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Game Dev with SDL2





<Excerpt>


On some platforms, we can set the opacity of our window.  In this lesson, we explore how to adjust the transparency of SDL2 windows using `SDL_SetWindowOpacity()` and `SDL_GetWindowOpacity()`. We’ll cover how to handle errors, retrieve the current opacity, and understand the practical uses of window transparency.


Opacity is represented by a floating-point number ranging from `0.0` to `1.0`.   The default value is `1.0`, which represents a completely opaque window.  A window with an opacity of `0.0` is fully transparent.


Intermediate values between `0.0` and `1.0` are used to make our windows semi-transparent.  The following window has an opacity of `0.5`:


</Excerpt>


<Img file=”trans_window.png” alt=”Screenshot showing a semitransparant window” width=”448” height=”246” blurHash=”L75}s,ofWBf8~qWBRjWC_3RjRjWV” />


## `SDL_SetWindowOpacity()`


To change a window's opacity, we use the `SDL_SetWindowOpacity()` function. This function takes two arguments: a pointer to the `SDL_Window` whose opacity we want to change and a `float` value representing the desired opacity.


The value should be between `0.0` (fully transparent) and `1.0` (fully opaque):


```c++
SDL_Window* Window{SDL_CreateWindow(
  "Window", 100, 200, 400, 200, 0)};

SDL_SetWindowOpacity(Window, 0.5);//<h>
```


### Error Handling


`SDL_SetWindowOpacity()` returns `0` if successful, or a negative error code if unsuccessful.  The function will fail if the `SDL_Window*` we provide doesn’t support opacity, or the platform itself doesn’t support window opacity.


As usual, we can call `SDL_GetError()` to retrieve an explanation for the failure:


```c++
if (SDL_SetWindowOpacity(nullptr, 0.5) < 0) {//<w>
  std::cout << "Error Setting Opacity: "
    << SDL_GetError();//<h>
}
```


```c++
//<o>
Error Setting Opacity: Invalid window
```


## `SDL_GetWindowOpacity()`


We can retrieve the current opacity of a window using the `SDL_GetWindowOpacity()` function.  We pass the `SDL_Window*` as the first argument, and a pointer to a floating point number as the second argument.  Our floating point number will be updated with the window’s opacity:


```c++
SDL_Window* Window{SDL_CreateWindow(
  "Window", 100, 200, 400, 200, 0)};

float Opacity;//<h>
SDL_GetWindowOpacity(Window, &Opacity);//<h>
std::cout << "Window Opacity: " << Opacity;

SDL_SetWindowOpacity(Window, 0.5);
SDL_GetWindowOpacity(Window, &Opacity);//<h>
std::cout << "\nWindow Opacity: " << Opacity;
```


```c++
//<o>
Window Opacity: 1
Window Opacity: 0.5
```


### Error Handling


`SDL_GetWindowOpacity()` returns `0` if it was successful, or a negative error code if it failed.  We can use `SDL_GetError()` to retrieve an explanation for the failure:


```c++
float Opacity;
if (SDL_GetWindowOpacity(nullptr, &Opacity) < 0) {//<w>
  std::cout << "Error Getting Opacity: "
    << SDL_GetError();//<h>
}
```


```c++
//<o>
Error Getting Opacity: Invalid window
```


If the platform does not support opacity, `SDL_GetWindowOpacity()` will not report an error. Instead, it will set the value pointed to by our opacity pointer to `1.0`, indicating the window is fully opaque.


<Aside>


### Preview: Window Visibility


If we want to make a window invisible, it’s generally preferred to hide it rather than setting its opacity to `0.0`.  We can hide and show a window by passing its `SDL_Window*` to `SDL_HideWindow()` or `SDL_ShowWindow()` respectively:


```c++
// Hide the window
SDL_HideWindow(Window);

// Show the window
SDL_ShowWindow(Window);
```


We cover window visibility in more detail in a dedicated lesson later in this chapter.


</Aside>


## Summary


In this lesson, we introduced how to manage window opacity in SDL2. We covered how to use `SDL_SetWindowOpacity()` and `SDL_GetWindowOpacity()` to set and retrieve transparency values, along with handling errors.  Key takeaways:

- Use `SDL_SetWindowOpacity()` to adjust a window’s transparency.
- Retrieve a window’s current opacity with `SDL_GetWindowOpacity()`.
- Consider platform-specific limitations by implementing error handling and using `SDL_GetError()` to debug issues.

<FAQ>


```javascript
//<m>
{
  slug: 'common-use-cases-transparent-windows',
  title: 'Use Cases for Transparent Windows',
  question: 'What are some common use cases for transparent windows?',
}
```


Transparent windows are commonly used in applications where visual layering or aesthetic appeal is important. Here are some key use cases:


### **Overlays and HUDs**


Transparent windows are often used for creating overlays, such as heads-up displays (HUDs) in games or real-time data monitoring applications. The transparency ensures that the main content remains visible while supplemental information is displayed on top.


### **Screen Recording Tools**


Screen recording and streaming software frequently use transparent windows to overlay controls, annotations, or indicators on the desktop without obstructing the user’s view.


### **Creative UI Design**


Applications with innovative or artistic user interfaces, such as media players or drawing tools, often use transparency for sleek, modern designs. For example, translucent control panels can make the interface visually appealing while keeping the background partially visible.


### **Focus and Context**


In productivity tools, transparency helps create focused views. For instance, a partially transparent task list might overlay on a calendar without completely obscuring it.


### **Debugging and Visualization**


Developers use transparency to visualize layouts or debug graphical layers in applications, as it makes overlapping elements easier to analyze.


In SDL, you might combine `SDL_SetWindowOpacity()` with event-driven updates to adjust transparency dynamically, creating practical and engaging interfaces.


</FAQ>


<FAQ>


```javascript
//<m>
{
  slug: 'platform-support-opacity',
  title: 'Why Some Platforms Lack Opacity',
  question: 'Why do some platforms not support window opacity?',
}
```


Platform limitations for window opacity arise due to differences in how operating systems handle windowing and graphics. Here are the main reasons:


### **Technical Constraints**


Some platforms use simpler windowing systems that don’t support advanced graphical features like alpha blending, which is required for transparency. These systems prioritize speed and simplicity, making transparency infeasible.


### **Compatibility**


Older platforms or lightweight window managers may lack support for transparency to maintain compatibility with legacy applications. Introducing opacity might break older software.


### **Design Choices**


Certain operating systems, like Linux distributions with minimal desktop environments (e.g., Xfce or Openbox), focus on performance over visual features. As a result, they exclude support for non-critical features like window opacity.


### **Security Concerns**


Transparency can pose security risks, such as phishing attacks that use invisible windows to mimic parts of the screen. Some platforms disable transparency entirely to mitigate these risks.


When developing cross-platform applications, always use `SDL_SetWindowOpacity()` cautiously and include fallbacks for systems that don’t support transparency. Check the return value of the function or use `SDL_GetError()` for diagnostics.


</FAQ>


<FAQ>


```javascript
//<m>
{
  slug: 'animating-opacity-changes',
  title: 'Animating Window Opacity',
  question: 'Can I animate opacity changes to create fade effects?',
}
```


Yes, you can animate opacity changes to create fade effects, but SDL does not provide built-in support for animations. You’ll need to implement this yourself by incrementally updating the opacity over time. Here’s an example:


### **Example: Fading a Window**


Here’s an example where we gradually fade a window over 50 iterations:


```c++
#include <SDL.h>
#include <iostream>
#include <thread>

void FadeWindow(
  SDL_Window* window,
  float start,
  float end,
  int duration_ms
) {
  // Number of opacity increments
  const int steps = 50;
  const float increment = (end - start) / steps;
  const int delay = duration_ms / steps;

  for (int i = 0; i <= steps; ++i) {
    float opacity = start + (increment * i);
    SDL_SetWindowOpacity(window, opacity);
    std::this_thread::sleep_for(
      std::chrono::milliseconds(delay));
  }
}

int main() {
  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
    std::cerr << "SDL Init Error: "
      << SDL_GetError() << "\n";
    return -1;
  }

  SDL_Window* window = SDL_CreateWindow(
    "Fading Window",
    SDL_WINDOWPOS_CENTERED,
    SDL_WINDOWPOS_CENTERED,
    400, 200, 0
  );

  if (!window) {
    std::cerr << "Window Creation Error: "
      << SDL_GetError() << "\n";
    SDL_Quit();
    return -1;
  }
  
  // Fade out in 2 seconds
  FadeWindow(window, 1.0f, 0.0f, 2000);

  SDL_DestroyWindow(window);
  SDL_Quit();
  return 0;
}
```


```c++
//<o>
Window fades to transparent over 2 seconds.
```


Animations add polish to applications, but remember to test for performance issues on slower devices.


</FAQ>


<FAQ>


```javascript
//<m>
{
  slug: 'check-opacity-support',
  title: 'Checking Opacity Support',
  question: 'How do I check if a platform supports opacity before using `SDL_SetWindowOpacity()`?',
}
```


You can determine whether a platform supports opacity by testing the return value of `SDL_SetWindowOpacity()` or `SDL_GetWindowOpacity()`. If these functions fail, the platform or the specific window likely does not support opacity. Here’s an example:


### **Example: Checking Opacity Support**


Here’s an example where we test if the platform we’re running on supports opacity:


```c++
#include <SDL.h>
#include <iostream>

int main() {
  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
    std::cerr << "SDL Init Error: "
      << SDL_GetError() << "\n";
    return -1;
  }

  SDL_Window* window = SDL_CreateWindow(
    "Opacity Check",
    SDL_WINDOWPOS_CENTERED,
    SDL_WINDOWPOS_CENTERED,
    400, 200, 0
  );

  if (!window) {
    std::cerr << "Window Creation Error: "
      << SDL_GetError() << "\n";
    SDL_Quit();
    return -1;
  }

  if (SDL_SetWindowOpacity(window, 0.5f) < 0) {
    std::cerr << "Opacity Not Supported: "
      << SDL_GetError() << "\n";
  } else {
    std::cout << "Opacity Supported!\n";
  }

  SDL_DestroyWindow(window);
  SDL_Quit();
  return 0;
}
```


```c++
//<o>
Opacity Not Supported: That operation is not supported
```


Testing is essential for writing cross-platform code that gracefully handles unsupported features.


</FAQ>


<FAQ>


```javascript
//<m>
{
  slug: 'partial-window-transparency',
  title: 'Partial Transparency in Windows',
  question: 'How do I make part of a window transparent but not the whole window?',
}
```


To make part of a window transparent while leaving the rest opaque, you’ll need to use SDL’s blending features or a related rendering library. SDL itself doesn’t directly support partial window transparency in the same way as full window opacity. Instead, you’ll need to render your window’s contents and use an alpha channel for specific regions.


### **Using SDL Textures for Partial Transparency**


You can achieve this by drawing a texture with an alpha channel over the transparent region of your window. Here’s an example:


```c++
#include <SDL.h>
#include <iostream>

// Create a semi-transparent rectangle
// on the window
void DrawTransparentRect(
  SDL_Renderer* renderer
) {
  SDL_SetRenderDrawBlendMode(renderer,
    SDL_BLENDMODE_BLEND);
  SDL_SetRenderDrawColor(renderer,
    255, 0, 0, 128); // RGBA (50% transparency)
  SDL_Rect rect = {100, 100, 200, 150};
  SDL_RenderFillRect(renderer, &rect);
}

int main() {
  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
    std::cerr << "SDL Init Error: "
      << SDL_GetError() << "\n";
    return -1;
  }

  SDL_Window* window = SDL_CreateWindow(
    "Partial Transparency",
    SDL_WINDOWPOS_CENTERED,
    SDL_WINDOWPOS_CENTERED,
    400, 300,
    SDL_WINDOW_SHOWN
  );

  if (!window) {
    std::cerr << "Window Creation Error: "
      << SDL_GetError() << "\n";
    SDL_Quit();
    return -1;
  }

  SDL_Renderer* renderer =
    SDL_CreateRenderer(window, -1, 0);

  SDL_SetRenderDrawColor(renderer,
    0, 0, 0, 255); // Black background
  SDL_RenderClear(renderer);

  DrawTransparentRect(renderer);
  SDL_RenderPresent(renderer);

  SDL_Delay(3000);

  SDL_DestroyRenderer(renderer);
  SDL_DestroyWindow(window);
  SDL_Quit();
  return 0;
}
```


```c++
//<o>
A red semi-transparent rectangle appears over a black background.
```


This approach allows for fine-grained control over transparency but requires handling all rendering manually.


</FAQ>


<FAQ>


```javascript
//<m>
{
  slug: 'window-id-vs-pointer',
  title: 'Why Use `SDL_GetWindowFromID()`?',
  question: 'Why should I use `SDL_GetWindowFromID()` instead of passing the `SDL_Window*`?',
}
```


`SDL_GetWindowFromID()` is particularly useful in scenarios where you are managing multiple windows and only have the window ID, not the pointer. While passing a `SDL_Window*` directly is simpler, here’s why you might prefer using `SDL_GetWindowFromID()`:


### **Events Are Window ID-Based**


When handling SDL events, window-related events, like `SDL_WINDOWEVENT`, provide only the window ID (`event.window.windowID`). To interact with the corresponding window, you must use `SDL_GetWindowFromID()` to retrieve its pointer.


### **Safety**


Using window IDs ensures that you’re accessing the correct window, especially if windows are dynamically created and destroyed. A pointer might become invalid if the window it references is destroyed.


### **Example Usage**


Here’s an example where we get a window ID from an `SDL_Window*`, and an `SDL_Window*` from a window ID:


```c++
#include <SDL.h>
#include <iostream>

int main() {
  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
    std::cerr << "SDL Init Error: "
      << SDL_GetError() << "\n";
    return -1;
  }

  SDL_Window* window = SDL_CreateWindow(
    "Window Example",
    SDL_WINDOWPOS_CENTERED,
    SDL_WINDOWPOS_CENTERED,
    400, 300,
    SDL_WINDOW_SHOWN
  );

  if (!window) {
    std::cerr << "Window Creation Error: "
      << SDL_GetError() << "\n";
    SDL_Quit();
    return -1;
  }

  Uint32 windowID = SDL_GetWindowID(window);//<h>

  SDL_Window* retrieved =//<h>
    SDL_GetWindowFromID(windowID);//<h>
    
  if (retrieved == window) {
    std::cout << "Successfully retrieved "
      "the window from ID!\n";
  }

  SDL_DestroyWindow(window);
  SDL_Quit();
  return 0;
}
```


```c++
//<o>
Successfully retrieved the window from ID!
```


Using IDs is safer and integrates better with SDL’s event-driven architecture.


</FAQ>


<FAQ>


```javascript
//<m>
{
  slug: 'opacity-fullscreen-interaction',
  title: 'Opacity and Fullscreen Windows',
  question: 'How does opacity interact with fullscreen windows?',
}
```


When a window is in fullscreen mode, opacity handling depends on the platform and the graphics driver. Many platforms do not support transparency in true fullscreen mode because the window is rendered directly to the screen, bypassing the compositor responsible for blending transparent surfaces.


### **Behavior in Fullscreen Modes**

1. **True Fullscreen (****`SDL_WINDOW_FULLSCREEN`****)**: Transparency is typically ignored, and the window appears fully opaque. This is because the application takes over the entire screen, and there’s no compositor to blend with the background.
2. **Fullscreen Desktop (****`SDL_WINDOW_FULLSCREEN_DESKTOP`****)**: Transparency might work because the window is treated as a borderless window covering the screen, allowing the compositor to manage blending.

### **Practical Notes**

- If you need transparency in fullscreen, prefer `SDL_WINDOW_FULLSCREEN_DESKTOP`.
- Keep in mind that performance might differ between true fullscreen and fullscreen desktop, especially for games or graphics-intensive applications.

### **Example: Testing Opacity in Fullscreen**


Below, we attempt to set the transparency of a fullscreen window:


```c++
#include <SDL.h>
#include <iostream>

int main() {
  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
    std::cerr << "SDL Init Error: "
      << SDL_GetError() << "\n";
    return -1;
  }

  SDL_Window* window = SDL_CreateWindow(
    "Fullscreen Opacity Test",
    SDL_WINDOWPOS_CENTERED,
    SDL_WINDOWPOS_CENTERED,
    800, 600,
    SDL_WINDOW_SHOWN
  );

  // Set transparency
  SDL_SetWindowOpacity(window, 0.5f); //<h>

  SDL_SetWindowFullscreen(window, //<h>
    SDL_WINDOW_FULLSCREEN_DESKTOP); //<h>
  SDL_Delay(3000);

  SDL_DestroyWindow(window);
  SDL_Quit();
  return 0;
}
```


```c++
//<o>
In fullscreen desktop mode, transparency works if supported by the platform.
```


Test fullscreen opacity behavior on your target platform to avoid unexpected results.


</FAQ>



Discover how to use SDL2 functions for controlling and retrieving window transparency settings.

Use Cases for Transparent Windows

Why Some Platforms Lack Opacity

Animating Window Opacity

Checking Opacity Support

Partial Transparency in Windows

Why Use `SDL_GetWindowFromID()`?

Opacity and Fullscreen Windows

Expanded window management lesson to a full chapter

`SDL_CreateWindow()`

`SDL_SetWindowOpacity()`

`SDL_GetWindowOpacity()`

`SDL_GetError()`

UPDATED FOR C++23 | Everything you need to know about SDL2 transparency: opacity settings, error handling, and practical examples | Clear explanations and simple code examples

Why Some Platforms Lack Opacity

Why do some platforms not support window opacity?

Technical Constraints

Compatibility

Design Choices

Security Concerns

Window Opacity

What are some common use cases for transparent windows?

Can I animate opacity changes to create fade effects?

How do I check if a platform supports opacity before using `SDL_SetWindowOpacity()`?

How do I make part of a window transparent but not the whole window?

Why should I use `SDL_GetWindowFromID()` instead of passing the `SDL_Window*`?

How does opacity interact with fullscreen windows?

Window Opacity

Game Dev with SDL2

This course includes:

Professional C++

Why Some Platforms Lack Opacity

Why do some platforms not support window opacity?

Technical Constraints

Compatibility

Design Choices

Security Concerns

Window Opacity

What are some common use cases for transparent windows?

Can I animate opacity changes to create fade effects?

How do I check if a platform supports opacity before using SDL_SetWindowOpacity()?

How do I make part of a window transparent but not the whole window?

Why should I use SDL_GetWindowFromID() instead of passing the SDL_Window*?

How does opacity interact with fullscreen windows?

Window Opacity

Game Dev with SDL2

This course includes:

Professional C++

How do I check if a platform supports opacity before using `SDL_SetWindowOpacity()`?

Why should I use `SDL_GetWindowFromID()` instead of passing the `SDL_Window*`?