.
• On success, use the returned R5 as the actual output byte count; it may be smaller than the allocated size.
• Pass -1 as the background colour when the conversion should use its natural background.

Some converters cannot know the output size cheaply. A successful size query may return R5 == -1; in that case, retry conversion with a guessed buffer size and grow it if ImageFileConvert returns the public "output buffer too small" error (&81ACCA). A common fallback is to start with the input size and increase geometrically, freeing failed buffers because their contents are not useful.

To obtain a dynamic sprite area from a Sprite destination conversion, request destination file type Sprite (&FF9) with Sprite flags clear so the converter returns a sprite file. Allocate four extra bytes before the returned data and store the total area size in the first word; the sprite-file header then follows at word 1.

Use the same packed file type pair:

unsigned int pair = (source_type << 16) | destination_type;

Caller-side register use is:

Typical call:

_swix(ImageFileConvert_Convert, _INR(0,6) | _OUT(5),
      flags, pair, src, srclen, dest, destlen, background, &used);

For file-to-file conversion:

1. Use OS_File 20 to read the source file type and length.
2. Load the source file into memory.
3. Resolve a textual destination type with OS_FSControl 31 if needed.
4. Pack the source and destination file types into R1.
5. Call ImageFileConvert_Convert first with R4 = 0 and R5 = 0 to query the required size.
6. If R5 returns a concrete size, allocate that many bytes and call again.
7. Save the converted data with OS_File 10 using the destination file type.

For a save or export dialogue driven by a shared UI object:

• use the stored converter pair to recover the destination file type when the user finally confirms the save
• set the save dialogue's file type from pair & 0xFFF
• derive the suggested leafname from the destination file type name, not from the current source filename alone
• when calling the conversion, pass the current source file type separately as a sanity check and prefer the destination from the enumerated pair rather than a free-form user-entered type

This keeps the eventual conversion consistent with the specific registered route that was offered to the user.

Do not assume the size query always returns a usable final length. In practice, some converters return R5 == -1 to mean that the final size is not yet known. In that case:

• start with a reasonable output buffer, commonly the input length
• call ImageFileConvert_Convert
• if the SWI returns public error &81ACCA, treat it as output buffer too small
• enlarge the buffer, commonly by doubling it
• retry until the conversion succeeds or a different error is returned

Use -1 in R6 unless the conversion specifically needs composition against a chosen background colour.

For Sprite destinations, bit 0 of R0 is still Converter_Convert_Sprite_CreateSprite:

• set it to request a bare sprite
• clear it to request a sprite file, including the 12-byte sprite-area header

For non-Sprite destinations, do not set Sprite-specific flags.

MiscOp Entry Point

For Sprite output converters, implement operation Converter_MiscOp_Sprite_ReadInfo (&400) where possible. It allows callers and render proxies to get image details without converting the full image.

For non-Sprite destinations, there may be no useful generic MiscOp. Returning err_ConverterUnknownMiscOp is valid when no operation is supported.

Entry:

Exit for ReadInfo:

Image flags include:

• Converter_MiscOp_Sprite_ReadInfo_Palette if the image has a palette.
• Converter_MiscOp_Sprite_ReadInfo_Colour for colour images.
• Converter_MiscOp_Sprite_ReadInfo_Mask if there is a mask or alpha channel.
• Converter_MiscOp_Sprite_ReadInfo_Interlace_None for normal non-interlaced images.

Return err_ConverterUnknownMiscOp for unrecognised operation numbers.

Calling ImageFileConvert_MiscOp

Client code can call ImageFileConvert_MiscOp when only lightweight image metadata is needed.

Use entry registers:

For Converter_MiscOp_Sprite_ReadInfo (&400), expect:

Prefer MiscOp over full conversion when the user only needs dimensions, bit depth, palette presence, mask presence, or interlace state.

Converter Pseudo-Errors

Converter handlers may return ImageFileConvert pseudo-error pointers from converters.h. ImageFileConvert maps them to its own public errors:

Use these rather than inventing local error blocks for ordinary converter failures.

For SWI callers, treat ordinary _kernel_oserror returns as the main failure path. Special-case &81ACCA only inside the resize-and-retry path for output buffers.

Caching And Repeated Conversion

ImageFileRender proxying and intermediate conversions may cause the same source data to be converted repeatedly. A common sequence is a size or bounding-box request followed by a real conversion. Expensive converters, especially vector or metafile converters, can cache the most recent converted output.

Practical cache guidance:

• Key the cache by input pointer, input length, and a checksum or content signature; pointer and length alone are not enough if buffers are reused.
• Clear the cache on failed conversion so stale output is never returned.
• If the cache grows dynamically, impose a sensible limit or use a dynamic area with a maximum size.
• If the conversion depends on display mode, font mapping, palettes, or other global rendering state, clear the cache on the relevant services such as Service_ModeChange.
• Always fall back to direct conversion if cache allocation or extension fails.

DrawFile Destination Reminders

When emitting DrawFiles, use riscos-output Draw and DrawFile references for object layout, coordinates, paths, text, sprites, and bounding boxes.

Useful patterns from WMF-style converters:

• Write the draw_fileheader first, then patch it again after all object bounding boxes have been accumulated.
• Initialise document and object bounding boxes to extreme values and merge each emitted object into the document bbox.
• Keep all object sizes word-aligned, including text strings and font tables.
• For path objects, emit MOVE, LINE, CURVE, CLOSE, and TERM elements exactly as required by DrawFile, and patch the path header after the element count and bbox are known.
• Convert embedded bitmap records into sprites and wrap them in draw_OBJSPRITE objects when a suitable bitmap-to-sprite converter is available.
• If text is emitted, maintain a DrawFile font table and use font mapping where possible; rotated text needs a transform and a transformed bbox.
• Document unsupported source primitives explicitly. Vector formats often contain operations that cannot be represented directly or are not worth supporting initially, such as clipping, raster operations, patterned fills, opacity, or complex brush modes.

Sprite Output Reminders

When emitting a Sprite or sprite file, use riscos-output for the sprite layout details. Important points:

• The sprite-file header is three words: sprite count, first sprite offset usually 16, and free offset.
• A bare sprite starts at the sprite header, without those 12 bytes.
• Paletted sprite palette entries are double words in &BBGGRR00 style.
• Set image and mask offsets relative to the start of the sprite header.
• If there is no mask, set mask == image.
• Preserve exact row stride and lbit/rbit semantics; do not assume byte-width rows for packed low-depth sprites.

Validation

Before declaring a converter complete:

• Build 32-bit with riscos-amu.
• Build 64-bit with riscos-amu BUILD64=1 when the project supports it.
• Test that the converter module loads successfully without ImageFileConvert present, if it is intended to be quiescent.
• Test with ImageFileConvert loaded and call both MiscOp &400 and Convert.
• Include at least one size-query call and one real output-buffer conversion.
• Check output headers and raw sizes, not only whether a renderer displays an image.