72 lines
2.3 KiB
Plaintext
72 lines
2.3 KiB
Plaintext
|
What's going on with drawing sharp, odd-pixel-width lines in Cairo?
|
||
|
|
-------------------------------------------------------------------
|
||
|
|
|
||
|
|
First read the Cairo FAQ entry:
|
||
|
|
|
||
|
|
https://www.cairographics.org/FAQ/#sharp_lines
|
||
|
|
|
||
|
|
Here is a slightly different explanation, based on our
|
||
|
|
understanding as of July 2023:
|
||
|
|
|
||
|
|
There are no pixels on integer positions; if you draw a
|
||
|
|
single pixel line on any such integer position, it will
|
||
|
|
(half) shade the row/col of pixels on either side of that
|
||
|
|
position. The pixels are actually at N.5.
|
||
|
|
|
||
|
|
If the line width is a multiple of two, then the row/col of
|
||
|
|
pixels on either side of the integer coordinate will both be
|
||
|
|
full shaded. So if you draw a 2-pixel wide line at 1.0, the
|
||
|
|
pixel rows/cols at 0.5 and 1.5 will both be fully shaded and
|
||
|
|
you get what you asked for: 2 rows/cols of pixels that are
|
||
|
|
visible as intended.
|
||
|
|
|
||
|
|
However, if you want to shade just a single (sharp) row/col
|
||
|
|
of pixels, you must do so at N.5, which is where the pixels
|
||
|
|
actually "are". The first such row/col is at 0.5. the
|
||
|
|
second row/col is at 1.5 etc. etc.
|
||
|
|
|
||
|
|
So ... to draw a single pixel line on the first row/col, the
|
||
|
|
relevant x/y coordinate is 0.5.
|
||
|
|
|
||
|
|
Consequently, if you are drawing lines that can ever be an
|
||
|
|
odd number of pixels in width, you MUST shift the
|
||
|
|
coordinates by + (line_width * 0.5) in order to shade only
|
||
|
|
the pixels at that position (and adjacent ones, if the line
|
||
|
|
is 3, 5, 7 etc. pixels wide).
|
||
|
|
|
||
|
|
|
||
|
|
Below is a tiny test program that will allow you to
|
||
|
|
experiment with 0.5 offset to see its effect (it generates
|
||
|
|
/tmp/out.png which you should look at with a tool that can
|
||
|
|
zoom in to view pixels easily)
|
||
|
|
|
||
|
|
// gcc -o cairo-test cairo-test.c `pkg-config --cflags --libs cairo`
|
||
|
|
#include <stdio.h>
|
||
|
|
#include <cairo/cairo.h>
|
||
|
|
|
||
|
|
int main (int argc, char **argv) {
|
||
|
|
char* fn = "/tmp/out.png";
|
||
|
|
|
||
|
|
cairo_surface_t* cs = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 100, 100);
|
||
|
|
cairo_t* cr = cairo_create (cs);
|
||
|
|
|
||
|
|
cairo_set_source_rgba (cr, 1, 1, 1, 1);
|
||
|
|
|
||
|
|
cairo_set_line_width (cr, 1);
|
||
|
|
|
||
|
|
/* Experiment with using 0., 0.5, -0.5 and more for the value
|
||
|
|
* of y, and check which rows of pixels are shaded as a result.
|
||
|
|
*/
|
||
|
|
|
||
|
|
double y = 0.5;
|
||
|
|
|
||
|
|
cairo_move_to (cr, 0, y);
|
||
|
|
cairo_line_to (cr, 100.5, y);
|
||
|
|
cairo_stroke (cr);
|
||
|
|
|
||
|
|
cairo_surface_write_to_png (cs, fn);
|
||
|
|
cairo_destroy (cr);
|
||
|
|
cairo_surface_destroy (cs);
|
||
|
|
return 0;
|
||
|
|
}
|