jpeg-format

name	jpeg-format
description	Use this skill when implementing, integrating, or reviewing JPEG read/write C code in wlframe, especially libjpeg-based parsing and generation in image, buffer, or related modules. Best for JPEG marker and colorspace handling, libjpeg lifecycle and error paths, scanline-based I/O, and wlframe-style ownership and cleanup review.

JPEG Format & libjpeg Integration Skill (wlframe/C Style)

When to Apply

Use this skill when implementing, integrating, or reviewing JPEG read/write C code in wlframe, especially in image/, buffer/, or related modules, or when using libjpeg for JPEG parsing/generation.

JPEG Format Essentials

• JPEG is a lossy compressed image format, widely used for photographs.
• Uses DCT, quantization, Huffman coding; supports grayscale and color (YCbCr).
• File starts with 0xFFD8 (SOI), ends with 0xFFD9 (EOI), contains various markers (APPn, DQT, DHT, SOF, SOS, etc.).

libjpeg Integration & API Design

• Use official libjpeg (https://github.com/libjpeg-turbo/libjpeg-turbo).
• Typical workflow:
  1. Create and configure jpeg_decompress_struct/jpeg_compress_struct.
  2. Set up error handling (setjmp/longjmp or custom error manager).
  3. Bind input/output (file or memory).
  4. Read/write headers.
  5. Read/write scanlines.
  6. Finish and destroy structs.
• Convert pixel data to/from wlframe's internal format as needed.
• API prefix: wlf_jpeg_. Use init/create and finish/destroy patterns.
• Struct layout: hot fields first, group pointers/handles/flags, hide implementation details via impl pointer.

Example API Design

struct wlf_jpeg_image {
	uint32_t width, height;
	uint8_t *pixels;
	int stride;
	int format; // e.g., WLF_PIXEL_FORMAT_RGBA8
	void *impl; // internal state (opaque)
};

bool wlf_jpeg_load(struct wlf_jpeg_image *out, const char *path);
bool wlf_jpeg_save(const struct wlf_jpeg_image *img, const char *path);
void wlf_jpeg_finish(struct wlf_jpeg_image *img);

Best Practices

• Check all allocation, I/O, and libjpeg API return values.
• Release resources in strict reverse order of allocation.
• Separate headers and implementation; expose only necessary API.
• Reference image/, buffer/ modules for style and error handling.

References

• https://www.w3.org/Graphics/JPEG/itu-t81.pdf
• https://libjpeg-turbo.org/Documentation/Documentation

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This skill combines wlframe, C, and struct-layout best practices for robust JPEG format and libjpeg integration.