feat: add configurable image input preprocessing (#2028)

docs/image_preprocessing.md

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+# Image preprocessing
+
+Use `--image-preprocess` to transform each image input once, before generation:
+
+```sh
+sd-cli ... \
+  --image-preprocess "target=init,mode=crop-resize,filter=lanczos,antialias=true" \
+  --image-preprocess "target=mask,filter=nearest-exact" \
+  --image-preprocess "target=ref,index=0,mode=fit-pad,width=768,height=768,filter=bicubic"
+```
+
+CLI and server image loaders decode at the original resolution. The generation
+entry point merges input defaults with user rules and prepares one transformed
+image per input. The original pipeline then consumes those images, including
+its mandatory canvas adaptation, reference resizing, and encoder preprocessing.
+
+```text
+native-resolution image
+  -> input defaults + user overrides
+  -> one input transform
+  -> original generation pipeline and model-specific processing
+```
+
+These rules do not override internal VAE, CLIP/VLM, ControlNet, or pixel-patch preprocessing.
+`--ref-image-args` retains its existing meaning and runs after this input transform.
+
+## Inputs and defaults
+
+| `target` | Input | Default geometry | Indexed? |
+| --- | --- | --- | --- |
+| `init` | img2img image or video first frame | Center crop to the generation aspect ratio, then resize | No |
+| `end` | Video last frame | Center crop, then resize | No |
+| `mask` | Inpainting mask | Inherit init geometry; otherwise center crop, then resize | No |
+| `control` | Control image | Center crop, then resize | No |
+| `ref` | Reference images | Preserve source dimensions | Yes |
+| `ip-adapter` | IP-Adapter image | Preserve source dimensions | No |
+| `id` | PhotoMaker identity images | Preserve source dimensions | Yes |
+| `control-frame` | Control video frames | Center crop, then resize | Yes |
+
+Canvas defaults use the aligned generation dimensions. Reference, IP-Adapter,
+and identity inputs use their original dimensions unless overridden. Default
+resampling is nearest for images and nearest-exact for masks.
+
+These defaults are shared by CLI, server, and C API. Moving geometry out of
+the loaders replaces the previous CLI/server BOX/sRGB resizing, so default
+pixels are not guaranteed to match earlier builds.
+
+Reference video and audio preprocessing are outside these image rules.
+Preprocessing options apply to `img_gen` and `vid_gen`, not standalone upscale
+or ADetailer mode. ADetailer clears the user's rules for its internal crops.
+
+## Rules
+
+Rules are comma-separated `key=value` lists. Repeat the CLI option or separate
+rules with semicolons. Every rule requires a `target` and at least one option.
+Rule syntax and input compatibility are checked when image/video generation
+starts. Unknown keys, invalid values, duplicate keys in a rule, missing images,
+and out-of-range indices cause generation to fail with an error log.
+
+Omit `index` to configure every image of that type; otherwise use a zero-based
+index. CLI directory inputs follow filename order. Indexed rules override
+type-wide rules field by field, regardless of order. At equal specificity,
+the last value for a field wins. `auto` selects the input preset.
+
+| `mode` | Input transform |
+| --- | --- |
+| `auto` | Use the input's default geometry |
+| `none` | Keep source dimensions without resizing, cropping, or padding |
+| `stretch` | Resize to the target dimensions |
+| `crop` | Crop a target-sized rectangle without resizing; fail if the source is too small |
+| `crop-resize` | Crop to the target aspect ratio, then resize |
+| `fit-pad` | Fit the entire image inside the target dimensions, preserving aspect ratio, then pad |
+
+`width` and `height` must be specified together as positive integers. They
+override the input transform's dimensions, not the generation or encoder size.
+For a native-size preset, specifying dimensions without a mode selects stretch.
+`mode=none` with explicit dimensions different from the source is contradictory
+and is rejected.
+
+`anchor=center|top|bottom|left|right` selects crop/padding placement.
+`pad_color=#RRGGBB` or `#RRGGBBAA` selects padding, defaulting to opaque black.
+A grayscale mask uses the first color component.
+
+`filter=auto|nearest|nearest-exact|bilinear|bicubic|lanczos` selects resampling.
+`antialias=auto|true|false` enables antialiasing automatically for filtered
+downscaling; explicit true requires bilinear, bicubic, or Lanczos.
+Filtered RGBA resizing uses premultiplied alpha.
+
+`canny=true|false` enables edge detection for any supported image target,
+defaulting to `false`. It runs once after geometry, before the original
+generation pipeline, including with `mode=none`. Grayscale, grayscale-alpha,
+RGB, and RGBA inputs are supported; alpha is preserved.
+
+Each input has its own Canny setting. Indexed rules can enable or disable it
+for individual references, identity images, or video control frames.
+
+```sh
+--image-preprocess "target=init,mode=fit-pad,canny=true"
+--image-preprocess "target=ref,index=0,mode=none,canny=true"
+--image-preprocess "target=control-frame,index=2,canny=true"
+```
+
+Init and mask sources must have the same dimensions. The mask inherits the
+init crop, resize, and padding coordinates, while retaining its own filter,
+padding value, and Canny setting. Conflicting mask geometry is rejected. An
+omitted mask remains absent until the original pipeline creates its default mask.
+
+## Downstream behavior
+
+`mode=none` only skips the input geometry transform. For example:
+
+```sh
+--image-preprocess "target=init,mode=none" \
+--image-preprocess "target=ref,mode=none"
+```
+
+The init image is still adapted to the generation canvas by the original
+pipeline. Reference images still follow `--ref-image-args` and model-specific
+resizing. CLIP retains its fixed input dimensions and normalization. HiDream-O1
+retains its original pixel-reference and visual preprocessing.
+
+Existing sharing between consumers is preserved: for example, Wan img2video
+uses the same adapted first frame for VAE conditioning and CLIP. High-resolution
+passes reuse the prepared images and apply their original size adaptation;
+they do not apply the user's crop a second time.
+
+To disable reference resizing before VAE encoding, use
+`--ref-image-args "resize_before_vae=false"` or the server field
+`"ref_image_args": "resize_before_vae=false"`. This is separate from
+`target=ref,mode=none`, which only skips input geometry. Model constraints
+still apply.
+
+## Server requests
+
+Native image/video requests and SDAPI accept `image_preprocess` as a string or
+an array of rule strings:
+
+```json
+{
+  "image_preprocess": [
+    "target=init,mode=fit-pad,filter=bicubic",
+    "target=mask,filter=nearest-exact",
+    "target=ref,index=0,mode=none"
+  ]
+}
+```
+
+OpenAI-compatible requests accept it through
+`<sd_cpp_extra_args>{...}</sd_cpp_extra_args>` in the prompt.
+Request rules replace server-default rules. Generation metadata records the
+user rules; image encodings and channel conventions are unchanged.
+
+## C API
+
+Set `image_preprocess` on the existing image/video generation parameters.
+The `generate_image()` and `generate_video()` signatures are unchanged:
+
+```c
+sd_img_gen_params_t params;
+sd_img_gen_params_init(&params);
+/* Set prompt, original-resolution input images, and generation options. */
+params.image_preprocess.rules = "target=init,mode=crop-resize,filter=lanczos;"
+                                "target=mask,filter=nearest-exact";
+bool ok = generate_image(ctx, &params, &images, &count);
+```
+
+Both generation parameter initializers set `image_preprocess.rules` to `NULL`,
+selecting input presets. Rule strings are borrowed for the synchronous call.
+The library owns temporary transformed pixels; caller images and arrays are
+not modified. Add `canny=true` to the desired target's rule in
+`image_preprocess.rules` to enable Canny.
+
+The parameter structs have grown; applications and bindings must be rebuilt.