FBO：绘制渲染纹理时渲染到纹理，错误的纹理映射

Question

我在Mac OS X应用程序上使用OpengGL在NSOpenGLView上绘制纹理。

该应用程序是一个电影播放器​​。它将电影帧解码为CVOpenGLTextureRef（这是OpenGL纹理），然后使用GL_QUAD将它们直接绘制到视图中。一切正常。

以下是代码的相关部分。

// "image" is the CVOpenGLTextureRef containing the movie frame as texture
GLenum textureTarget = CVOpenGLTextureGetTarget(image);
GLuint textureName = CVOpenGLTextureGetName(image);

glEnable(textureTarget);
glBindTexture(textureTarget, textureName); 

glBegin(GL_QUADS);
// Draw the quads
//Note: textureTagret is NOT GL_TEXTURE_2D, therefore texture coordinates
//are NOT scaled to [0, 1] 

glTexCoord2f(0.0f, imageRect.size.height);
glVertex2f (0.0f, 0.0f);

glTexCoord2f(0.0f, 0.0f); 
glVertex2f (0.0f, windowRect.size.height); 

glTexCoord2f(imageRect.size.width, 0.0f);
glVertex2f (windowRect.size.width, windowRect.size.height);

glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (windowRect.size.width,0.0f); 

glEnd();
glDisable(textureTarget);
glFlush();

它工作正常，我可以调整窗口大小，纹理正确映射到较小的窗口。

请点击此处查看从全屏到500x280像素的不同窗口大小：

我现在想使用FBO渲染纹理，我开始制作一个非常简单的实现，包括将电影帧渲染到屏幕外的FBO（纹理），然后将该纹理绑定到屏幕上绘制。

以下是代码：

// "image" is the CVOpenGLTextureRef containing the movie frame as texture
GLenum textureTarget = CVOpenGLTextureGetTarget(image);
GLuint textureName = CVOpenGLTextureGetName(image);

////////////////////////////////////////////////////////////////////
// the creation on the FBO is done only once on program start:

GLuint fboId;
GLuint textureId;
float targetWidth = 2048;
float targetHeight = 2048;

// create a texture object
glGenTextures(1, &textureId);
glBindTexture(textureTarget, textureId);
glTexParameterf(textureTarget, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(textureTarget, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(textureTarget, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameterf(textureTarget, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexParameteri(textureTarget, GL_GENERATE_MIPMAP, GL_TRUE); // automatic mipmap
glTexImage2D(textureTarget, 0, GL_RGBA8, targetWidth, targetHeight, 0,
                 GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glBindTexture(textureTarget, 0);

// create a framebuffer object
glGenFramebuffersEXT(1, &fboId);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);

// attach the texture to FBO color attachment point
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT,
                           textureTarget, textureId, 0);

// check FBO status
GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if(status != GL_FRAMEBUFFER_COMPLETE_EXT)
    return FALSE;

// switch back to window-system-provided framebuffer
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
///////////////////////////////////////////////////////////////////////////////

// Render to texture

glEnable(textureTarget);
glBindTexture(textureTarget, textureName);

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
glClear(GL_COLOR_BUFFER_BIT);   

glBegin(GL_QUADS);
// Draw the quads

glTexCoord2f(0.0f, imageRect.size.height);
glVertex2f (0.0f, 0.0f);        

glTexCoord2f(0.0f, 0.0f); 
glVertex2f (0.0f,imageRect.size.height);

glTexCoord2f(imageRect.size.width, 0.0f);
glVertex2f (imageRect.size.width, imageRect.size.height);

glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (imageRect.size.width,0.0f);

glEnd();
glFlush();

// Bind newly rendered texture
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
glClear(GL_COLOR_BUFFER_BIT);
glBindTexture(textureTarget, textureId);
glGenerateMipmapEXT(textureTarget);

// draw on-screen
glBegin(GL_QUADS);
// Draw the quads
//Note: textureTagret is NOT GL_TEXTURE_2D, therefore texture coordinates
//are NOT scaled to [0, 1] 

glTexCoord2f(0.0f, imageRect.size.height);
glVertex2f (0.0f, 0.0f);

glTexCoord2f(0.0f, 0.0f); 
glVertex2f (0.0f, windowRect.size.height); 

glTexCoord2f(imageRect.size.width, 0.0f);
glVertex2f (windowRect.size.width, windowRect.size.height);

glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (windowRect.size.width,0.0f); 

glEnd();
glDisable(textureTarget);
glFlush();

代码无法正常工作，因为图像/纹理不仅是颠倒的，而且纹理映射也是错误的。它只在窗口全屏时才能正常工作，否则表现得非常奇怪。

见下图：

正如您所看到的，纹理在窗口内正确缩放，但它会与全屏窗口大小和实际窗口大小之间的差异成比例地“裁剪”。

我尝试过一切都没有成功。

由于这是我第一次使用OpenGL，我错过了什么？我变得疯了......

Answer 1

// Render to texture

glEnable(textureTarget);
glBindTexture(textureTarget, textureName);

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
glClear(GL_COLOR_BUFFER_BIT);   

我没有在这里设置视口和帧缓冲对象的投影。

glBegin(GL_QUADS);
// Draw the quads

glTexCoord2f(0.0f, imageRect.size.height);
g    lVertex2f (0.0f, 0.0f);        

glTexCoord2f(0.0f, 0.0f); 
glVertex2f (0.0f,imageRect.size.height);

glTexCoord2f(imageRect.size.width, 0.0f);
glVertex2f (imageRect.size.width, imageRect.size.height);

glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (imageRect.size.width,0.0f);

glEnd();
glFlush();