问题

我目前正在为一个库添加对SDL2窗口系统的支持，该库目前使用GLUT。这包括渲染文本。问题的关键部分是，我无法在文本渲染发生的位置访问Window或Renderer。以前，使用GLUT的glutBitmapCharacter函数渲染文本。

我非常接近解决这个问题，通过使用glDrawPixels，但输出看起来像这样：

似乎纹理被翻转，上方渲染了一些噪音。我尝试了一些GL_UNPACK_XXX函数的设置，但无法找出如何修复这个问题。你有什么想法吗？

以下是一个能够复现此问题的示例应用程序：

main.cpp:

#include <SDL2/SDL.h>
#include <SDL2/SDL_ttf.h>
#include <GL/glew.h>
#include <string>
#include <stdint.h>
#include <chrono>
#include <thread>

const int width = 400;
const int height = 400;

TTF_Font* font = nullptr;

// 此函数是我唯一可以更改的部分！
void drawText(std::string text) {
  SDL_Color color = {
    static_cast<Uint8>(255),
    static_cast<Uint8>(0),
    static_cast<Uint8>(255),
    static_cast<Uint8>(255)
  };

  SDL_Surface* textRender = TTF_RenderUTF8_Blended(font, text.c_str(), color);
  SDL_Surface* surface = SDL_ConvertSurfaceFormat(textRender, SDL_PIXELFORMAT_RGBA8888, 0);
  SDL_FreeSurface(textRender);

  glPushAttrib(GL_BLEND);
  glPushAttrib(GL_BLEND_EQUATION);
  glPushAttrib(GL_CLIENT_PIXEL_STORE_BIT);

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

  glPixelStorei(GL_UNPACK_SWAP_BYTES,  GL_FALSE);
  glPixelStorei(GL_UNPACK_LSB_FIRST,   GL_FALSE);
  glPixelStorei(GL_UNPACK_ROW_LENGTH,  surface->pitch);
  glPixelStorei(GL_UNPACK_SKIP_ROWS,   0);
  glPixelStorei(GL_UNPACK_SKIP_PIXELS, 0);
  glPixelStorei(GL_UNPACK_ALIGNMENT,   1);

  glDrawPixels(surface->w, surface->h, GL_RGBA, GL_UNSIGNED_BYTE, surface->pixels);

  glPopAttrib();
  glPopAttrib();
  glPopAttrib();

  SDL_FreeSurface(surface);
}

// 下面的一切只是为了提供一个最小的可复现示例！

void loop(int64_t frame) {
  glClearColor(1.0, 1.0, 1.0, 1.0);
  glClear(GL_COLOR_BUFFER_BIT);
  glRasterPos2f(-1.0, 0);
  drawText(std::string("Hello World!") + std::to_string(frame));
}

int main() {
  SDL_InitSubSystem(SDL_INIT_VIDEO);
  TTF_Init();

  auto* window = SDL_CreateWindow("sdl-text", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, width, height, SDL_WINDOW_OPENGL);
  auto* context = SDL_GL_CreateContext(window);
  glewInit();

  bool quit = false;
  int64_t frame = 0;

  font = TTF_OpenFont("arial.ttf", 42);
  if (!font) {
    quit = true;
  }

  while (!quit) {
    loop(frame++);

    SDL_GL_SwapWindow(window);

    SDL_Event e;
    while (SDL_PollEvent(&e)) {
      switch(e.type) {
        case SDL_QUIT:
          quit = true;
          break;
      }
    }

    std::this_thread::sleep_for(std::chrono::milliseconds(16));
  }

  TTF_Quit();
  SDL_GL_DeleteContext(context);
  SDL_DestroyWindow(window);
  SDL_QuitSubSystem(SDL_INIT_VIDEO);

  return 0;
}

使用以下命令编译：

g++ main.cpp -lSDL2 -lSDL2_ttf -lGLEW -lGL

<details>
<summary>英文:</summary>
I am currently adding support for a SDL2 windowing system to a library, that currently uses GLUT. This includes rendering text. The problematic part is that I don&#39;t have access to the `Window` or `Renderer` at the position where text rendering happens. Previously text was rendered with GLUT using the `glutBitmapCharacter` function.
I am very close to solving this, by using `glDrawPixels`, but the output looks like this:
[![Output][1]][1]
It seems, that the texture is flipped and some noise is rendered above. I tried playing with the `GL_UNPACK_XXX` functions, but couldn&#39;t find out how to fix that. Do you have some idea?
Here is a sample application, that reproduces this issuse:
`main.cpp:`
```c++
#include &lt;SDL2/SDL.h&gt;
#include &lt;SDL2/SDL_ttf.h&gt;
#include &lt;GL/glew.h&gt;
#include &lt;string&gt;
#include &lt;stdint.h&gt;
#include &lt;chrono&gt;
#include &lt;thread&gt;
const int width = 400;
const int height = 400;
TTF_Font* font = nullptr;
// THIS FUNCTION IS THE ONLY THING I CAN CHANGE!
void drawText(std::string text) {
SDL_Color color = {
static_cast&lt;Uint8&gt;(255),
static_cast&lt;Uint8&gt;(0),
static_cast&lt;Uint8&gt;(255),
static_cast&lt;Uint8&gt;(255)
};
SDL_Surface* textRender = TTF_RenderUTF8_Blended(font, text.c_str(), color);
SDL_Surface* surface = SDL_ConvertSurfaceFormat(textRender, SDL_PIXELFORMAT_RGBA8888, 0);
SDL_FreeSurface(textRender);
glPushAttrib(GL_BLEND);
glPushAttrib(GL_BLEND_EQUATION);
glPushAttrib(GL_CLIENT_PIXEL_STORE_BIT);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glPixelStorei(GL_UNPACK_SWAP_BYTES,  GL_FALSE);
glPixelStorei(GL_UNPACK_LSB_FIRST,   GL_FALSE);
glPixelStorei(GL_UNPACK_ROW_LENGTH,  surface-&gt;pitch);
glPixelStorei(GL_UNPACK_SKIP_ROWS,   0);
glPixelStorei(GL_UNPACK_SKIP_PIXELS, 0);
glPixelStorei(GL_UNPACK_ALIGNMENT,   1);
glDrawPixels(surface-&gt;w, surface-&gt;h, GL_RGBA, GL_UNSIGNED_BYTE, surface-&gt;pixels);
glPopAttrib();
glPopAttrib();
glPopAttrib();
SDL_FreeSurface(surface);
}
// EVERYTHING BELOW HERE IS ONLY TO PROVIDE A MINIMUM REPRODUCABLE EXAMPLE!
void loop(int64_t frame) {
glClearColor(1.0, 1.0, 1.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT);
glRasterPos2f(-1.0, 0);
drawText(std::string(&quot;Hello World!&quot;) + std::to_string(frame));
}
int main() {
SDL_InitSubSystem(SDL_INIT_VIDEO);
TTF_Init();
auto* window = SDL_CreateWindow(&quot;sdl-text&quot;, SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, width, height, SDL_WINDOW_OPENGL);
auto* context = SDL_GL_CreateContext(window);
glewInit();
bool quit = false;
int64_t frame = 0;
font = TTF_OpenFont(&quot;arial.ttf&quot;, 42);
if (!font) {
quit = true;
}
while (!quit) {
loop(frame++);
SDL_GL_SwapWindow(window);
SDL_Event e;
while (SDL_PollEvent(&amp;e)) {
switch(e.type) {
case SDL_QUIT:
quit = true;
break;
}
}
std::this_thread::sleep_for(std::chrono::milliseconds(16));
}
TTF_Quit();
SDL_GL_DeleteContext(context);
SDL_DestroyWindow(window);
SDL_QuitSubSystem(SDL_INIT_VIDEO);
return 0;
}

compiled with the following command:

g++ main.cpp -lSDL2 -lSDL2_ttf -lGLEW -lGL

答案1

得分: 1

好的，以下是已翻译的部分：

"Ok, it seems like OpenGL takes the row length in pixel and SDL provides it in bytes. This is easily solved."

"我们还不需要将表面转换为RGBA。我们只需告诉OpenGL绘制BGRA（尽管SDL说它提供AGBR，但它也有效）。

"Finally we flip the image before drawing with glPixelZoom(1, -1). To offset the flip we have to move it up by the height."

"The solution is the following:"

"void drawText(std::string text) {"

"SDL_Color color = {"

"static_cast(255),"

"static_cast(0),"

"static_cast(0),"

"};"

"// We don't need to convert the surface format."

"SDL_Surface* surface = TTF_RenderUTF8_Blended(font, text.c_str(), color);"

"glPushAttrib(GL_BLEND);"

"glPushAttrib(GL_BLEND_EQUATION);"

"glPushAttrib(GL_CLIENT_PIXEL_STORE_BIT);"

"glEnable(GL_BLEND);"

"glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);"

"// SDL uses bytes, while OpenGL uses pixels."

"glPixelStorei(GL_UNPACK_ROW_LENGTH, surface->pitch / surface->format->BytesPerPixel);"

"// Get the current raster position."

"std::array<float, 4> pos;"

"glGetFloatv(GL_CURRENT_RASTER_POSITION, pos.data());"

"// Because the texture is upside down we move it up by the height and flip it."

"glWindowPos2f(pos[0], pos1 + surface->h);"

"glPixelZoom(1, -1);"

"// Use BGRA as a format."

"glDrawPixels(surface->w, surface->h, GL_BGRA, GL_UNSIGNED_BYTE, surface->pixels);"

"glPopAttrib();"

"glPopAttrib();"

"glPopAttrib();"

"SDL_FreeSurface(surface);"

"}"

void drawText(std::string text) {
  SDL_Color color = {
    static_cast<Uint8>(255),
    static_cast<Uint8>(0),
    static_cast<Uint8>(0),
  };

  // We dont't need to convert the surface format.
  SDL_Surface* surface = TTF_RenderUTF8_Blended(font, text.c_str(), color);

  glPushAttrib(GL_BLEND);
  glPushAttrib(GL_BLEND_EQUATION);
  glPushAttrib(GL_CLIENT_PIXEL_STORE_BIT);

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

  // SDL uses bytes, while OpenGL uses pixels.
  glPixelStorei(GL_UNPACK_ROW_LENGTH, surface->pitch / surface->format->BytesPerPixel);

  // Get the current raster position.
  std::array<float, 4> pos;
  glGetFloatv(GL_CURRENT_RASTER_POSITION, pos.data());

  // Because the texture is upside down we move it up by the height and flip it.
  glWindowPos2f(pos[0], pos[1] + surface->h);
  glPixelZoom(1, -1);

  // Use BGRA as a format.
  glDrawPixels(surface->w, surface->h, GL_BGRA, GL_UNSIGNED_BYTE, surface->pixels);

  glPopAttrib();
  glPopAttrib();
  glPopAttrib();

  SDL_FreeSurface(surface);
}