Check whether a JPEG is progressive or baseline by reading its marker stream locally. This is useful when you need to verify if an export preset, CMS, or CDN is actually delivering progressive images.
SOF0 and SOF2 markers
The file extension or visual appearance cannot prove how a JPEG is encoded. The encoding is determined by the Start of Frame marker inside the file structure.
The checker reads the marker stream and identifies the encoding. An SOF0 marker means the image is a baseline JPEG, while an SOF2 marker means it is a progressive JPEG.
Scan count and structure
Progressive JPEGs are divided into multiple scans, allowing browsers to render a rough version first and refine it as data arrives. Baseline JPEGs typically have a much simpler, single-scan structure.
The scan table displays each Start of Scan segment. You can inspect component IDs, spectral selection, successive approximation, and Huffman table selectors to understand exactly how the image data is layered.
Metadata and subsampling
Beyond the encoding type, the inspector reports the chroma subsampling, quantization tables, and Huffman table counts. It also checks for JFIF presence and EXIF orientation.
By loading multiple files at once, you can compare the scan structures and metadata of JPEGs generated by different encoders without recompressing them.