Generating High‑Quality Vector PDFs with Cairo’s PDF Surface

Why Cairo’s PDF Surface Matters

When a PDF is generated from raster data, every line, curve, and glyph becomes a pixel block. That means scaling the file up or down degrades quality, and editing text or shapes in downstream tools is impossible. Cairo’s cairo_pdf_surface_t solves this by emitting true vector content: paths, text outlines, and embedded images remain resolution‑independent and editable. For applications that need crisp, scalable reports or cross‑platform printouts, the PDF surface is a production‑ready choice.

Creating a Vector PDF in Practice

Setting Up the Surface

/* Compile with: gcc -o pdf_example pdf_example.c `pkg-config --cflags --libs cairo` */
#include <cairo.h>
#include <stdio.h>

int main(void) {
    /* Create a 595x842 PDF surface (A4, 72 dpi). */
    cairo_surface_t *surface = cairo_pdf_surface_create("output.pdf", 595, 842);
    if (cairo_surface_status(surface) != CAIRO_STATUS_SUCCESS) {
        fprintf(stderr, "Failed to create PDF surface\n");
        return 1;
    }
    cairo_t *cr = cairo_create(surface);

    /* ---- drawing code will go here ---- */

    cairo_destroy(cr);
    cairo_surface_destroy(surface);
    return 0;
}

Drawing Primitives

Once the surface is created, Cairo’s normal drawing API applies. Every cairo_move_to, cairo_line_to, cairo_arc, etc., becomes a vector path in the PDF.

    /* Draw a blue rectangle */
    cairo_set_source_rgb(cr, 0, 0, 1); // blue
    cairo_rectangle(cr, 50, 50, 200, 100);
    cairo_fill(cr);

    /* Draw a red circle */
    cairo_set_source_rgb(cr, 1, 0, 0); // red
    cairo_arc(cr, 300, 200, 50, 0, 2 * M_PI);
    cairo_stroke(cr);

Embedding Images

Raster images can be inserted via cairo_image_surface_create_from_png. The image is embedded as a bitmap inside the PDF, but the surrounding drawing remains vector. Note that the PDF surface does not support alpha transparency; all image data is treated as opaque.

    cairo_surface_t *img = cairo_image_surface_create_from_png("logo.png");
    cairo_set_source_surface(cr, img, 400, 50);
    cairo_paint(cr);
    cairo_surface_destroy(img);

Adding Text

Text is rendered by Cairo’s font system. Cairo pulls system fonts; if a requested font is missing, it falls back to a default. The resulting glyphs are converted to outlines, preserving vector quality.

    cairo_select_font_face(cr, "Sans", CAIRO_FONT_SLANT_NORMAL, CAIRO_FONT_WEIGHT_BOLD);
    cairo_set_font_size(cr, 24);
    cairo_move_to(cr, 50, 300);
    cairo_show_text(cr, "Hello, Cairo PDF!");

What You’ll Lose (and How to Mitigate)

• No alpha transparency: All drawing is opaque. If translucency is required, pre‑blend the image or use a different format.
• Font availability: Missing fonts are replaced, which can alter appearance. Bundle fonts or embed them using a font‑embedding library if consistency is critical.
• Advanced patterns and clipping: Complex patterns or clipping masks may not be preserved. Stick to simple fills and strokes for guaranteed fidelity.

Verifying Your PDF

1. Check surface creation:

   if (cairo_surface_status(surface) != CAIRO_STATUS_SUCCESS) {
       /* handle error */
   }
   

2. After writing, open the file with a viewer (e.g., Acrobat, Okular) and confirm that shapes can be selected and resized without pixelation.
3. Use pdftotext or pdfinfo to verify the page count and basic metadata.

   pdftotext output.pdf -
   pdfinfo output.pdf
   

4. Measure performance: compare cairo_pdf_surface_create rendering time against a raster surface for a large document. Expect a CPU‑bound slowdown for very large PDFs.

Bottom Line

Cairo’s PDF surface delivers fully vector PDFs that scale cleanly and remain editable, making it a solid choice for report generators, print workflows, and cross‑platform document export. Be mindful of the lack of alpha support and font dependencies, and verify the output with simple tools before shipping to users.