C-使用FBO和纹理的屏幕外渲染呈黑屏

Question

我正在Mac OS X（10.12）上开发一个简单的屏幕外渲染应用程序，但始终有黑屏。我确认着色器编译并链接没问题，并且可以将纹理更改为单个颜色。然后，我将有关深度信息相关的代码发表评论，但无效。单步调试时，找不到glGetError返回错误。以下是代码，请问什么问题？

GLuint program;
if (compile_link_shader(vertex_shader_source, fragment_shader_source, &program) < 0) {
   base_error_log("call compile_link_shader failedn");
   return -1;
}
glViewport(0, 0, target->width, target->height);
GLuint vao, vbo, ibo, loc_attr;
glGenVertexArrays(1, &vao);
glGenBuffers(1, &vbo);
glGenBuffers(1, &ibo);
glBindVertexArray(vao);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
glBufferData(GL_ARRAY_BUFFER, sizeof(pos_coord), pos_coord, GL_STATIC_DRAW);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(element_index), element_index, GL_STATIC_DRAW);
loc_attr = _static_cast(GLuint) glGetAttribLocation(program, "vertexPosition");
glVertexAttribPointer(loc_attr, 2, GL_FLOAT, GL_FALSE,
                          4 * sizeof(GLfloat), _static_cast(GLvoid*) (0 * 2 * sizeof(GLfloat)));
glEnableVertexAttribArray(loc_attr);
loc_attr = (GLuint) glGetAttribLocation(program, "textureCoordinate");
glVertexAttribPointer(loc_attr, 2, GL_FLOAT, GL_FALSE,
                          4 * sizeof(GLfloat), (GLvoid*) (1 * 2 * sizeof(GLfloat)));
glEnableVertexAttribArray(loc_attr);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
GLuint texture;
glGenTextures(1, &texture);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
/* glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE); // Removed from GL 3.1 and above */
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
GLenum format = origin->channels == 3 ? GL_RGB : GL_RGBA;
glTexImage2D(GL_TEXTURE_2D, 0, format, origin->width, origin->height, 0,
                 format, GL_UNSIGNED_BYTE, origin->image);
glGenerateMipmap(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, 0);
GLuint fbo;
GLuint depthBufferName;
glGenRenderbuffers(1, &depthBufferName);
glBindRenderbuffer(GL_RENDERBUFFER, depthBufferName);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT32, origin->width, origin->height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthBufferName);
if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE) {
        base_error_log("glCheckFramebufferStatus not GL_FRAMEBUFFER_COMPLETEn");
    return -1;
}
glUseProgram(program);
GLint loc_uniform = glGetUniformLocation(program, "texture");
glUniform1i(loc_uniform, texture);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glCullFace(GL_BACK);
glFrontFace(GL_CCW);
glEnable(GL_CULL_FACE);
glEnable(GL_DEPTH_TEST);
glBindTexture(GL_TEXTURE_2D, texture);
glBindVertexArray(vao);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
glDrawElements(GL_TRIANGLES, 2 * 3, GL_UNSIGNED_INT, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
glReadBuffer(GL_COLOR_ATTACHMENT0);
target->image = malloc(_static_cast(size_t) (target->width * target->height * target->channels));
if (!target->image) {
     base_error_log("malloc target->image failedn");
     return -1;
}
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glReadPixels(0, 0, target->width, target->height, (target->channels == 3 ? GL_RGB : GL_RGBA),
                 GL_UNSIGNED_BYTE, target->image);
glBindTexture(GL_TEXTURE_2D, 0);
glBindFramebuffer(GL_FRAMEBUFFER, 0);

另一部分是：

#define GL_DO_NOT_WARN_IF_MULTI_GL_VERSION_HEADERS_INCLUDED
#define GLEW_STATIC
#include <GL/glew.h>
#include <OpenGL/OpenGL.h>
#include <OpenGL/gl3.h>
#include <OpenGL/gl3ext.h>
static GLfloat pos_coord[] = {
        // Vertex Positions     // Texture Coordinates
        -1.0f,  1.0f,           0.0f, 1.0f, // Left Top
         1.0f,  1.0f,           1.0f, 1.0f, // Right Top
         1.0f, -1.0f,           1.0f, 0.0f, // Right Bottom
        -1.0f, -1.0f,           0.0f, 0.0f, // Left Bottom
};
static GLuint element_index[] = { // Note that we start from 0 with GL_CCW!
        0, 3, 2, // First Triangle
        0, 2, 1, // Second Triangle
};
static const char *vertex_shader_source = ""
        "#version 330 coren"
        "precision mediump float;n"
        "in vec2 vertexPosition;n"
        "in vec2 textureCoordinate;n"
        "out vec2 varyingTextureCoordinate;n"
        "void main() {n"
        "    gl_Position = vec4(vertexPosition, 0.0, 1.0);n"
        "    varyingTextureCoordinate = textureCoordinate;n"
        "}";
static const char *default_fragment_shader_source = ""
        "#version 330 coren"
        "precision mediump float;n"
        "in vec2 varyingTextureCoordinate;n"
        "out vec4 fragmentColor;n"
        "uniform sampler2D texture;n"
        "void main() {n"
        "    fragmentColor = texture(texture, varyingTextureCoordinate);n"
       /* "    fragmentColor = vec4(1.0, 1.0, 0.0, 1.0);n" */ // this line work fine
        "}";

以下代码具有相同的效果：黑屏。是由于同样的问题吗？

glViewport(0, 0, target->width, target->height);
GLuint vao, vbo, ibo, loc_attr;
glGenVertexArrays(1, &vao);
glGenBuffers(1, &vbo);
glGenBuffers(1, &ibo);
glBindVertexArray(vao);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
glBufferData(GL_ARRAY_BUFFER, sizeof(pos_coord), pos_coord, GL_STATIC_DRAW);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(element_index), element_index, GL_STATIC_DRAW);
loc_attr = _static_cast(GLuint) glGetAttribLocation(program, "vertexPosition");
glVertexAttribPointer(loc_attr, 2, GL_FLOAT, GL_FALSE,
                      4 * sizeof(GLfloat), _static_cast(GLvoid*) (0 * 2 * sizeof(GLfloat)));
glEnableVertexAttribArray(loc_attr);
loc_attr = (GLuint) glGetAttribLocation(program, "textureCoordinate");
glVertexAttribPointer(loc_attr, 2, GL_FLOAT, GL_FALSE,
                      4 * sizeof(GLfloat), (GLvoid*) (1 * 2 * sizeof(GLfloat)));
glEnableVertexAttribArray(loc_attr);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
glActiveTexture(GL_TEXTURE0);
GLuint texture;
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
/* glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE); // Removed from GL 3.1 and above */
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
GLenum format = origin->channels == 3 ? GL_RGB : GL_RGBA;
glTexImage2D(GL_TEXTURE_2D, 0, format, origin->width, origin->height, 0,
             format, GL_UNSIGNED_BYTE, origin->image);
glGenerateMipmap(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, 0);
GLuint fbo;
GLuint colorBufferName, depthBufferName;
glGenRenderbuffers(1, &colorBufferName);
glBindRenderbuffer(GL_RENDERBUFFER, colorBufferName);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA, origin->width, origin->height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
glGenRenderbuffers(1, &depthBufferName);
glBindRenderbuffer(GL_RENDERBUFFER, depthBufferName);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT32, origin->width, origin->height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, colorBufferName);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthBufferName);
if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE) {
    base_error_log("glCheckFramebufferStatus not GL_FRAMEBUFFER_COMPLETEn");
    return -1;
}
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glUseProgram(program);
GLint loc_uniform = glGetUniformLocation(program, "texture");
glUniform1i(loc_uniform, texture);
glBindTexture(GL_TEXTURE_2D, texture);
glBindVertexArray(vao);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
glDrawElements(GL_TRIANGLES, 2 * 3, GL_UNSIGNED_INT, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
glBindTexture(GL_TEXTURE_2D, 0);
glDrawBuffer(GL_COLOR_ATTACHMENT0);
glReadBuffer(GL_COLOR_ATTACHMENT0);
target->image = malloc(_static_cast(size_t) (target->width * target->height * target->channels));
if (!target->image) {
    base_error_log("malloc target->image failedn");
    return -1;
}
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glReadPixels(0, 0, target->width, target->height, (target->channels == 3 ? GL_RGB : GL_RGBA),
             GL_UNSIGNED_BYTE, target->image);
glBindFramebuffer(GL_FRAMEBUFFER, 0);

Answer 1

texture是框架缓冲区的颜色附件，该框架被写成，纹理绑定到读取的纹理采样器。这是一个未定义的行为。

Khronos OpenGL Documantation Framebuffer对象 - 反馈循环说：

可以将纹理绑定到FBO，将相同纹理绑定到着色器，然后尝试同时渲染。
只要您防止自己从该图像中取样，就可以将一个图像从纹理绑定到FBO，然后用该纹理渲染是完全有效的。如果您确实尝试读取并写入同一图像，则会获得未定义的结果。这意味着它可能会做您想要的，采样器可能会获取旧数据，采样器可能会获得一半的旧数据和一半的新数据，或者可能会获取垃圾数据。这些都是可能的结果。
不要这样做。您将获得的是未定义的行为。

由于texture是活动帧缓冲器的颜色附件，因此 glClear命令将清除纹理：

glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);
.....
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

如果要呈现纹理，则必须创建第二个纹理，将其连接到框架缓冲颜色平面并写入。

。

另请参见：

• OpenGL读写到相同的纹理
• framebuffer对象扩展示例

伸展到答案

您必须将纹理单元的索引设置为纹理采样器统一，而不是纹理对象本身。纹理单元由glActiveTexture和glBindTexture设置，将纹理绑定到目标（例如GL_TEXTURE_2D）和活动纹理单元。
顺便说一句，您必须重命名统一texture的名称，因为它与GLSL函数texture的名称相同。在以下代码中，我使用u_texture。

GLint loc_uniform = glGetUniformLocation( program, "u_texture" );
.....
int textureUnitIndex = 0; // 0, 1, 2 ...
glActiveTexture( GL_TEXTURE0 + textureUnitIndex );
glBindTexture( GL_TEXTURE_2D, texture );
.....
glUseProgram( program );
glUniform1i( loc_uniform, textureUnitIndex ); // <-- index of the texture unit
                                              //     NOT the texture object