Drawing lines with alpha doesn't work

[smcameron]

You can attempt:

sng_set_foreground_alpha(RED, 0.5);
sng_current_draw_line(x1, y1, x2, y2);

But it will be drawn with full opacity.

The alpha value appears to be plumbed nearly all the way through, apart from this, in graph_dev_opengl.c:

void graph_dev_draw_line(float x1, float y1, float x2, float y2)
{
        make_room_in_vertex_buffer_2d(2);

        add_vertex_2d(x1, y1, sgc.hue, 255, GL_LINES);
        add_vertex_2d(x2, y2, sgc.hue, 255, GL_LINES);
}

I quickly attempted the patch below, but it didn't work. There might be a good reason (performance?) why it's not implemented.

$ stg show enable-drawing-lines-with-alpha
commit f46a48cd8bbe9952bb01f3179bbc5f2f796b3943
Author: Stephen M. Cameron <[email protected]>
Date:   Mon Mar 10 14:27:12 2025 -0400

    Enable drawing lines with alpha
    
    Signed-off-by: Stephen M. Cameron <[email protected]>

diff --git a/graph_dev_opengl.c b/graph_dev_opengl.c
index 64f63751..3ce82a4f 100644
--- a/graph_dev_opengl.c
+++ b/graph_dev_opengl.c
@@ -1204,6 +1204,8 @@ static void draw_vertex_buffer_2d(void)
                GLint start = 0;
                GLbyte mode = sgc.vertex_type_2d[0];
 
+               glEnable(GL_BLEND);
+               BLEND_FUNC(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
                for (i = 0; i < sgc.nvertex_2d; i++) {
                        if (mode != sgc.vertex_type_2d[i]) {
                                GLsizei count;
@@ -1235,6 +1237,7 @@ static void draw_vertex_buffer_2d(void)
                        glDrawArrays(mode, start, count);
                        /* printf("glDrawArrays 2 mode=%d start=%d count=%d\n", mode, start, i - start); */
                }
+               glDisable(GL_BLEND);
 
                sgc.nvertex_2d = 0;
 
@@ -3006,8 +3009,9 @@ void graph_dev_draw_line(float x1, float y1, float x2, float y2)
 {
        make_room_in_vertex_buffer_2d(2);
 
-       add_vertex_2d(x1, y1, sgc.hue, 255, GL_LINES);
-       add_vertex_2d(x2, y2, sgc.hue, 255, GL_LINES);
+       unsigned char alpha = (unsigned char) (sgc.alpha * 255.0);
+       add_vertex_2d(x1, y1, sgc.hue, alpha, GL_LINES);
+       add_vertex_2d(x2, y2, sgc.hue, alpha, GL_LINES);
 }
 
 void graph_dev_draw_rectangle(int filled, float x, float y, float width, float height)

I was trying to make the pull down menus fade away instead of just disappearing.