JPEG Format Boundary

JPEG is a visualization and interchange boundary, not a quantitatively exact archival format. It is appropriate for previews, overlays, reports, and lightweight exports after metric-sensitive computation has completed.

The native facade infers the format from the path:

let preview = ritk_io::read_image_native("aligned_preview.jpg")?;
ritk_io::write_image_native("aligned_preview_copy.jpg", &preview)?;

JPEG quantization changes intensities, so a round trip must use a bounded error or perceptual check rather than bitwise equality. Keep NIfTI, NRRD, or MetaImage as the quantitative source for registration and metric evaluation. After decoding, the result is an ordinary RITK image and can enter the same filter pipeline as any other input.

Quality-75 reconstruction oracle

The writer uses the Annex K luminance table and the quality scaling implemented by the locked jpeg-encoder 0.7.1 provider. At quality 75 the scale is 200 - 2 * 75 = 50, and each quantizer is Q = clamp((K * 50 + 50) / 100, 1, 255). The executable oracle checks the complete emitted table and the coefficients needed by four analytical 8 x 8 blocks: Q(0,0) = 8, Q(0,4) = 12, Q(4,0) = 9, and Q(4,4) = 34.

A constant centered level a has DCT coefficient F(0,0) = 8a. The fourth orthonormal basis has signs +--++--+; horizontal, vertical, and product blocks therefore have F(0,4), F(4,0), or F(4,4) = 8a. The coefficients 8a = 96, 72, and 136 are exact multiples of those quantizers, so forward quantization and inverse DCT reconstruct every integer sample exactly: the derived error bound is zero. Tests also require exact agreement with the independent decoder already used by the crate. A second oracle applies the Annex A forward DCT, emitted quality-75 quantizers, and inverse DCT to the rounded source gradient. Its exact decoded samples preserve source-sensitive coverage across the gradient’s odd-frequency coefficients. A changed quality table fails the table oracle, while an inverse-DCT normalization, covered basis, or level-shift mutation fails the analytical pixels.

The file reader maps the JPEG frame’s full encoded range to 0–255 with nearest integer rounding before grayscale conversion or RGB image construction. This display policy applies equally to low-precision lossless and 12-bit DCT input. DICOM decoding uses the clinical path instead: it preserves full integer sample values and requires the frame precision to match BitsStored.

Example Summary