开罗充满透明度 [英] Cairo Fill with Transparency

问题描述

我是开罗的新手，试图用透明的颜色和笔触创建文字。
笔触颜色的透明度有效但文本填充颜色透明度 transparency_value 不起作用。
如果我减少 transparency_value ，文字颜色会变暗（黑色）并且增加 transparency_value 会使文字颜色更亮（在我的情况下为绿色）

  cairo_surface_t * surface = cairo_image_surface_create（CAIRO_FORMAT_ARGB32,640,480）; 
 cairo_t * cairo = cairo_create（surface）; 
 cairo_set_font_face（cairo，myfont_face）; 
 cairo_set_font_size（cairo，25）; 
 cairo_text_extents_t范围; 
 cairo_text_extents（cairo，Hello World，& extents）; 
 cairo_move_to（cairo，200,200）; 
 cairo_text_path（cairo，Hello World）; 
 double transparency_value = 0.5; 
 cairo_set_source_rgba（cairo，0,1,0，transparency_value）; //透明度不起作用
 // cairo_fill（cairo）; //这没有什么区别
 cairo_fill_preserve（cairo）; 
 cairo_set_source_rgba（cairo，0.56,0.76,0.96,0.5）; //透明度有效
 cairo_set_line_width（cairo，1.5）; 
 cairo_stroke（cairo）; 
  

解决方案

可能是你在外面画你的文字表面？在下面的示例中，我添加了对 cairo_move_to（cr，200,200）的调用，现在我得到以下结果。 （这是用Lua编写的，使用

编辑：这是我将 transparency_value 更改为0.1时的结果。显然，结果是不同的，透明度正常工作（当放大时，你仍会在中间看到一些微弱的绿色）。

I'm new to Cairo, trying to create text with transparent color and stroke.
stroke color's transparency works but text fill color transparency transparency_value doesn't work. If i reduce transparency_value , text color just gets darker(black) and increasing transparency_value makes text color brighter (green in my case)

 cairo_surface_t* surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 640, 480);
 cairo_t* cairo = cairo_create(surface);
 cairo_set_font_face(cairo, myfont_face);
 cairo_set_font_size(cairo, 25);
 cairo_text_extents_t extents;  
 cairo_text_extents(cairo, "Hello World", &extents);
 cairo_move_to(cairo, 200, 200);        
 cairo_text_path(cairo, "Hello World"); 
 double transparency_value = 0.5;
 cairo_set_source_rgba(cairo, 0,1,0,transparency_value ); //transparency doesn't work
 //cairo_fill(cairo);  //this didn't make a difference
 cairo_fill_preserve(cairo);
 cairo_set_source_rgba(cairo, 0.56, 0.76, 0.96, 0.5); //transparency works
 cairo_set_line_width(cairo, 1.5);
 cairo_stroke(cairo);

解决方案

Could it be that you are drawing your text outside of the surface? In the following example I added a call to cairo_move_to(cr, 200, 200) and now I get the following result. (This is written in Lua and uses https://github.com/pavouk/lgi to call into cairo; comments indicate things that I changed compared to your version)

local cairo = require("lgi").cairo
local surface = cairo.ImageSurface.create(cairo.Format.ARGB32, 640, 480)
local cr = cairo.Context(surface)
local myfont_face = cr:get_font_face() -- I have to get this from somewhere
cr:move_to(200, 200) -- I added this line to make something appear
cr:set_font_face(myfont_face)
cr:set_font_size(25)
cr:text_path("Hello World")
local transparency_value = 0.5
cr:set_source_rgba(0, 1, 0, transparency_value)
-- cr:fill()
cr:fill_preserve()
cr:set_source_rgba(0.65, 0.76, 0.96, 0.5)
cr:set_line_width(5) -- changed from 1.5 to 5 to make it more prominent
cr:stroke()
surface:write_to_png("/tmp/out.png")

Edit: And this is the result when I change transparency_value to 0.1. Clearly, the result is different and transparency works correctly (when zooming in, you still see some faint green in the middle).

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