🖼️ JPG to PS Converter

Frequently Asked Technical Questions

Question 1: How does the DCTDecode embedding work technically?

Answer: The original JPEG binary data is read from the file using the FileReader API as an ArrayBuffer. It is then encoded to ASCII85 (a base-85 binary-to-text encoding) and embedded inline in the PostScript image operator. The PS interpreter applies two filters in sequence: /ASCII85Decode converts the ASCII85 text back to binary, then /DCTDecode decompresses the JPEG data stream to recover the original pixels. This pipeline preserves the original JPEG quality exactly — no re-encoding occurs.

Question 2: What PostScript level does the output conform to?

Answer: The output conforms to PostScript Language Level 2, which introduced the DCTDecode filter for native JPEG decompression. Level 2 has been supported by virtually all PostScript interpreters since 1991 (GhostScript, Adobe CPSI, etc.). The DSC (Document Structuring Conventions) comments follow the 3.0 specification.

Question 3: How does ASCII85 encoding work and why is it used?

Answer: ASCII85 encodes every 4 bytes of binary data as 5 ASCII characters (base-85), resulting in only 25% size overhead compared to ASCIIHex's 100%. The character set is printable ASCII 33–117 plus 'z' for zero groups. It's the standard encoding for binary data in PostScript and PDF. The delimiters <~ and ~> mark the start and end of the encoded stream.

Question 4: How is the image positioned on the page in PostScript coordinates?

Answer: PostScript uses a coordinate system where (0,0) is the bottom-left corner of the page, X increases rightward, and Y increases upward. The converter calculates the image's bottom-left position based on the selected position preset (center, top-left, etc.), then uses the translate and scale operators to place and size the image. The ImageMatrix [W 0 0 -H 0 H] flips the Y axis because image data is stored top-down while PS renders bottom-up.

Question 5: Why is the ImageMatrix [W 0 0 -H 0 H] instead of [W 0 0 H 0 0]?

Answer: JPEG image data is stored top-down (first row = top of image). PostScript's image operator maps data from bottom to top by default. The negative Y scale (-H) and Y offset (H) in the ImageMatrix flip the image vertically so that the first row of JPEG data appears at the top of the rendered image. Without this flip, the image would appear upside down.

Question 6: Does the PS file support CMYK color for printing?

Answer: The generated PS file uses /DeviceRGB color space for 3-component JPEGs and /DeviceGray for grayscale. RGB is suitable for most inkjet and laser printers, which handle the RGB-to-CMYK conversion internally. For professional offset printing requiring explicit CMYK, you would need to convert the image to CMYK first and modify the PS color space to /DeviceCMYK with the appropriate Decode array.

Question 7: How does the "Fill Page" scaling mode work?

Answer: "Fill Page" scales the image to cover the entire printable area, maintaining aspect ratio but potentially cropping edges. It calculates the scale factor as max(printableWidth/imageWidth, printableHeight/imageHeight). The image is then centered within the printable area, and any overflow is clipped by the page boundaries. This is ideal for full-bleed printing where the image must cover the entire page.

Question 8: Can I create multi-page PS files with one image per page?

Answer: The current converter creates one PS file per image, each containing a single page. For multi-page PS documents, you would need to merge the individual PS files or use a tool like GhostScript: gs -sDEVICE=pswrite -o multipage.ps page1.ps page2.ps page3.ps. Batch conversion with this tool plus GhostScript merging is the recommended workflow.

Question 9: Is there a maximum file size or resolution limit for conversion?

Answer: There is no artificial limit. Practical limits are governed by your device's available browser memory (RAM), since the entire JPEG file must be read into memory for ASCII85 encoding. Very large images (50+ megapixels or 20MB+ files) may cause slower processing on lower-memory devices, but the ASCII85 encoding is stream-based and efficient.

Question 10: How do I convert the PS output to PDF?

Answer: Use GhostScript: gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -o output.pdf input.ps. On macOS, you can open the PS file in Preview and choose File → Export as PDF. Online PS-to-PDF converters also work, but remember that uploading to an online service means your file leaves your device.

Question 11: Does the converter handle EXIF-rotated JPEGs correctly?

Answer: When a JPEG is loaded into an HTML Image element, the browser automatically applies EXIF orientation. The canvas preview reflects the correct orientation. However, the raw JPEG data embedded in the PS file may still contain the original EXIF orientation tag. Most modern PS viewers do not interpret EXIF rotation, so the image may appear un-rotated in some viewers. For critical use, pre-rotate the JPEG before converting.

Question 12: What is the difference between PS and EPS?

Answer: PS (PostScript) is a full page description language for complete documents with page setup. EPS (Encapsulated PostScript) is a subset designed for embedding graphics in other documents — it includes a BoundingBox but no page setup commands. This tool generates standard PS files. For EPS, the PostScript would need to be restructured with a %%BoundingBox header and without showpage or setpagedevice.