71 lines
3.7 KiB
Markdown
71 lines
3.7 KiB
Markdown
# How iris renders an unbounded number of images
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Iris cannot require Vulkan descriptor indexing. The Android Vulkan Profile
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2025, covering 80.1% of active Vulkan-capable Android devices as of October
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2025, does not require `VK_EXT_descriptor_indexing` or its bindless texture
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features ([Android Vulkan profiles](https://developer.android.com/ndk/guides/graphics/android-vulkan-profile)).
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Arm guarantees the extension only on Valhall and fifth-generation GPUs
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([Arm Vulkan guidance](https://developer.arm.com/mobile-graphics-and-gaming/vulkan-api-best-practices-on-arm-gpus)).
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The emulator also rejects wgpu requests for `TEXTURE_BINDING_ARRAY`,
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`SAMPLED_TEXTURE_AND_STORAGE_BUFFER_ARRAY_NON_UNIFORM_INDEXING`, and
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`PARTIALLY_BOUND_BINDING_ARRAY`. Those features therefore must not enter
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Iris's required device feature set.
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## Current design
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Glyph atlas pages are layers of one `texture_2d_array`. `GpuTextures` doubles
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the array when it runs out of layers, copies the old layers on the GPU, and
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rebuilds every bind group that referenced the old view. Page numbers are
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assigned synchronously by `Textures::add_page` because glyph insertion needs
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the layer before the renderer processes queued texture updates.
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Standalone images each own a bind group and are not placed in the glyph
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array. Each render layer keeps ordinary rect/glyph instances separately from
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image instances. It draws the ordinary batch once, then binds and draws each
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standalone image. This removes any fixed image count at the cost of one bind
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and draw call per visible image, which is the appropriate tradeoff for phone
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transcripts containing a modest number of screenshots.
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The masks storage buffer appears in every image bind group. If that buffer or
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the atlas array is reallocated, all affected bind groups must be rebuilt;
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retaining a bind group across either reallocation would leave it pointing at
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the old GPU resource.
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Texture updates accumulate their rebuild requirement with OR. A patch must
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never clear a rebuild requested by an earlier push in the same batch.
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Within a render layer, images are drawn after rects and glyphs. Both primitive
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lists use `swap_remove`, so no code may infer draw adjacency from arena
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adjacency after a free.
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Standalone images currently use `NonFiltering` sampling. Thumbnail scaling
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and filtering remain image-widget decisions, not texture-storage decisions.
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## Colour convention
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Palette literals and decoded image bytes enter Iris as straight-alpha sRGB.
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Solid paints are converted once when registered and stored in a separate GPU
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paint table as linear `vec4<f32>`; rect and glyph primitives carry only the
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paint-table index. `Paints::set` rewrites a slot in place, so a shared theme
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can change without rebuilding widgets or primitive buffers. A rect may also
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hold a `Paint` definition and resolve it to a `PaintId` on its first draw;
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independently constructed definitions deliberately receive independent slots.
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Standalone images, colour glyphs, and atlas pages use
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`Rgba8UnormSrgb`, which makes sampling decode their RGB channels to linear
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light. Shaders therefore always return linear values. Both window backends
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render through an sRGB texture view in `SurfaceColorSpace::Srgb`, which
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encodes exactly once on output; the clear colour is linear too. A surface
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without an advertised RGBA/BGRA sRGB view and sRGB output space is rejected
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rather than silently displaying a different colour pipeline. The readback
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test in `iris/tests/color_space.rs` guards the solid, image, and in-place
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theme-update paths end to end.
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## Verification rig
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`scripts/rigs/gpu-probe` requests Iris's exact feature and limit set without a
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window. Run it on the target device when changing renderer requirements. A
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successful desktop adapter is not evidence that the same feature is available
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on Android hardware.
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