refactor(render): TextBackend owns its atlas, and staged glyphs carry a rotation basis

src/render/shaders.cpp

@@ -73,28 +73,38 @@ void main() {
 )";
 
 const char* const kTextVS = R"(#version 330 core
-layout(location = 0) in vec2 a_pos;  // per-instance glyph quad top-left, pixels
-layout(location = 1) in vec2 a_uv;   // per-instance glyph atlas texel origin
+layout(location = 0) in vec2 a_pos;   // per-instance glyph cell top-left, pixels
+layout(location = 1) in vec2 a_uv;    // per-instance glyph rect top-left, normalised
+layout(location = 2) in vec2 a_axis;  // per-instance cell +x axis, unit, pixels
 
 uniform vec2 u_viewport;  // render target size, pixels
-uniform vec2 u_px_range;  // pixels per glyph cell on screen
+uniform vec2 u_px_range;  // glyph cell on screen, pixels (width, height)
+uniform vec2 u_glyph_uv;  // glyph cell in the atlas, normalised (width, height)
 
-out vec2 v_texel;  // atlas texel coordinate of this fragment
-
-const vec2 kGlyphTexels = vec2(8.0);  // BitmapFont::kGlyphPx
+out vec2 v_uv;  // normalised atlas coordinate of this fragment
 
 void main() {
+  // The cell's two axes: the run direction the instance carries and its
+  // perpendicular, so the cell follows the basis instead of the screen axes. The
+  // perpendicular is the cell's own +v axis -- with an unrotated basis
+  // (1.0, 0.0) it is (0.0, 1.0), i.e. down the screen, which is the direction v
+  // grows in the atlas -- and building it this way keeps the quad's orientation:
+  // a mirrored perpendicular would draw every glyph flipped about its diagonal.
+  vec2 ux = a_axis;
+  vec2 uy = vec2(-ux.y, ux.x);
   vec2 corner = vec2(float(gl_VertexID & 1), float((gl_VertexID >> 1) & 1));
-  vec2 px = a_pos + corner * u_px_range;
+  vec2 px = a_pos + corner.x * u_px_range.x * ux + corner.y * u_px_range.y * uy;
   // Rect is top-left origin, GL is bottom-left: the one and only y flip.
   vec2 ndc = vec2(px.x / u_viewport.x * 2.0 - 1.0, 1.0 - px.y / u_viewport.y * 2.0);
   gl_Position = vec4(ndc, 0.0, 1.0);
-  v_texel = a_uv + corner * kGlyphTexels;
+  // The same handedness as the cell, so the glyph's rect lands on it the right
+  // way round: u runs along the basis, v down the glyph.
+  v_uv = a_uv + corner * u_glyph_uv;
 }
 )";
 
 const char* const kTextFS = R"(#version 330 core
-in vec2 v_texel;  // atlas texel coordinate of this fragment
+in vec2 v_uv;  // normalised atlas coordinate of this fragment
 
 uniform sampler2D u_atlas;  // GL_R8 glyph atlas, GL_NEAREST
 uniform vec2 u_tex_size;    // atlas size in texels
@@ -106,7 +116,7 @@ void main() {
   // Sample the centre of the texel the fragment lands in. GL_NEAREST alone
   // would tie at a texel boundary, where the driver may pick the neighbouring
   // glyph cell; rounding to a centre removes the tie by construction.
-  vec2 texel = clamp(floor(v_texel), vec2(0.0), u_tex_size - vec2(1.0));
+  vec2 texel = clamp(floor(v_uv * u_tex_size), vec2(0.0), u_tex_size - vec2(1.0));
   float coverage = texture(u_atlas, (texel + vec2(0.5)) / u_tex_size).r;
   o_color = vec4(u_color.rgb, u_color.a * coverage);
 }