# Riscos Output > Details of how the RISC OS output system works. Use when creating text and graphics output using text positioning, graphics, drawing shapes, sprites, fonts, sprite areas, palette programming, ImageFileRender, or ColourTrans translation tables. It is not needed for plain textual output. - Skill: `gerph/riscos-output` (Agent Skill, multi-file: 32 files) - Install (CLI): `npx skillmds@latest add gerph/riscos-output` - Raw SKILL.md: https://api.skillmd.com/api/skills/gerph/riscos-output/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Marketing & Growth - License: MIT - Author: gerph (https://skillmd.com/u/gerph) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/gerph/riscos-output --- # RISC OS text and graphics output This skill covers writing text, graphics, and fonts to the screen or other output devices on RISC OS. For graphical text, font, rectangle, and line rendering, the GContext library can be used. ## Boundaries Use this skill when the task involves: * VDU output and control sequences * `OS_Plot` graphics * Draw and DrawFile rendering * Font Manager usage * sprite creation, plotting, redirection, or file layout * palette programming or `ColourTrans` * screen mode selection or mode decoding * `ImageFileRender` It is not needed for plain textual output that does not involve positioning, graphics, fonts, sprites, or palette handling. ## Terminology * VDU output covers character and control codes written through `OS_WriteC`, `OS_WriteN`, and `OS_Write0`. * Graphics output covers `OS_Plot`, sprite rendering, font rendering, and related drawing systems. * Draw provides path-based vector graphics through `Draw_*` SWIs. * Outline fonts are provided through the Font Manager via `Font_*` SWIs. * Sprites are raster images rendered through `OS_SpriteOp`. * Sprite areas are in-memory collections of sprites; sprite files are their on-disc representation. * `ImageFileRender` renders complete image files without the caller needing to know whether the source is JPEG, DrawFile, Sprite, PNG, or another supported format. * Modes define resolutions, pixel geometry, and colour depth. * `ColourTrans` is used for colour selection and translation in mode-independent code and for sprite translation tables. ## Coordinate system ### Graphics Graphics coordinates on RISC OS use: * Y=0 at the bottom, positive upwards * X=0 at the left, positive rightwards * units which are usually double the screen resolution Before declaring graphics code complete, check that the coordinates really are correct. ### Text Text coordinates on RISC OS use: * Y=0 at the top, positive downwards * X=0 at the left, positive rightwards To write text at a specific text-cell position, use `VDU 31, , `. Text characters are 8 pixels by 8 pixels. ## Common gotchas * Text and graphics coordinates do not share the same origin. * Sprite plotting normally needs a `ColourTrans` translation table to match the destination mode correctly. * Sprite row 0 is the bottom row, not the top row. * If code reads or writes the framebuffer directly, remove cursors first and restore them afterwards. * While output is redirected to a sprite, do not call `Wimp_Poll`. * If a saved screen image is paletted, preserve and restore the palette block as well as the pixel data. ## Task selection Choose the smallest useful subsystem for the task: * For VDU control sequences and textual cursor movement, read [references/vdu-codes.md](references/vdu-codes.md). * For simple line, point, rectangle, and shape plotting, read [references/os_plot.md](references/os_plot.md). * For Draw paths, filled shapes, and DrawFile object layout, read [references/draw.md](references/draw.md) and [references/drawfile.md](references/drawfile.md). If the task is a viewer or plug-in that must fit a DrawFile into a Wimp redraw box, read `references/drawfile.md` early. * For Font Manager usage, sizing, rendering, and colour handling, start with [references/fonts-overview.md](references/fonts-overview.md), then read the focused font references listed there. * For `ColourTrans`, start with [references/colourtrans-overview.md](references/colourtrans-overview.md), then read the focused colour references listed there. * For screen modes and mode decoding, start with [references/modes-overview.md](references/modes-overview.md), then read the focused mode references listed there. * For sprites, start with [references/sprites-overview.md](references/sprites-overview.md), then read the focused sprite references listed there. * For framebuffer access, pointer-driver quirks, or palette restoration, read [references/framebuffer-and-palettes.md](references/framebuffer-and-palettes.md). * For general image-file rendering, read [references/imagefilerender.md](references/imagefilerender.md). ## Cross-skill routing * When implementing image converter modules, use `riscos-graphics-converters` as well as this skill. * When using the Toolbox ImageFileGadget, use `using-toolbox-imagefilegadget` as well as this skill. * For higher-level graphics primitives and font handling through a library rather than raw SWIs, consider `using-libgcontext`. ## Image File Converters This skill covers the sprite, palette, mode, and rendering details needed by image converters, but it does not define the `ImageFileConvert` registration interface. When creating modules which convert image formats to Sprite, use the `riscos-graphics-converters` skill as well. When creating converters whose destination is DrawFile, use the Draw and DrawFile references in this skill for object layout, coordinate conversion, paths, text objects, sprite objects, font tables, and bounding-box handling. Use `riscos-graphics-converters` for registration, converter entry points, caching, and validation workflow. ## ImageFileRender Use `ImageFileRender` for general image files that should be drawn to the screen, printer, or a Wimp redraw context without the caller needing to know whether the file is a JPEG, DrawFile, Sprite, PNG, or a third-party format. For direct screen rendering, load the file into memory, derive the RISC OS filetype from the load address, choose one of the transform modes, and call `ImageFileRender_Render`. Use `ImageFileRender_BBox` when layout code needs the natural or transformed extent first, and set a graphics window (`VDU 24`) or use redraw rectangles for clipping. For new renderers, implement and register an ImageFileRender renderer. Read [references/imagefilerender.md](references/imagefilerender.md) for the calling sequences, structures, flags, and renderer registration workflow. For Toolbox ImageFileGadget method usage, use the `using-toolbox-imagefilegadget` skill. The public master documentation is: https://gerph.github.io/riscos-prminxml-staging/prm/html/select/graphics/imagefilerender.html ## Reference index * VDU control codes: [references/vdu-codes.md](references/vdu-codes.md) * `OS_Plot` operations: [references/os_plot.md](references/os_plot.md) * Draw paths: [references/draw.md](references/draw.md) * DrawFiles: [references/drawfile.md](references/drawfile.md) * Font quick reference: [references/fonts-overview.md](references/fonts-overview.md) * `ColourTrans` quick reference: [references/colourtrans-overview.md](references/colourtrans-overview.md) * Mode quick reference: [references/modes-overview.md](references/modes-overview.md) * Sprite quick reference: [references/sprites-overview.md](references/sprites-overview.md) * Framebuffer and palette handling: [references/framebuffer-and-palettes.md](references/framebuffer-and-palettes.md) * ImageFileRender: [references/imagefilerender.md](references/imagefilerender.md)