feat: return generation metadata with core results (#2206)

docs/generation_metadata.md

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+# Generation results and metadata
+
+`generate_image()`, `generate_video()`, `adetail_image()` and `upscale()` return
+owned results containing the media and its generation record. This is a breaking
+C API change: applications and bindings must update their calls and rebuild.
+
+```c
+sd_img_gen_params_t params;
+sd_img_gen_params_init(&params);
+params.prompt = "a small house";
+
+sd_image_generation_result_t* result = NULL;
+if (generate_image(ctx, &params, &result)) {
+    for (int i = 0; i < result->count; ++i) {
+        const sd_image_result_t* item = &result->items[i];
+        /* Consume item->image and item->generation_info together. */
+    }
+    free_sd_image_generation_result(result);
+}
+```
+
+`result_out` is required and is set to `NULL` on failure. A successful result
+owns its pixels and UTF-8 JSON strings. The video result additionally owns its
+audio and contains the effective `fps` and `frame_count`. Results remain valid
+after freeing the generation context. Use the matching library result destructor;
+do not separately free fields unless transferring ownership and clearing them.
+Both result destructors accept `NULL`.
+
+## Reproduction parameters
+
+The `sdcpp.generation/v1` JSON is a snapshot of generation parameters for the
+same loaded context and the same input images, masks, reference media and other
+assets. It is created after `GenerationRequest` resolves the request, before
+preprocessing or sampling. A failed call returns no result or metadata.
+
+Fields follow the C generation parameter structs: `prompt`, `negative_prompt`,
+`width`, `height`, `seed`, `sample_params`, `hires`, `cache`,
+`vae_tiling_params`, `image_preprocess`, and the image/video-specific options.
+`kind` distinguishes image and video parameters. There is no inferred `task`.
+Enums use their API string names. Seeds are decimal strings to preserve their
+full integer range. Omitted optional fields retain the parameter initializer's
+defaults, including automatic values represented by infinity in the C API.
+
+Disabled optional features are omitted entirely: `cache`, `hires`, VAE tiling,
+zero/nonpositive `shifted_timestep`, inactive SLG, false circular axes, and empty
+preprocessing/reference arguments. SLG layers remain present when `slg_uncond`
+is enabled even with zero SLG scale. Enabled caches store their mode's parameters;
+TaylorSeer retains the shared DBCache thresholds and limits used by the runtime.
+Image CFG at its neutral value of 1 is omitted and resolves back to 1 by default.
+
+Image strength, ControlNet/IP-Adapter strength and PhotoMaker settings are stored
+when their corresponding input images are supplied. PuLID requires an embedding
+path. Layer count is stored for layered generation; video VACE and high-noise
+settings require those models. High-noise settings omit scheduler, custom sigmas,
+flow shift and timestep shifting because that pass shares the base configuration.
+Explicit nonempty argument strings are retained without rewriting their contents.
+Core sampling values such as seed, sampler, scheduler, steps and guidance remain
+explicit even when equal to their defaults. ADetailer always retains its inpaint
+strength because its input crops are created later.
+Model-required VAE tiling is resolved before serialization, so forced tiling
+(such as MiniMax-H3) remains present even if the caller did not enable it.
+
+The request fixes the seed, dimensions, clip skip, guidance, sampler, scheduler
+and eta before taking the snapshot. `custom_sigmas` is stored only when supplied
+as an input. Automatically computed schedules, preprocessing transforms,
+conditioning/cache decisions, detector regions and output statistics are not
+stored: they are recomputed from the same parameters and inputs.
+
+Each image records the seed used to sample its latent. `batch_count` is omitted;
+replay image parameters with `batch_count = 1`. Layers or frames decoded from
+the same latent share its seed, so layered generation reproduces that group of
+layers. Video frames share the video request. Partial batch cancellation returns
+metadata only for completed outputs. Full cancellation returns no result.
+
+The current image pipeline prepares input latents once using the starting seed
+and shares them across a batch. With a stochastic VAE encoder, replaying a later
+image alone using its sampling seed can change those input latents. Per-image
+seeds therefore do not guarantee independent reproduction for these paths.
+
+To replay a record, initialize the matching C parameter struct, populate its
+fields from the JSON, restore the same input buffers, and call generation with
+the same context. Keep custom-sigma/SLG arrays and string storage alive for the
+call. Paths supplied as generation parameters (LoRAs, hires models and identity
+embeddings) are retained. `models` identifies successfully loaded context model
+files by component (`model`, `diffusion_model`, `clip_l`, `vae`, etc.), using
+filenames with extensions and omitting directories. Optional components disabled
+for the request and preview-only TAE are omitted. WebUI text includes `Model`
+(the diffusion model when supplied separately, otherwise the checkpoint, without
+the extension) and `VAE` for a separate active VAE. These names are identifiers,
+not hashes or sufficient configuration to rebuild a context; replay still requires
+the same model files and context settings. The JSON does not embed media buffers
+or model weights.
+
+When `--lora-model-dir` is configured, CLI/server output records express LoRA
+paths relative to that directory, preserving subdirectories and file extensions
+and using `/` separators. This applies to file metadata, API responses and
+ADetailer records. Resolve these paths against the same LoRA directory before
+passing them back to the C API. Core results retain the supplied loading paths.
+Files outside the directory retain `..` components; paths on another volume,
+or records without a configured LoRA directory, retain their original paths.
+
+## Postprocessing
+
+Upscale records `upscale.upscale_factor`. ADetailer records `adetailer` and
+`inpaint_params`, with a concrete starting seed chosen before detection. These
+operations require the same upscaler/detector context and source image on replay.
+They do not store detection results or per-region generation traces.
+
+The CLI/server append operation parameter snapshots to `postprocess` in call
+order. The source generation parameters remain unchanged. Frame postprocessing
+uses the same operation parameters for each video frame. Standalone
+postprocessing does not invent source diffusion parameters for an input image.
+
+## File metadata
+
+Image encoders accept the native record and derive WebUI infotext from it.
+Images use the existing metadata locations and combined text layout:
+`<WebUI infotext>, SDCPP: <native JSON>`. Standalone postprocessing without
+source diffusion parameters stores only native JSON. Video metadata stores
+only native JSON.
+
+| Container | Metadata location |
+| --- | --- |
+| PNG | `tEXt`, keyword `parameters` |
+| JPEG | COM segment, `parameters` label followed by a NUL byte |
+| WebP image / animated WebP | XMP property `sdcpp:parameters` |
+| AVI | RIFF INFO `ICMT` |
+| WebM | `COMMENT` tag |
+
+Inspect images or videos with `sd-cli -M metadata --image <file>`; the same
+command supports all containers above, with `--metadata-format json` for a
+structured report. Video metadata inspection does not require a model or video
+decoder.
+Matroska containers are also supported, including `.webm` outputs whose audio
+codec causes the muxer to select the Matroska document type.
+
+The XMP namespace is `https://github.com/leejet/stable-diffusion.cpp/ns/1.0/`.
+PNG and JPEG retain the existing UTF-8 text payload convention. Memory encoding
+and file saving use the same implementation. The core result and API
+`generation_info` fields remain pure JSON; the combined layout applies only
+to embedded image metadata.
+
+WebUI conversion is lossy: known samplers and schedules have explicit name
+mappings; unsupported ones use `SDCPP Sampler`/`SDCPP Schedule` fields. Hires
+keeps the base request size and separate configured hires size/steps. The native
+record retains the full generation parameters. Standalone postprocessing has no WebUI
+infotext because its original diffusion settings are unknown.
+
+WebUI prompt text appends ordinary LoRAs as `<lora:filename_without_extension:weight>`.
+Directory components are omitted to match WebUI's filename lookup; the native
+`loras` array retains relative paths and weights. High-noise-only LoRAs and names
+that cannot be represented by this tag syntax remain in the native record only.
+The native prompt is not modified, and postprocessing LoRAs are not added to the
+source generation prompt.
+
+JPEG requires the COM payload, including the label and terminating NUL bytes,
+to fit within 65,533 bytes. Larger records fail encoding with an error rather
+than being truncated. Use PNG or WebP for larger records.
+
+Disabling file metadata omits both image representations, but does not disable
+the core generation record or its API response. Native async image results and
+OpenAI image response items contain `generation_info`; video results contain a
+sequence-level `generation_info`. The SD API `info` object contains per-output
+`generation_info` alongside derived `infotexts` and each image's seed. Its outer
+`parameters` field continues to echo the request.