(C + GTK + Cairo) Can't get linear gradients to work - c

Question

I have a sine wave that I generate with a random function. The resulting points are put into an array where the index is 0 to 799 (corresponding to my x value) and the value of said index is corresponding to the position of y.
Now, I'm trying to create a gradient that starts from green with 100% alpha and goes down 100px to green 20% alpha. Would look like the wave fades out downwards basically. I'm doing that by drawing a 1px wide x 100px high gradient starting at the y position of each point on my line.
Here is the code in my drawing function.
void *do_draw(void *ptr)
{
/*** Prepare SIGALRM ***/
siginfo_t info;
sigset_t sigset;
sigemptyset(&sigset);
sigaddset(&sigset, SIGALRM);
while(1)
{
while(sigwaitinfo(&sigset, &info) > 0)
{
currently_drawing = 1;
int width, height;
gdk_threads_enter();
gdk_drawable_get_size(pixmap, &width, &height);
gdk_threads_leave();
/*** Create surface to draw on ***/
cairo_surface_t *cst = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, width, height);
cairo_t *cr = cairo_create(cst);
/*** Draw stuff ***/
static int i = 0;
++i; i = i % 800;
cairo_set_source_rgba (cr, .0, .0, .0, 1);
cairo_paint(cr);
int j = 0;
for (j = 0; j < 799; ++j)
{
double y = (double)(height + sine[j]) / 2;
cairo_pattern_t *pat1;
pat1 = cairo_pattern_create_linear(j, y, j, y + 100);
cairo_pattern_add_color_stop_rgba(pat1, 0.1, 0, 1, 0, 1);
cairo_pattern_add_color_stop_rgba(pat1, 0.9, 0, 1, 0, 0.2);
cairo_rectangle(cr, j, y, j, y + 100);
cairo_set_source(cr, pat1);
cairo_fill(cr);
cairo_pattern_destroy(pat1);
}
cairo_destroy(cr);
gdk_threads_enter();
cairo_t *cr_pixmap = gdk_cairo_create(pixmap);
cairo_set_source_surface(cr_pixmap, cst, 0, 0);
cairo_paint(cr_pixmap);
cairo_destroy(cr_pixmap);
gdk_threads_leave();
cairo_surface_destroy(cst);
currently_drawing = 0;
}
}
}
The result I get at the moment is just a 20% alpha green 100px thick line but it does follow my coordinates. How do I get this gradient to work? I'm not understanding how the gradient space works I think.

Answer 1

I turned this into some freestanding code that can actually be compiled and tested and then noticed that your call to cairo_rectangle() is wrong. The arguments to this functions are:
x
y
width
height
You are passing in:
j
y
j
y + 100
So larger values result in larger and larger rectangles being used. I assume you want these arguments instead:
j
y
1
100
For reference, here is my code:
#include <cairo.h>
#include <math.h>
int main(void)
{
int width = 800, height = 800;
int sine[800];
int k;
for (k = 0; k < 800; k++) {
sine[k] = height * sin(k*M_PI/180);
}
/*** Create surface to draw on ***/
cairo_surface_t *cst = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, width, height);
cairo_t *cr = cairo_create(cst);
/*** Draw stuff ***/
static int i = 0;
++i; i = i % 800;
cairo_set_source_rgba (cr, .0, .0, .0, 1);
cairo_paint(cr);
int j = 0;
for (j = 0; j < 799; ++j)
{
double y = (double)(height + sine[j]) / 2;
cairo_pattern_t *pat1;
pat1 = cairo_pattern_create_linear(j, y, j, y + 100);
cairo_pattern_add_color_stop_rgba(pat1, 0.1, 0, 1, 0, 1);
cairo_pattern_add_color_stop_rgba(pat1, 0.9, 0, 1, 0, 0.2);
cairo_rectangle(cr, j, y, 1, 100);
cairo_set_source(cr, pat1);
cairo_fill(cr);
cairo_pattern_destroy(pat1);
}
cairo_destroy(cr);
cairo_surface_write_to_png(cst, "t.png");
cairo_surface_destroy(cst);
return 0;
}