我编写了一个简单的 OpenGL 程序来采样 256x256 PNG 纹理并将其渲染到窗口中的矩形上。为此,我严重依赖教程。当我几乎直接按照教程进行操作时(进行最小的更改以便将 SDL2 用于 [x]),纹理没有正确渲染,并且似乎以某种方式缩小了 2 倍。这是源代码的第一个版本,以及结果的屏幕截图:
// Link statically with GLEW
#define GLEW_STATIC
// Headers
#include <GL/glew.h>
#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <SDL2/SDL_opengl.h>
// Shader sources
const GLchar* vertexSource = R"glsl(
#version 150 core
in vec2 position;
in vec2 color;
out vec2 Color;
void main()
{
Color = color;
gl_Position = vec4(position, 0.0, 1.0);
}
)glsl";
const GLchar* fragmentSource = R"glsl(
#version 150 core
in vec2 Color;
out vec4 outColor;
uniform sampler2D tex;
void main()
{
outColor = texture(tex,Color);
}
)glsl";
int main()
{
SDL_Init(SDL_INIT_VIDEO);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 4);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);
SDL_Window* window = SDL_CreateWindow("OpenGL", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, SDL_WINDOW_OPENGL | SDL_WINDOW_ALLOW_HIGHDPI);
SDL_GLContext context = SDL_GL_CreateContext(window);
glViewport(0, 0, 800, 600);
// Initialize GLEW
glewExperimental = GL_TRUE;
glewInit();
// Create Vertex Array Object
GLuint vao;
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);
// Create a Vertex Buffer Object and copy the vertex data to it
GLuint vbo;
glGenBuffers(1, &vbo);
GLfloat vertices[] = {
// x y u v
-0.5f, 0.5f, 0.0f, 0.0f, // Top-left
0.5f, 0.5f, 1.0f, 0.0f, // Top-right
0.5f, -0.5f, 1.0f, 1.0f, // Bottom-right
-0.5f, -0.5f, 0.0f, 1.0f // Bottom-left
};
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
// Create an element array
GLuint ebo;
glGenBuffers(1, &ebo);
GLuint elements[] = {
0, 1, 2,
2, 3, 0
};
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(elements), elements, GL_STATIC_DRAW);
// Create and compile the vertex shader
GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
glShaderSource(vertexShader, 1, &vertexSource, NULL);
glCompileShader(vertexShader);
// Create and compile the fragment shader
GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(fragmentShader, 1, &fragmentSource, NULL);
glCompileShader(fragmentShader);
// Link the vertex and fragment shader into a shader program
GLuint shaderProgram = glCreateProgram();
glAttachShader(shaderProgram, vertexShader);
glAttachShader(shaderProgram, fragmentShader);
glBindFragDataLocation(shaderProgram, 0, "outColor");
glLinkProgram(shaderProgram);
glUseProgram(shaderProgram);
// Specify the layout of the vertex data
GLint posAttrib = glGetAttribLocation(shaderProgram, "position");
glEnableVertexAttribArray(posAttrib);
glVertexAttribPointer(posAttrib, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(GLfloat), 0);
GLint colAttrib = glGetAttribLocation(shaderProgram, "color");
glEnableVertexAttribArray(colAttrib);
glVertexAttribPointer(colAttrib, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(GLfloat), (void*)(2 * sizeof(GLfloat)));
GLuint tex;
glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_2D, tex);
SDL_Surface* surface = IMG_Load("../textures/16x16-sb-ascii.png");
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, surface->w, surface->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, surface->pixels);
SDL_FreeSurface(surface);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
bool running = true;
while (running)
{
SDL_Event windowEvent;
while (SDL_PollEvent(&windowEvent))
{
switch (windowEvent.type)
{
case SDL_QUIT:
running = false;
break;
}
}
// Clear the screen to black
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
// Draw a rectangle from the 2 triangles using 6 indices
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
// Swap buffers
SDL_GL_SwapWindow(window);
}
glDeleteProgram(shaderProgram);
glDeleteShader(fragmentShader);
glDeleteShader(vertexShader);
glDeleteBuffers(1, &ebo);
glDeleteBuffers(1, &vbo);
glDeleteVertexArrays(1, &vao);
SDL_Quit();
return 0;
}
Initial results
作为引用,这里是实际的纹理文件:
Original texture file
如您所见,矩形的不同象限中的纹理似乎已经翻了两番,下面的两个拷贝莫名其妙地被彻底破坏了。
正如人们所预料的那样,将所有纹理坐标归一化为 0.5 会在预期的比例下生成一个纹理拷贝,尽管仍然可以看到许多奇怪的黄色/蓝色伪影,边缘周围还有一些非常细微的噪声伪影:
GLfloat vertices[] = {
// x y u v
-0.5f, 0.5f, 0.0f, 0.0f, // Top-left
0.5f, 0.5f, 0.5f, 0.0f, // Top-right
0.5f, -0.5f, 0.5f, 0.5f, // Bottom-right
-0.5f, -0.5f, 0.0f, 0.5f // Bottom-left
};
Results after scaling down the coordinates.
我在预装了 Ubuntu 18.04.4 LTS 的戴尔笔记本电脑上执行此操作。我的显卡是 Intel UHD Graphics 630 (Coffeelake 3x8 GT2)。
运行 glxinfo | grep "OpenGL version"给出以下结果:OpenGL 版本字符串:3.0 Mesa 19.2.8。
任何有关缩放或蓝色/黄色伪影的帮助将不胜感激。谢谢!
最佳答案
生成的纹理图像是亮度/alpha 图像,只有一个颜色 channel 和一个 alpha channel 。因此,您必须使用格式 GL_RG
当您通过 glTexImage2D
指定二维纹理图像时:
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RG, surface->w, surface->h, 0,
GL_RG, GL_UNSIGNED_BYTE, surface->pixels);
要么设置 Swizzle mask从红色 channel 读取绿色和蓝色以及从绿色 channel 读取 alpha 值:
GLint swizzleMask[] = {GL_RED, GL_RED, GL_RED, GL_GREEN};
glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_RGBA, swizzleMask);
或使用 Swizzling在片段着色器中进行纹理查找时获取红色 channel 和 alpha channel :
#version 150 core
in vec2 Color;
out vec4 outColor;
uniform sampler2D tex;
void main()
{
outColor = texture(tex,Color).rrrg;
}
关于c++ - 在 OpenGL/SDL2 教程中,纹理采样坐标似乎按 2 倍缩放,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/62206541/