Enable depth buffer, add depth to shader

shaders/cell.v.glsl

@@ -35,6 +35,17 @@ layout (location = 5) in vec4 bg_color_in;
 // the entire terminal grid in a single GPU pass.
 layout (location = 6) in uint mode_in;
 
+// The Z-depth. This is between 0.0 and 1.0. This is used to paint newer
+// values on top of older ones to limit the amount of data that needs to
+// be sent to the GPU.
+//
+// 0 is further back/away, 1 is front/near
+//
+// TODO: It would be better to just send the Z value as part of grid_coord
+// and normalize it to NDC and then linearly transform it but this was easy
+// to get going more quickly.
+layout (location = 7) in float grid_z;
+
 // The background or foreground color for the fragment, depending on
 // whether this is a background or foreground pass.
 flat out vec4 color;
@@ -82,6 +93,11 @@ void main() {
     // Example: (1,0) with a 30 wide cell is converted to (30,0)
     vec2 cell_pos = cell_size * grid_coord;
 
+    // Our Z value. For now we just use grid_z directly but we pull it
+    // out here so the variable name is more uniform to our cell_pos and
+    // in case we want to do any other math later.
+    float cell_z = grid_z;
+
     // Turn the cell position into a vertex point depending on the
     // gl_VertexID. Since we use instanced drawing, we have 4 vertices
     // for each corner of the cell. We can use gl_VertexID to determine
@@ -103,7 +119,7 @@ void main() {
         // one cell up and to the left. (Do the math to verify yourself)
         cell_pos = cell_pos + cell_size * position;
 
-        gl_Position = projection * vec4(cell_pos, 0.0, 1.0);
+        gl_Position = projection * vec4(cell_pos, cell_z, 1.0);
         color = bg_color_in / 255.0;
         break;
 
@@ -117,7 +133,7 @@ void main() {
 
         // Calculate the final position of the cell.
         cell_pos = cell_pos + glyph_size * position + glyph_offset_calc;
-        gl_Position = projection * vec4(cell_pos, 0.0, 1.0);
+        gl_Position = projection * vec4(cell_pos, cell_z, 1.0);
 
         // We need to convert our texture position and size to normalized
         // device coordinates (0 to 1.0) by dividing by the size of the texture.
@@ -134,7 +150,7 @@ void main() {
         // Same as background since we're taking up the whole cell.
         cell_pos = cell_pos + cell_size * position;
 
-        gl_Position = projection * vec4(cell_pos, 0.0, 1.0);
+        gl_Position = projection * vec4(cell_pos, cell_z, 1.0);
         color = bg_color_in / 255.0;
         break;
 
@@ -145,7 +161,7 @@ void main() {
         // Same as background since we're taking up the whole cell.
         cell_pos = cell_pos + cell_size * position;
 
-        gl_Position = projection * vec4(cell_pos, 0.0, 1.0);
+        gl_Position = projection * vec4(cell_pos, cell_z, 1.0);
         color = bg_color_in / 255.0;
         break;
 
@@ -156,7 +172,7 @@ void main() {
         // Same as background since we're taking up the whole cell.
         cell_pos = cell_pos + bar_size * position;
 
-        gl_Position = projection * vec4(cell_pos, 0.0, 1.0);
+        gl_Position = projection * vec4(cell_pos, cell_z, 1.0);
         color = bg_color_in / 255.0;
         break;
 
@@ -174,7 +190,7 @@ void main() {
         // above the bottom.
         cell_pos = cell_pos + underline_offset - underline_size * position;
 
-        gl_Position = projection * vec4(cell_pos, 0.0, 1.0);
+        gl_Position = projection * vec4(cell_pos, cell_z, 1.0);
         color = fg_color_in / 255.0;
         break;
     }

src/Grid.zig

@@ -100,6 +100,10 @@ const GPUCell = struct {
 
     /// uint mode
     mode: u8,
+
+    /// float grid_z. This is normalized to a float by dividing by the
+    /// max int value.
+    grid_z: u16,
 };
 
 pub fn init(alloc: Allocator, config: *const Config) !Grid {
@@ -198,6 +202,8 @@ pub fn init(alloc: Allocator, config: *const Config) !Grid {
     try vbobind.attributeAdvanced(5, 4, gl.c.GL_UNSIGNED_BYTE, false, @sizeOf(GPUCell), offset);
     offset += 4 * @sizeOf(u8);
     try vbobind.attributeIAdvanced(6, 1, gl.c.GL_UNSIGNED_BYTE, @sizeOf(GPUCell), offset);
+    offset += 1 * @sizeOf(u8);
+    try vbobind.attributeAdvanced(7, 1, gl.c.GL_UNSIGNED_SHORT, true, @sizeOf(GPUCell), offset);
     try vbobind.enableAttribArray(0);
     try vbobind.enableAttribArray(1);
     try vbobind.enableAttribArray(2);
@@ -205,6 +211,7 @@ pub fn init(alloc: Allocator, config: *const Config) !Grid {
     try vbobind.enableAttribArray(4);
     try vbobind.enableAttribArray(5);
     try vbobind.enableAttribArray(6);
+    try vbobind.enableAttribArray(7);
     try vbobind.attributeDivisor(0, 1);
     try vbobind.attributeDivisor(1, 1);
     try vbobind.attributeDivisor(2, 1);
@@ -212,6 +219,7 @@ pub fn init(alloc: Allocator, config: *const Config) !Grid {
     try vbobind.attributeDivisor(4, 1);
     try vbobind.attributeDivisor(5, 1);
     try vbobind.attributeDivisor(6, 1);
+    try vbobind.attributeDivisor(7, 1);
 
     // Build our texture
     const tex = try gl.Texture.create();
@@ -342,6 +350,7 @@ pub fn updateCells(self: *Grid, term: Terminal) !void {
                     .mode = 1,
                     .grid_col = @intCast(u16, x),
                     .grid_row = @intCast(u16, y),
+                    .grid_z = 0,
                     .glyph_x = 0,
                     .glyph_y = 0,
                     .glyph_width = 0,
@@ -380,6 +389,7 @@ pub fn updateCells(self: *Grid, term: Terminal) !void {
                     .mode = 2,
                     .grid_col = @intCast(u16, x),
                     .grid_row = @intCast(u16, y),
+                    .grid_z = 1,
                     .glyph_x = glyph.atlas_x,
                     .glyph_y = glyph.atlas_y,
                     .glyph_width = glyph.width,
@@ -402,6 +412,7 @@ pub fn updateCells(self: *Grid, term: Terminal) !void {
                     .mode = 6, // underline
                     .grid_col = @intCast(u16, x),
                     .grid_row = @intCast(u16, y),
+                    .grid_z = 1,
                     .glyph_x = 0,
                     .glyph_y = 0,
                     .glyph_width = 0,
@@ -435,6 +446,9 @@ pub fn updateCells(self: *Grid, term: Terminal) !void {
             .bg_g = 0xFF,
             .bg_b = 0xFF,
             .bg_a = 255,
+
+            // The cursor is always at the very front
+            .grid_z = 255,
         });
     }
 }

src/Window.zig

@@ -200,6 +200,14 @@ pub fn create(alloc: Allocator, loop: libuv.Loop, config: *const Config) !*Windo
     gl.c.glEnable(gl.c.GL_BLEND);
     gl.c.glBlendFunc(gl.c.GL_SRC_ALPHA, gl.c.GL_ONE_MINUS_SRC_ALPHA);
 
+    // Depth test since we use the Z-buffer as an optimization to send
+    // only changed cells to the GPU. As cells are updated, we bump their
+    // Z value to be closer, so that over time new values are rendered "over"
+    // values (in reality old values are culled by the depth test, so they
+    // aren't ever "rendered").
+    gl.c.glEnable(gl.c.GL_DEPTH_TEST);
+    gl.c.glDepthFunc(gl.c.GL_LESS);
+
     // Create our terminal grid with the initial window size
     const window_size = try window.getSize();
     var grid = try Grid.init(alloc, config);
@@ -960,7 +968,7 @@ fn renderTimerCallback(t: *libuv.Timer) void {
         .a = win.bg_a,
     };
     gl.clearColor(gl_bg.r, gl_bg.g, gl_bg.b, gl_bg.a);
-    gl.clear(gl.c.GL_COLOR_BUFFER_BIT);
+    gl.clear(gl.c.GL_COLOR_BUFFER_BIT | gl.c.GL_DEPTH_BUFFER_BIT);
 
     // Update the cells for drawing
     win.grid.updateCells(win.terminal) catch |err|