How GCRflow converts a print file

What the conversion does, and does not do, to each kind of object in a PDF; how TIFF files, output intents and RGB are handled; and the published operating range of the ColorField 1 conversion kernel.

PDF objects: what is done and what is not

Text, vector shapes and gradients

Does
Converts their colors with GCR as PDF objects. Text and shapes in exactly RGB black (#000000) or DeviceGray black come out as single-ink K100, whatever the RGB profile. Near-blacks are converted like any other color.
Does not
Rasterize them. Convert text drawn with the PDF’s implicit default color (no color set, as in some Figma exports): that text stays RGB.

Images

Does
Converts images as image objects. Black in RGB images comes out as rich black; DeviceGray grayscale images in a PDF are converted to K only.
Does not
Rasterize the page around them. Turn black in RGB images into single-ink black.

Objects already in CMYK

Does
Converts them with GCR at the strength you choose, so their values can change. CMYK values are treated as values of the paper standard you choose.
Does not
Use a CMYK profile from the source file for the conversion. Leave CMYK values as they are, except in the API’s mode without ink saving (mode=cmyk_only), where RGB gets a standard ICC separation and CMYK values stay as they are.

Spot colors

Does
Keeps Separation and DeviceN spot colors as spot colors. A duotone image (black + spot) keeps its spot plate and gains CMYK channels.
Does not
Convert the tint values of spot colors.

Overprint

Does
Gives the number of K-only objects that overprint (OPM 1) in the Conversion Info report.
Does not
Promise that K-only objects that overprint will print as before: on top of GCR-converted colors they can print lighter. Check overprint in your preflight as usual.

Blend modes

Does
Converts objects with Normal blending. Gives the number of objects with other blend modes in the Conversion Info report.
Does not
Keep the result of Multiply, Overlay and other blend modes unchanged: blended over GCR-converted colors, it can differ from the original.

Files with blend modes

If a file uses Multiply, Overlay or other blend modes over colored areas, turn on “Strict blend-mode color” in the converter’s advanced settings (blend_strict in the API). The areas under those objects are then kept out of GCR, with no ink saving there. Check the result in the separation preview (first page) and on a proof.

TIFF files

  • RGB and CMYK TIFF files are converted pixel by pixel, in a separate raster pipeline, and written as 8-bit CMYK (16-bit input is reduced to 8 bits).
  • Converted TIFF files embed a CMYK ICC profile; TIFF has no output intent.
  • Grayscale TIFF files are returned unchanged. TIFF files with other bit depths or color spaces cannot be converted.
  • The low-resolution preview PDF and the Conversion Info report are made for PDF files only.

Output intent and document IDs

  • PDF: the ICC profile of the paper standard you choose (GRACoL 2013, Fogra 27, 39 or 51) is embedded as the output intent and replaces the output intents of the source file.
  • A PDF/X declaration in the source is left as it is; GCRflow does not check PDF/X conformance.
  • Output PDFs are written with deterministic document IDs instead of random ones.

RGB handling

RGB with an embedded ICC profile is converted per object using that profile; untagged RGB defaults to ECI-RGB V1.0 and can be switched to sRGB, Adobe RGB or Display P3 (the same rule applies to PDF and TIFF). The RGB rendering intent defaults to relative colorimetric with black point compensation and can be set to perceptual.

In the API, untagged RGB is either ECI-RGB V1.0 or sRGB (option rgb_source, see the API page).

Checking the result

  • Separation preview in the job list: the first page before and after, side by side, as CMY, CMYK or K only.
  • Low-resolution preview PDF (100 dpi, watermarked) before you download, for PDF files only.
  • Conversion Info report for each PDF job: the settings used, total ink before and after, and the number of blend-mode objects and of K-only objects that overprint (a count, not a list).
  • Proof the file before press as you would any other.

ColorField 1 — Verified Operating Range

  • GCR strength from GCR0 to GCR100. Official presets: GRACoL 2013 at TAC 320%. Custom presets: GRACoL 2013, Fogra 27, Fogra 39 or Fogra 51, total ink limit 260%–340%, within the limit offered for each paper standard.
  • With the same kernel build and parameters, repeated bakes are byte-identical. The integrated interpolation path also passed its bit-identical golden checks.
  • On the audited synthetic device-space gradients, second-difference continuity was 22–53× smoother than the previous production path. The multiplier depends on the test gradient and sampling density; it is not a universal constant.
  • Does not promise to repair banding already rasterized into the source file.
  • The audited conversion path introduced no new banding on the test set.
  • Measured improvement was observed where gradient breaks came from the prior color-separation path.

Boundary: ColorField 1 is published only for the range above — coated paper standards only (no uncoated or newsprint profiles), and no custom ICC upload.