Help with Cairo graphics from data

**hikerFreak** · 04-02-2017

Hi folks,

I have been trying to create an image from pixel data using cairo, but it is not working.

Here is a simple code that generates a .png file using cairo:

Code:

//cc -o hello $(pkg-config --cflags --libs cairo) hello.c

#include <cairo.h>

#define ROWS 240
#define COLS 80



int
main (int argc, char *argv[])
{
        cairo_surface_t *surface =
            cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 240, 80);
        cairo_t *cr =
            cairo_create (surface);


        cairo_select_font_face (cr, "serif", CAIRO_FONT_SLANT_NORMAL, CAIRO_FONT_WEIGHT_BOLD);
        cairo_set_font_size (cr, 32.0);
        cairo_set_source_rgb (cr, 0.0, 0.0, 1.0);
        cairo_move_to (cr, 10.0, 50.0);
        cairo_show_text (cr, "Hello, world");


        cairo_destroy (cr);
        cairo_surface_write_to_png (surface, "hello.png");
        cairo_surface_destroy (surface);
        return 0;
}

Here is the pixel data array bw that I want to use in the code above:

Code:

 unsigned char bw[ROWS * COLS] = {0};
  int r, c;
 
  for (r = 0; r < ROWS; r++)
    for (c = 0; c < COLS; c++)
      if (r / 20 % 2 && c / 20 % 2)
        bw[r*COLS + c] = 255;

Is this the function I should use to plot the array bw: cairo_image_surface_create_for_data (https://www.cairographics.org/manual-1.0.2/cairo-Image-Surfaces.html#cairo-image-surface-create-for-data)?

If so what is the value I should use for int stride?

Should cairo_format_t format be assigned CAIRO_FORMAT_ARGB32?

**algorism** · 04-02-2017

The code you've posted just creates a png file. It doesn't render to the screen. Do you just want a png file of the grid pattern? Easy enough.

You need to pass 32-bit rgba values to cairo. You can tell the surface to ignore the alpha value by specifying the RGB24 format, but pixel data still needs to be 32-bit (4 byte) values.

Code:

//gcc -o cairo cairo.c $(pkg-config --cflags --libs cairo)

#include <cairo.h>
#include <stdlib.h>

#define ROWS 200
#define COLS 300

typedef unsigned char uchar;

int main (int argc, char *argv[])
{
    cairo_format_t format = CAIRO_FORMAT_RGB24;
    int stride = cairo_format_stride_for_width(format, COLS);
    uchar *pix = calloc(stride * ROWS, 1);

    for (int r = 0; r < ROWS; r++)
        for (int c = 0; c < COLS; c++)
            if (r / 20 % 2 && c / 20 % 2) {
                uchar *p = &pix[r*stride + c*4];
                p[0] = p[1] = p[2] = p[3] = 255;
                // p[3] is the alpha, which is unused with RGB24
            }

    cairo_surface_t *surface =
        cairo_image_surface_create_for_data(
            pix, format, COLS, ROWS, stride);

    cairo_surface_write_to_png (surface, "grid.png");
    cairo_surface_destroy (surface);
    free(pix);
    return 0;
}

**hikerFreak** · 04-03-2017

Once again algorism, thank you!

You've been very helpful, both for this question and for my previous one on plotting images based on data arrays.

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