[example] Writing into the depth buffer (#2836)

* Add a depth buffer example.

* Fixed a typo

4 files changed, 179 insertions, 1 deletions

diff --git a/examples/Makefile b/examples/Makefile
index 76a33149..7beff293 100644
--- a/examples/Makefile
+++ b/examples/Makefile
@@ -520,7 +520,8 @@ SHADERS = \
     shaders/shaders_spotlight \
     shaders/shaders_hot_reloading \
     shaders/shaders_mesh_instancing \
-    shaders/shaders_multi_sample2d
+    shaders/shaders_multi_sample2d \
+    shaders/shaders_write_depth
 
 AUDIO = \
     audio/audio_module_playing \
diff --git a/examples/shaders/resources/shaders/glsl100/write_depth.fs b/examples/shaders/resources/shaders/glsl100/write_depth.fs
new file mode 100644
index 00000000..5167efb8
--- /dev/null
+++ b/examples/shaders/resources/shaders/glsl100/write_depth.fs
@@ -0,0 +1,15 @@
+#version 100             
+#extension GL_EXT_frag_depth : enable          
+precision mediump float;                // Precision required for OpenGL ES2 (WebGL)
+
+varying vec2 fragTexCoord;
+varying vec4 fragColor;
+
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+void main()
+{
+    vec4 texelColor = texture2D(texture0, fragTexCoord);
+    gl_FragColor = texelColor*colDiffuse*fragColor;
+	gl_FragDepthEXT = 1.0 - gl_FragCoord.z;
+}
\ No newline at end of file
diff --git a/examples/shaders/resources/shaders/glsl330/write_depth.fs b/examples/shaders/resources/shaders/glsl330/write_depth.fs
new file mode 100644
index 00000000..36c70fe4
--- /dev/null
+++ b/examples/shaders/resources/shaders/glsl330/write_depth.fs
@@ -0,0 +1,13 @@
+#version 330
+
+in vec2 fragTexCoord;
+in vec4 fragColor;
+
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+void main()
+{
+    vec4 texelColor = texture2D(texture0, fragTexCoord);
+    gl_FragColor = texelColor*colDiffuse*fragColor;
+	gl_FragDepth = 1.0 - gl_FragCoord.z;
+}
\ No newline at end of file
diff --git a/examples/shaders/shaders_write_depth.c b/examples/shaders/shaders_write_depth.c
new file mode 100644
index 00000000..28c605c7
--- /dev/null
+++ b/examples/shaders/shaders_write_depth.c
@@ -0,0 +1,149 @@
+/*******************************************************************************************
+*
+*   raylib [core] example - Basic window
+*
+*   Example originally created with raylib 4.2, last time updated with raylib 4.2
+*
+*   Example contributed by Buğra Alptekin Sarı (@BugraAlptekinSari) and reviewed by Ramon Santamaria (@raysan5)
+*
+*   Example licensed under an unmodified zlib/libpng license, which is an OSI-certified,
+*   BSD-like license that allows static linking with closed source software
+*
+*   Copyright (c) 2022 Buğra Alptekin Sarı (@BugraAlptekinSari)
+*
+********************************************************************************************/
+
+#include "raylib.h"
+#include "rlgl.h"
+
+#if defined(PLATFORM_DESKTOP)
+#define GLSL_VERSION            330
+#else   // PLATFORM_RPI, PLATFORM_ANDROID, PLATFORM_WEB
+#define GLSL_VERSION            100
+#endif
+//------------------------------------------------------------------------------------
+// Customized render texture function to create a writable render buffer
+RenderTexture2D LoadRenderTextureMOD(int width, int height)
+{
+    RenderTexture2D target = { 0 };
+
+    target.id = rlLoadFramebuffer(width, height);   // Load an empty framebuffer
+
+    if (target.id > 0)
+    {
+        rlEnableFramebuffer(target.id);
+
+        // Create color texture (default to RGBA)
+        target.texture.id = rlLoadTexture(0, width, height, PIXELFORMAT_UNCOMPRESSED_R8G8B8A8, 1);
+        target.texture.width = width;
+        target.texture.height = height;
+        target.texture.format = PIXELFORMAT_UNCOMPRESSED_R8G8B8A8;
+        target.texture.mipmaps = 1;
+
+        // Create depth rendertexture
+        target.depth.id = rlLoadTextureDepth(width, height, false);
+        target.depth.width = width;
+        target.depth.height = height;
+        target.depth.format = 19;       //DEPTH_COMPONENT_24BIT?
+        target.depth.mipmaps = 1;
+
+        // Attach color texture and depth renderbuffer/texture to FBO
+        rlFramebufferAttach(target.id, target.texture.id, RL_ATTACHMENT_COLOR_CHANNEL0, RL_ATTACHMENT_TEXTURE2D, 0);
+        rlFramebufferAttach(target.id, target.depth.id, RL_ATTACHMENT_DEPTH, RL_ATTACHMENT_TEXTURE2D, 0);
+
+        // Check if fbo is complete with attachments (valid)
+        if (rlFramebufferComplete(target.id)) TRACELOG(LOG_INFO, "FBO: [ID %i] Framebuffer object created successfully", target.id);
+
+        rlDisableFramebuffer();
+    }
+    else TRACELOG(LOG_WARNING, "FBO: Framebuffer object can not be created");
+
+    return target;
+}
+
+// Unload render texture from GPU memory (VRAM)
+void UnloadRenderTextureMOD(RenderTexture2D target)
+{
+    if (target.id > 0)
+    {
+        // Color texture attached to FBO is deleted
+        rlUnloadTexture(target.texture.id);
+        rlUnloadTexture(target.depth.id);
+
+        // NOTE: Depth texture/renderbuffer is automatically
+        // queried and deleted before deleting framebuffer
+        rlUnloadFramebuffer(target.id);
+    }
+}
+//------------------------------------------------------------------------------------
+// Program main entry point
+//------------------------------------------------------------------------------------
+int main(void)
+{
+    // Initialization
+    //--------------------------------------------------------------------------------------
+    const int screenWidth = 800;
+    const int screenHeight = 450;
+
+    InitWindow(screenWidth, screenHeight, "raylib [core] example - basic window");
+
+    // The shader inverst the depth buffer by writing into it by `gl_FragDepth = 1 - gl_FragCoord.z;`
+    Shader shader = LoadShader(0, TextFormat("resources/shaders/glsl%i/write_depth.fs", GLSL_VERSION));
+
+    //Use Customized function to create writable depth buffer
+    RenderTexture2D target = LoadRenderTextureMOD(screenWidth, screenHeight);
+
+    Camera camera = {    // Define the camera to look into our 3d world
+        .position = (Vector3){ 2.0f, 2.0f, 3.0f },    // Camera position
+        .target = (Vector3){ 0.0f, 0.5f, 0.0f },      // Camera looking at point
+        .up = (Vector3){ 0.0f, 1.0f, 0.0f },          // Camera up vector (rotation towards target)
+        .fovy = 45.0f,                                // Camera field-of-view Y
+        .projection = CAMERA_PERSPECTIVE             // Camera mode type
+    };
+    SetCameraMode(camera, CAMERA_ORBITAL);
+    SetTargetFPS(60);
+    //--------------------------------------------------------------------------------------
+
+    // Main game loop
+    while (!WindowShouldClose())    // Detect window close button or ESC key
+    {
+        // Update
+        //----------------------------------------------------------------------------------
+        UpdateCamera(&camera);
+        // Draw
+        //----------------------------------------------------------------------------------
+        // Draw FBO
+        BeginTextureMode(target);
+        ClearBackground(WHITE);
+        BeginMode3D(camera);
+        BeginShaderMode(shader);
+        DrawCubeWiresV((Vector3) { 0.0f, 0.5f, 1.0f }, (Vector3) { 1.0f, 1.0f, 1.0f }, RED);
+        DrawCubeV((Vector3) { 0.0f, 0.5f, 1.0f }, (Vector3) { 1.0f, 1.0f, 1.0f }, PURPLE);
+        DrawCubeWiresV((Vector3) { 0.0f, 0.5f, -1.0f }, (Vector3) { 1.0f, 1.0f, 1.0f }, DARKGREEN);
+        DrawCubeV((Vector3) { 0.0f, 0.5f, -1.0f }, (Vector3) { 1.0f, 1.0f, 1.0f }, YELLOW);
+        DrawGrid(10, 1.0f);
+        EndShaderMode();
+        EndMode3D();
+        EndTextureMode();
+
+        // Draw Screen
+        BeginDrawing();
+
+        ClearBackground(RAYWHITE);
+        DrawTextureRec(target.texture, (Rectangle) { 0, 0, screenWidth, -screenHeight }, (Vector2) { 0, 0 }, WHITE);
+        DrawFPS(0, 0);
+
+        EndDrawing();
+        //----------------------------------------------------------------------------------
+    }
+
+    // De-Initialization
+    //--------------------------------------------------------------------------------------
+    UnloadRenderTextureMOD(target);
+    UnloadShader(shader);
+
+    CloseWindow();        // Close window and OpenGL context
+    //--------------------------------------------------------------------------------------
+
+    return 0;
+}