CT1 Converter

What are CT1 files?

CT1 (CALS Raster Type 1) is a bilevel raster format specified by MIL-R-28002 for the exchange of scanned technical documents. The file begins with a 2048-byte header made of sixteen 128-byte ASCII records, each holding a tagged field: source and destination document identifiers, text and figure identifiers, document class, raster type (fixed to 1), orientation, pixel count, and density. The orientation field gives the pel path and line progression angles, such as 000,270 for a standard upright page, while the pixel count field gives the width and height as zero-padded six-digit numbers, and the density field gives the resolution, commonly 200, 300, or 400 dpi. The header is followed by a single image compressed with CCITT Group 4 (T.6), in which each line is encoded as differences from the line above, producing very compact files for line art. Conversion to graphics requires decoding the T.6 stream, rotating the image according to the orientation field, and applying the density as image resolution. The resulting 1-bit bitmap can be saved as a TIFF with Group 4 compression to preserve the original size, as a PDF, or as PNG or JPEG, where it is usually expanded to 8-bit grayscale. Supported by IrfanView, XnView, and Quick View Plus.