buffer pointer offset support

Author: wkjarosz

include/shader.h

@@ -150,6 +150,12 @@ class Shader
     */
     void set_buffer_divisor(const std::string &name, size_t divisor);
 
+    /**
+        Set the pointer offset for call to glVertexAttribPointer. Useful in instance drawing to start drawing instances
+        from a certain index in an attribute buffer.
+    */
+    void set_buffer_pointer_offset(const std::string &name, size_t offset);
+
     // /**
     //  * \brief Associate a texture with a named shader parameter
     //  *
@@ -240,6 +246,7 @@ class Shader
         size_t       shape[3]{0, 0, 0};
         size_t       size             = 0;
         size_t       instance_divisor = 0;
+        size_t       pointer_offset   = 0;
         bool         dirty            = false;
 
         std::string to_string() const;

src/app.cpp

@@ -1248,7 +1248,8 @@ void SampleViewer::draw_2D_points_and_grid(const float4x4 &mvp, int2 dims, int p
     m_2d_point_shader->set_uniform("color", m_point_color);
     int2 range = get_draw_range();
 
-    m_2d_point_shader->set_buffer("center", m_2d_points, m_subset_count * plot_index + range.x, range.y);
+    m_2d_point_shader->set_buffer_pointer_offset("center",
+                                                 size_t(m_subset_count * plot_index + range.x) * sizeof(float3));
     m_2d_point_shader->set_buffer_divisor("center", 1); // one center per quad/instance
 
     if (range.y > 0)

src/shader.cpp

@@ -384,10 +384,22 @@ void Shader::set_buffer_divisor(const std::string &name, size_t divisor)
 {
     auto it = m_buffers.find(name);
     if (it == m_buffers.end())
-        throw std::runtime_error("Shader::set_buffer(): could not find argument named \"" + name + "\"");
+        throw std::runtime_error("Shader::set_buffer_divisor(): could not find argument named \"" + name + "\"");
 
     Buffer &buf          = m_buffers[name];
     buf.instance_divisor = divisor;
+    buf.dirty            = true;
+}
+
+void Shader::set_buffer_pointer_offset(const std::string &name, size_t offset)
+{
+    auto it = m_buffers.find(name);
+    if (it == m_buffers.end())
+        throw std::runtime_error("Shader::set_buffer_pointer_offset(): could not find argument named \"" + name + "\"");
+
+    Buffer &buf        = m_buffers[name];
+    buf.pointer_offset = offset;
+    buf.dirty          = true;
 }
 
 // void Shader::set_texture(const std::string &name, Texture *texture)
@@ -461,7 +473,8 @@ void Shader::begin()
                                          "\" has an invalid shapeension (expected ndim=2, got " +
                                          std::to_string(buf.ndim) + ")");
 
-            CHK(glVertexAttribPointer(buf.index, (GLint)buf.shape[1], gl_type, GL_FALSE, 0, nullptr));
+            CHK(glVertexAttribPointer(buf.index, (GLint)buf.shape[1], gl_type, GL_FALSE, 0,
+                                      (const void *)buf.pointer_offset));
             CHK(glVertexAttribDivisor(buf.index, (GLuint)buf.instance_divisor));
             break;