时间戳

首页 > 解决方案 > OpenGL改变统一改变先前的绘制

c++ - OpenGL改变统一改变先前的绘制

问题描述

我有这个示例代码

// EBO is just a rectangle
// copypasted from learnopengl
glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

glUseProgram(shaderId);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);

glUniform1f(glGetUniformLocation(shaderId, shiftName), 0); //shiftName is string "shift"
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

glUniform1f(glGetUniformLocation(shaderId, shiftName), 0.5);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

glFlush();
SDL_GL_SwapWindow(window);

我拥有的着色器：
顶点：

#version 330 core
layout (location = 0) in vec3 pos;

out vec4 vertexColor;
uniform float shift = 0;

void main()
{
    gl_Position = vec4(pos.x, pos.y - shift, pos.z, 1.0);
    vertexColor = vec4(0, shift, 0, 1.0);
}

分段：

#version 330 core
out vec4 FragColor;

in vec4 vertexColor;

void main()
{
    FragColor = vertexColor;
}

据我了解，对于每个 glDraw 调用，我应该得到两个矩形，一个在另一个之下，有两种颜色。但相反，我得到一个矩形进行第二次绘制。
我假设，两个绘图调用实际上都为我绘制了一个相同的矩形。但我显然不明白为什么。
我尝试在两者之间进行刷新，在其间创建第二个缓冲区、glUseProgram 等。您可以在此处
查看完整代码

标签： c++openglsdlglad

解决方案

触发此问题的实际源代码是这样的：

shader.Select();
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
shader.Set("shift", 0);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);        
shader.Set("shift", 0.5f);         
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

结合一些包含这些方法的着色器类：

void Shader::Set(const std::string& name, bool value) const {
  glUniform1i(glGetUniformLocation(id, name.c_str()), (int32_t)value);
}

void Shader::Set(const std::string& name, int32_t value) const {
  glUniform1i(glGetUniformLocation(id, name.c_str()), value);
}

void Shader::Set(const std::string& name, float value) const {
  glUniform1f(glGetUniformLocation(id, name.c_str()), value);
}

This means that shader.Set("shift", 0); does call the int32 overload, resulting in trying to use glUniform1i to set a float uniform, which will just produce a GL error and not change the uniform at all. THe first frame will acutally be correct since the uniform is intialized to 0 by default, but after that, it will stay on 0.5 forever. Use shader.Set("shift", 0.0f);, but IMO, that kind of overloads is doing more harm than good.

Side note: Those glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO); calls are not necessary, the VAO will store the GL_ELEMENT_ARRAY_BINDING.

推荐阅读