Clipping hides; it does not remove

The crop that is not a crop

Place a photograph in a layout, drag the frame in until only a face shows, export a PDF. Most people believe the rest of the photograph is gone. It is not. The frame is a clipping path: an instruction that says render only the part of this object that falls inside this shape. The object itself (every pixel of the original photograph, at its original size) is still in the file.

Three consequences follow, and the third one has ended careers.

File size. A 40 MB photograph cropped to a thumbnail is still 40 MB in the exported PDF unless you explicitly downsample or crop-on-export. This is why a twelve-page brochure ships at 480 MB.

RIP time. The press’s raster image processor decodes the whole image and then throws most of it away, on every page, every time.

Recovery. Anyone with a PDF tool can release the clip and see what you hid. Newspapers have published redacted documents where the black rectangle was an object drawn over the text rather than the text being removed; the text was extractable with copy-and-paste. The same mechanism applies to a face cropped out of a photo, a price list with the competitor’s column covered, a contract with a name masked.

If you need something to be gone, delete the pixels in a raster editor and place the edited file. A clip, a mask, a white box and a black box are all just drawings on top.

Clip versus mask

• A clip path is geometric and binary: inside is shown, outside is not. Hard edges only.
• A mask uses a greyscale or luminance image: white shows, black hides, grey is partially transparent. This is how a soft fade into a background is made.

Applying them:

• Inkscape: select the object and the shape, Object ▸ Clip ▸ Set, or Object ▸ Mask ▸ Set. Release with the same menu.
• Illustrator: Ctrl+7 for a clipping mask; the Transparency panel’s Make Mask for an opacity mask.
• CorelDRAW: Object ▸ PowerClip ▸ Place Inside Frame. Extract contents to release.
• Scribus: an image frame is itself the clip.

CorelDRAW’s PowerClip is worth a separate warning: it nests happily, and a file can end up with PowerClips inside PowerClips several levels deep, each one carrying full-size contents. A file that opens slowly for no visible reason usually has this.

Transparency groups and the knockout you did not ask for

Opacity has two scopes and the difference is visible.

• Object opacity applies to each object separately. Two overlapping 50% circles show a darker patch where they cross, because the second is composited onto the first.
• Group opacity composites the group internally at full strength, then fades the whole result. The overlap disappears.

That is the setting people hunt for when a logo made of two overlapping shapes develops a visible seam at 50% opacity. In Illustrator it is the Transparency panel’s Isolate Blending; in SVG it is putting the opacity on the <g> rather than on each <path>.

A knockout group goes further: inside the group, later objects punch through earlier ones rather than blending with them. It is rarely needed and, when set by accident, produces artwork that looks correct on screen and prints with holes in it.

Before the file leaves

Three checks, thirty seconds:

1. Release clips on anything going to a machine. A cutting plotter or laser follows paths, and a clipped path still carries its hidden geometry: the blade will happily cut the part you thought was cropped away.
2. Crop images properly for press. Most PDF export dialogs have a crop image data to frames option. Turn it on and watch the file size fall. In Acrobat Pro the equivalent is the PDF Optimizer’s Discard cropped image data; Ghostscript will not do it for you.
3. Never use a clip, a mask or a covering rectangle for redaction. Flatten to raster, or remove the content at source. If you must prove something is gone, export the page to a 300 ppi image, check it, and place that.

A quick way to see what a PDF is carrying: pdfimages -list job.pdf lists every embedded image with its true pixel dimensions. If a picture you cropped to a 20 mm square is listed at 6000 × 4000 pixels, the crop was a clip and the rest of the photograph is still travelling with the file.