From f13ff0ad6a7581cd99fb00797f4492de3525ace4 Mon Sep 17 00:00:00 2001
From: HampusM <hampus@hampusmat.com>
Date: Sun, 29 Mar 2026 21:12:30 +0200
Subject: feat(opengl-bindings): add VertexArray::draw_elements vertex offset
 param

---
 opengl-bindings/Cargo.toml          |  2 +-
 opengl-bindings/src/vertex_array.rs | 99 ++++++++++++++++++++++++-------------
 2 files changed, 67 insertions(+), 34 deletions(-)

diff --git a/opengl-bindings/Cargo.toml b/opengl-bindings/Cargo.toml
index b21cd74..a2f7ade 100644
--- a/opengl-bindings/Cargo.toml
+++ b/opengl-bindings/Cargo.toml
@@ -22,7 +22,7 @@ gl_commands = [
 	"NamedBufferSubData",
 	"CreateVertexArrays",
 	"DrawArrays",
-	"DrawElements",
+	"DrawElementsBaseVertex",
 	"VertexArrayElementBuffer",
 	"VertexArrayVertexBuffer",
 	"EnableVertexArrayAttrib",
diff --git a/opengl-bindings/src/vertex_array.rs b/opengl-bindings/src/vertex_array.rs
index 3dc5cb6..5de59d8 100644
--- a/opengl-bindings/src/vertex_array.rs
+++ b/opengl-bindings/src/vertex_array.rs
@@ -29,33 +29,37 @@ impl VertexArray
     ///
     /// # Errors
     /// Returns `Err` if:
-    /// - `start_index` is too large
-    /// - `cnt` is too large
+    /// - `vertex_offset` is too large
+    /// - `vertex_cnt` is too large
     pub fn draw_arrays(
         current_context: &CurrentContextWithFns<'_>,
         primitive_kind: PrimitiveKind,
-        start_index: u32,
-        cnt: u32,
+        vertex_offset: u32,
+        vertex_cnt: u32,
     ) -> Result<(), DrawError>
     {
-        let start_index: crate::sys::types::GLint =
-            start_index
-                .try_into()
-                .map_err(|_| DrawError::StartIndexValueTooLarge {
-                    value: start_index,
+        let vertex_offset: crate::sys::types::GLint =
+            vertex_offset.try_into().map_err(|_| {
+                DrawError::VertexOffsetValueTooLarge {
+                    value: vertex_offset,
                     max_value: crate::sys::types::GLint::MAX as u32,
-                })?;
-
-        let cnt: crate::sys::types::GLsizei =
-            cnt.try_into().map_err(|_| DrawError::CountValueTooLarge {
-                value: cnt,
-                max_value: crate::sys::types::GLsizei::MAX as u32,
+                }
             })?;
 
+        let vertex_cnt: crate::sys::types::GLsizei =
+            vertex_cnt
+                .try_into()
+                .map_err(|_| DrawError::VertexCountValueTooLarge {
+                    value: vertex_cnt,
+                    max_value: crate::sys::types::GLsizei::MAX as u32,
+                })?;
+
         unsafe {
-            current_context
-                .fns()
-                .DrawArrays(primitive_kind.into_gl(), start_index, cnt);
+            current_context.fns().DrawArrays(
+                primitive_kind.into_gl(),
+                vertex_offset,
+                vertex_cnt,
+            );
         }
 
         Ok(())
@@ -67,26 +71,40 @@ impl VertexArray
     /// Returns `Err` if `cnt` is too large.
     pub fn draw_elements(
         current_context: &CurrentContextWithFns<'_>,
-        primitive_kind: PrimitiveKind,
-        start_index: u32,
-        cnt: u32,
+        DrawElementsOptions {
+            primitive_kind,
+            element_offset,
+            element_cnt,
+            vertex_offset,
+        }: DrawElementsOptions,
     ) -> Result<(), DrawError>
     {
-        let cnt: crate::sys::types::GLsizei =
-            cnt.try_into().map_err(|_| DrawError::CountValueTooLarge {
-                value: cnt,
-                max_value: crate::sys::types::GLsizei::MAX as u32,
+        let element_cnt: crate::sys::types::GLsizei =
+            element_cnt
+                .try_into()
+                .map_err(|_| DrawError::ElementCountValueTooLarge {
+                    value: element_cnt,
+                    max_value: crate::sys::types::GLsizei::MAX as u32,
+                })?;
+
+        let vertex_offset: crate::sys::types::GLint =
+            vertex_offset.try_into().map_err(|_| {
+                DrawError::VertexOffsetValueTooLarge {
+                    value: vertex_offset,
+                    max_value: crate::sys::types::GLint::MAX as u32,
+                }
             })?;
 
         unsafe {
-            current_context.fns().DrawElements(
+            current_context.fns().DrawElementsBaseVertex(
                 primitive_kind.into_gl(),
-                cnt,
+                element_cnt,
                 crate::sys::UNSIGNED_INT,
                 // TODO: Make this not sometimes UB. DrawElements expects a actual
                 // pointer to a memory location when no VBO is bound.
                 // See: https://stackoverflow.com/q/21706113
-                std::ptr::without_provenance::<c_void>(start_index as usize),
+                std::ptr::without_provenance::<c_void>(element_offset as usize),
+                vertex_offset,
             );
         }
 
@@ -212,7 +230,7 @@ impl VertexArray
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy)]
 pub enum PrimitiveKind
 {
     Triangles,
@@ -261,17 +279,32 @@ pub struct VertexBufferSpec
     pub vertex_size: usize,
 }
 
+#[derive(Debug, Clone)]
+pub struct DrawElementsOptions
+{
+    pub primitive_kind: PrimitiveKind,
+    pub element_offset: u32,
+    pub element_cnt: u32,
+    pub vertex_offset: u32,
+}
+
 #[derive(Debug, thiserror::Error)]
 pub enum DrawError
 {
-    #[error("Start index value {value} is too large. Must be < {max_value}")]
-    StartIndexValueTooLarge
+    #[error("Vertex offset value {value} is too large. Must be < {max_value}")]
+    VertexOffsetValueTooLarge
+    {
+        value: u32, max_value: u32
+    },
+
+    #[error("Vertex count value {value} is too large. Must be < {max_value}")]
+    VertexCountValueTooLarge
     {
         value: u32, max_value: u32
     },
 
-    #[error("Count value {value} is too large. Must be < {max_value}")]
-    CountValueTooLarge
+    #[error("Element count value {value} is too large. Must be < {max_value}")]
+    ElementCountValueTooLarge
     {
         value: u32, max_value: u32
     },
-- 
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