windows下3D文字

简单概述

需要在每一帧的视频图像上面添加3D文字，文字可以自由移动位置，变换各种字体属性，还能进行一些简单动画。然后把处理好的视频图像传个下一个步骤去处理。做的过程中参考了GitHub上的一个开源项目^[1],这个项目有一点内存泄露，需要修改一下。我们的项目主要思路利用opengl+freetype实现，首先使用freetype生成文字的3d顶点，然后使用曲面细分^[2]，生成更细致的顶点数据，接着计算顶点的法线，便于后面光照计算。渲染过程，首先把视频图像纹理绘制，然后开启混合，画上3d文字，这些都离屏渲染到一个Framebuffer,使用glBlitFramebuffer转移到 draw Framebuffer上，最后使用glReadPixels读取渲染后的图像数据。

一、文字与图像的融合

开始是先绘制的图像纹理，然后切换shader program，开启混合：

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);

结果在vs和mingw环境下都是只有视频图像，没有文字，接着我改了一下先绘制文字后面绘制视频图像，开启这种混合：glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_DST_ALPHA);,在vs和mingw都可以显示正常了。目前还没搞清楚为什么前一种情况，文字没有融合进去。

二、离屏渲染

项目中很多过程都是在cpu里面实现的，所以我们需要把3d文字进行离屏渲染，然后从GPU里面读到内存，这会消耗一定的性能。因为文字的字体大小可变，当放大时候，如果不进行抗锯齿处理，文字会很难看。生成read framebuffer和draw framebuffer,绘制后读取图像数据。

//生成两个fbo
enum {color , depth, buffer_count};
GLuint back_buffers[buffer_count];
int msaa_level = 4;
glGenFramebuffers(1, &m_backfbo);
glBindFramebuffer(GL_FRAMEBUFFER, m_backfbo);
glGenRenderbuffers(buffer_count, back_buffers);
glBindRenderbuffer(GL_RENDERBUFFER, back_buffers[color]);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8, m_canvasWidth, m_canvasHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER,back_buffers[color]);

glBindRenderbuffer(GL_RENDERBUFFER, back_buffers[depth]);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, m_canvasWidth,m_canvasHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, back_buffers[depth]);

GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
if (status != GL_FRAMEBUFFER_COMPLETE)
	std::cout << "frame buffer status fail
";

GLuint render_buffers[buffer_count];
glGenFramebuffers(1, &m_renderfbo);
glBindFramebuffer(GL_FRAMEBUFFER, m_renderfbo);
glGenRenderbuffers(buffer_count, render_buffers);
glBindRenderbuffer(GL_RENDERBUFFER, render_buffers[color]);
glRenderbufferStorageMultisample(GL_RENDERBUFFER, msaa_level, GL_RGBA8, m_canvasWidth, m_canvasHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, render_buffers[color]);
glBindRenderbuffer(GL_RENDERBUFFER, render_buffers[depth]);
glRenderbufferStorageMultisample(GL_RENDERBUFFER, msaa_level,
	GL_DEPTH_COMPONENT24, m_canvasWidth, m_canvasHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, render_buffers[depth]);

status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
if (status != GL_FRAMEBUFFER_COMPLETE)
	std::cout << "frame buffer status fail
";
//....after draw

glBindFramebuffer(GL_READ_FRAMEBUFFER, m_renderfbo);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, m_backfbo);
glBlitFramebuffer(0, 0, m_canvasWidth, m_canvasHeight, 0, 0, m_canvasWidth, m_canvasHeight, GL_COLOR_BUFFER_BIT, GL_NEAREST);
glBindFramebuffer(GL_FRAMEBUFFER, m_backfbo);
glReadPixels(0, 0, m_canvasWidth, m_canvasHeight, GL_RGBA, GL_UNSIGNED_BYTE, offscreen_image); //get offscreen image

最后效果如下：
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