[rtextures] Created `ImageFromChannel()` (#4105)

* created ImageFromChannel

Adds the possibility to extract a specific channel from an image

* naming convention

* example window height

* removed threshold

* removed alpha channel

* channel example organization

* updated channel example image

2 files changed, 203 insertions, 0 deletions

diff --git a/src/raylib.h b/src/raylib.h
index c3c546c3..26cf03e8 100644
--- a/src/raylib.h
+++ b/src/raylib.h
@@ -1334,6 +1334,7 @@ RLAPI Image ImageCopy(Image image);
 RLAPI Image ImageFromImage(Image image, Rectangle rec);                                                  // Create an image from another image piece
 RLAPI Image ImageText(const char *text, int fontSize, Color color);                                      // Create an image from text (default font)
 RLAPI Image ImageTextEx(Font font, const char *text, float fontSize, float spacing, Color tint);         // Create an image from text (custom sprite font)
+RLAPI Image ImageFromChannel(Image image, int selectedChannel);                                          // Create an image from a selected channel of another image
 RLAPI void ImageFormat(Image *image, int newFormat);                                                     // Convert image data to desired format
 RLAPI void ImageToPOT(Image *image, Color fill);                                                         // Convert image to POT (power-of-two)
 RLAPI void ImageCrop(Image *image, Rectangle crop);                                                      // Crop an image to a defined rectangle
diff --git a/src/rtextures.c b/src/rtextures.c
index e914db41..9d547cd0 100644
--- a/src/rtextures.c
+++ b/src/rtextures.c
@@ -1630,6 +1630,208 @@ Image ImageTextEx(Font font, const char *text, float fontSize, float spacing, Co
     return imText;
 }
 
+// Create an image from a selected channel of another image
+Image ImageFromChannel(Image image, int selectedChannel)
+{
+    Image result = { 0 };
+
+    // Security check to avoid program crash
+    if ((image.data == NULL) || (image.width == 0) || (image.height == 0))
+        return result;
+
+    // Check selected channel
+    if (selectedChannel < 0)
+    {
+        TRACELOG(LOG_WARNING, "Channel cannot be negative. Setting channel to 0.");
+        selectedChannel = 0;
+    }
+    if (image.format == PIXELFORMAT_UNCOMPRESSED_GRAYSCALE
+            || image.format == PIXELFORMAT_UNCOMPRESSED_R32
+            || image.format == PIXELFORMAT_UNCOMPRESSED_R16
+            )
+    {
+        if (selectedChannel > 0)
+        {
+            TRACELOG(LOG_WARNING, "This image has only 1 channel. Setting channel to it.");
+            selectedChannel = 0;
+        }
+    }
+    else if (image.format == PIXELFORMAT_UNCOMPRESSED_GRAY_ALPHA)
+    {
+        if (selectedChannel > 1)
+        {
+            TRACELOG(LOG_WARNING, "This image has only 2 channels. Setting channel to alpha.");
+            selectedChannel = 1;
+        }
+    }
+    else if (image.format == PIXELFORMAT_UNCOMPRESSED_R5G6B5
+            || image.format == PIXELFORMAT_UNCOMPRESSED_R8G8B8
+            || image.format == PIXELFORMAT_UNCOMPRESSED_R32G32B32
+            || image.format == PIXELFORMAT_UNCOMPRESSED_R16G16B16
+            )
+    {
+        if (selectedChannel > 2)
+        {
+            TRACELOG(LOG_WARNING, "This image has only 3 channels. Setting channel to red.");
+            selectedChannel = 0;
+        }
+    }
+
+    // formats rgba
+    if (selectedChannel > 3)
+    {
+        TRACELOG(LOG_WARNING, "ImageFromChannel supports channels 0 to 3 (rgba). Setting channel to alpha.");
+        selectedChannel = 3;
+    }
+
+    result.format = PIXELFORMAT_UNCOMPRESSED_GRAYSCALE;
+    result.height = image.height;
+    result.width = image.width;
+    result.mipmaps = 1;
+
+    unsigned char *pixels = (unsigned char *)RL_CALLOC(image.width * image.height, sizeof(unsigned char)); // values 0 to 255
+
+    if (image.format >= PIXELFORMAT_COMPRESSED_DXT1_RGB) TRACELOG(LOG_WARNING, "IMAGE: Pixel data retrieval not supported for compressed image formats");
+    else
+    {
+        for (int i = 0, k = 0; i < image.width * image.height; ++i)
+        {
+            float imageValue = -1;
+            switch (image.format)
+            {
+                case PIXELFORMAT_UNCOMPRESSED_GRAYSCALE:
+                {
+                    imageValue = (float)((unsigned char *)image.data)[i + selectedChannel]/255.0f;
+
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_GRAY_ALPHA:
+                {
+                    imageValue = (float)((unsigned char *)image.data)[k + selectedChannel]/255.0f;
+
+                    k += 2;
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R5G5B5A1:
+                {
+                    unsigned short pixel = ((unsigned short *)image.data)[i];
+
+                    if (selectedChannel == 0)
+                    {
+                        imageValue = (float)((pixel & 0b1111100000000000) >> 11)*(1.0f/31);
+                    }
+                    else if (selectedChannel == 1)
+                    {
+                        imageValue = (float)((pixel & 0b0000011111000000) >> 6)*(1.0f/31);
+                    }
+                    else if (selectedChannel == 2)
+                    {
+                        imageValue = (float)((pixel & 0b0000000000111110) >> 1)*(1.0f/31);
+                    }
+                    else if (selectedChannel == 3)
+                    {
+                        imageValue = ((pixel & 0b0000000000000001) == 0)? 0.0f : 1.0f;
+                    }
+
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R5G6B5:
+                {
+                    unsigned short pixel = ((unsigned short *)image.data)[i];
+
+                    if (selectedChannel == 0)
+                    {
+                        imageValue = (float)((pixel & 0b1111100000000000) >> 11)*(1.0f/31);
+                    }
+                    else if (selectedChannel == 1)
+                    {
+                        imageValue = (float)((pixel & 0b0000011111100000) >> 5)*(1.0f/63);
+                    }
+                    else if (selectedChannel == 2)
+                    {
+                        imageValue = (float)(pixel & 0b0000000000011111)*(1.0f/31);
+                    }
+
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R4G4B4A4:
+                {
+                    unsigned short pixel = ((unsigned short *)image.data)[i];
+
+                    if (selectedChannel == 0)
+                    {
+                        imageValue = (float)((pixel & 0b1111000000000000) >> 12)*(1.0f/15);
+                    }
+                    else if (selectedChannel == 1)
+                    {
+                        imageValue = (float)((pixel & 0b0000111100000000) >> 8)*(1.0f/15);
+                    }
+                    else if (selectedChannel == 2)
+                    {
+                        imageValue = (float)((pixel & 0b0000000011110000) >> 4)*(1.0f/15);
+                    }
+                    else if (selectedChannel == 3)
+                    {
+                        imageValue = (float)(pixel & 0b0000000000001111)*(1.0f/15);
+                    }
+
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R8G8B8A8:
+                {
+                    imageValue = (float)((unsigned char *)image.data)[k + selectedChannel]/255.0f;
+
+                    k += 4;
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R8G8B8:
+                {
+                    imageValue = (float)((unsigned char *)image.data)[k + selectedChannel]/255.0f;
+
+                    k += 3;
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R32:
+                {
+                    imageValue = ((float *)image.data)[k];
+
+                    k += 1;
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R32G32B32:
+                {
+                    imageValue = ((float *)image.data)[k + selectedChannel];
+
+                    k += 3;
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R32G32B32A32:
+                {
+                    imageValue = ((float *)image.data)[k + selectedChannel];
+
+                    k += 4;
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R16:
+                {
+                    imageValue = HalfToFloat(((unsigned short *)image.data)[k]);
+
+                    k += 1;
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R16G16B16:
+                {
+                    imageValue = HalfToFloat(((unsigned short *)image.data)[k+selectedChannel]);
+
+                    k += 3;
+                } break;
+                case PIXELFORMAT_UNCOMPRESSED_R16G16B16A16:
+                {
+                    imageValue = HalfToFloat(((unsigned short *)image.data)[k + selectedChannel]);
+
+                    k += 4;
+                } break;
+                default: break;
+            }
+
+            pixels[i] = imageValue * 255;
+        }
+    }
+
+    result.data = pixels;
+
+    return result;
+}
+
 // Resize and image to new size using Nearest-Neighbor scaling algorithm
 void ImageResizeNN(Image *image,int newWidth,int newHeight)
 {