From 00fa82937ac5dbaa5b5bac04b2e84fa38c59e6c2 Mon Sep 17 00:00:00 2001
From: Philipp Oppermann <dev@phil-opp.com>
Date: Fri, 29 Dec 2023 17:11:46 +0100
Subject: [PATCH] Write blue rectangle to the screen

---
 kernel/src/framebuffer.rs | 49 +++++++++++++++++++++++++++++++++++++++
 kernel/src/main.rs        | 17 ++++++++++++--
 2 files changed, 64 insertions(+), 2 deletions(-)
 create mode 100644 kernel/src/framebuffer.rs

diff --git a/kernel/src/framebuffer.rs b/kernel/src/framebuffer.rs
new file mode 100644
index 00000000..0c82e1b9
--- /dev/null
+++ b/kernel/src/framebuffer.rs
@@ -0,0 +1,49 @@
+use bootloader_api::info::{FrameBuffer, PixelFormat};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Position {
+    pub x: usize,
+    pub y: usize,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Color {
+    pub red: u8,
+    pub green: u8,
+    pub blue: u8,
+}
+
+pub fn set_pixel_in(framebuffer: &mut FrameBuffer, position: Position, color: Color) {
+    let info = framebuffer.info();
+
+    // calculate offset to first byte of pixel
+    let byte_offset = {
+        // use stride to calculate pixel offset of target line
+        let line_offset = position.y * info.stride;
+        // add x position to get the absolute pixel offset in buffer
+        let pixel_offset = line_offset + position.x;
+        // convert to byte offset
+        pixel_offset * info.bytes_per_pixel
+    };
+
+    // set pixel based on color format
+    let pixel_buffer = &mut framebuffer.buffer_mut()[byte_offset..];
+    match info.pixel_format {
+        PixelFormat::Rgb => {
+            pixel_buffer[0] = color.red;
+            pixel_buffer[1] = color.green;
+            pixel_buffer[2] = color.blue;
+        }
+        PixelFormat::Bgr => {
+            pixel_buffer[0] = color.blue;
+            pixel_buffer[1] = color.green;
+            pixel_buffer[2] = color.red;
+        }
+        PixelFormat::U8 => {
+            // use a simple average-based grayscale transform
+            let gray = color.red / 3 + color.green / 3 + color.blue / 3;
+            pixel_buffer[0] = gray;
+        }
+        other => panic!("unknown pixel format {other:?}"),
+    }
+}
diff --git a/kernel/src/main.rs b/kernel/src/main.rs
index 3904431e..d616a8dc 100644
--- a/kernel/src/main.rs
+++ b/kernel/src/main.rs
@@ -5,12 +5,25 @@ use core::panic::PanicInfo;
 
 use bootloader_api::BootInfo;
 
+mod framebuffer;
+
 bootloader_api::entry_point!(kernel_main);
 
 fn kernel_main(boot_info: &'static mut BootInfo) -> ! {
     if let Some(framebuffer) = boot_info.framebuffer.as_mut() {
-        for byte in framebuffer.buffer_mut() {
-            *byte = 0x90;
+        let color = framebuffer::Color {
+            red: 0,
+            green: 0,
+            blue: 255,
+        };
+        for x in 0..100 {
+            for y in 0..100 {
+                let position = framebuffer::Position {
+                    x: 20 + x,
+                    y: 100 + y,
+                };
+                framebuffer::set_pixel_in(framebuffer, position, color);
+            }
         }
     }
     loop {}