ghostty/src/renderer/shaders/cell.metal

using namespace metal;

// The possible modes that a shader can take.
enum Mode : uint8_t {
    MODE_BG = 1u,
    MODE_FG = 2u,
    MODE_FG_COLOR = 7u,
    MODE_STRIKETHROUGH = 8u,
};

struct Uniforms {
  float4x4 projection_matrix;
  float2 cell_size;
  float strikethrough_position;
  float strikethrough_thickness;
  float min_contrast;
};

struct VertexIn {
  // The mode for this cell.
  uint8_t mode [[ attribute(0) ]];

  // The grid coordinates (x, y) where x < columns and y < rows
  float2 grid_pos [[ attribute(1) ]];

  // The width of the cell in cells (i.e. 2 for double-wide).
  uint8_t cell_width [[ attribute(6) ]];

  // The color. For BG modes, this is the bg color, for FG modes this is
  // the text color. For styles, this is the color of the style.
  uchar4 color [[ attribute(5) ]];

  // The fields below are present only when rendering text (fg mode)

  // The background color of the cell. This is used to determine if
  // we need to render the text with a different color to ensure
  // contrast.
  uchar4 bg_color [[ attribute(7) ]];

  // The position of the glyph in the texture (x,y)
  uint2 glyph_pos [[ attribute(2) ]];

  // The size of the glyph in the texture (w,h)
  uint2 glyph_size [[ attribute(3) ]];

  // The left and top bearings for the glyph (x,y)
  int2 glyph_offset [[ attribute(4) ]];
};

struct VertexOut {
  float4 position [[ position ]];
  float2 cell_size;
  uint8_t mode;
  float4 color;
  float2 tex_coord;
};

//-------------------------------------------------------------------
// Color Functions
//-------------------------------------------------------------------
#pragma mark - Colors

// https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
float luminance_component(float c) {
    if (c <= 0.03928f) {
        return c / 12.92f;
    } else {
        return pow((c + 0.055f) / 1.055f, 2.4f);
    }
}

float relative_luminance(float3 color) {
  color.r = luminance_component(color.r);
  color.g = luminance_component(color.g);
  color.b = luminance_component(color.b);
  float3 weights = float3(0.2126f, 0.7152f, 0.0722f);
  return dot(color, weights);
}

// https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
float contrast_ratio(float3 color1, float3 color2) {
  float l1 = relative_luminance(color1);
  float l2 = relative_luminance(color2);
  return (max(l1, l2) + 0.05f) / (min(l1, l2) + 0.05f);
}

// Return the fg if the contrast ratio is greater than min, otherwise
// return a color that satisfies the contrast ratio. Currently, the color
// is always white or black, whichever has the highest contrast ratio.
float4 contrasted_color(float min, float4 fg, float4 bg) {
    float3 fg_premult = fg.rgb * fg.a;
    float3 bg_premult = bg.rgb * bg.a;
    float ratio = contrast_ratio(fg_premult, bg_premult);
    if (ratio < min) {
        float white_ratio = contrast_ratio(float3(1.0f), bg_premult);
        float black_ratio = contrast_ratio(float3(0.0f), bg_premult);
        if (white_ratio > black_ratio) {
            return float4(1.0f);
        } else {
            return float4(0.0f, 0.0f, 0.0f, 1.0f);
        }
    }

    return fg;
}

//-------------------------------------------------------------------
// Terminal Grid Cell Shader
//-------------------------------------------------------------------
#pragma mark - Terminal Grid Cell Shader

vertex VertexOut uber_vertex(
  unsigned int vid [[ vertex_id ]],
  VertexIn input [[ stage_in ]],
  constant Uniforms &uniforms [[ buffer(1) ]]
) {
  // Convert the grid x,y into world space x, y by accounting for cell size
  float2 cell_pos = uniforms.cell_size * input.grid_pos;

  // Scaled cell size for the cell width
  float2 cell_size_scaled = uniforms.cell_size;
  cell_size_scaled.x = cell_size_scaled.x * input.cell_width;

  // Turn the cell position into a vertex point depending on the
  // vertex ID. Since we use instanced drawing, we have 4 vertices
  // for each corner of the cell. We can use vertex ID to determine
  // which one we're looking at. Using this, we can use 1 or 0 to keep
  // or discard the value for the vertex.
  //
  // 0 = top-right
  // 1 = bot-right
  // 2 = bot-left
  // 3 = top-left
  float2 position;
  position.x = (vid == 0 || vid == 1) ? 1.0f : 0.0f;
  position.y = (vid == 0 || vid == 3) ? 0.0f : 1.0f;

  VertexOut out;
  out.mode = input.mode;
  out.cell_size = uniforms.cell_size;
  out.color = float4(input.color) / 255.0f;
  switch (input.mode) {
  case MODE_BG:
    // Calculate the final position of our cell in world space.
    // We have to add our cell size since our vertices are offset
    // one cell up and to the left. (Do the math to verify yourself)
    cell_pos = cell_pos + cell_size_scaled * position;

    out.position = uniforms.projection_matrix * float4(cell_pos.x, cell_pos.y, 0.0f, 1.0f);
    break;

  case MODE_FG:
  case MODE_FG_COLOR: {
    float2 glyph_size = float2(input.glyph_size);
    float2 glyph_offset = float2(input.glyph_offset);

    // The glyph_offset.y is the y bearing, a y value that when added
    // to the baseline is the offset (+y is up). Our grid goes down.
    // So we flip it with `cell_size.y - glyph_offset.y`.
    glyph_offset.y = cell_size_scaled.y - glyph_offset.y;

    // Calculate the final position of the cell which uses our glyph size
    // and glyph offset to create the correct bounding box for the glyph.
    cell_pos = cell_pos + glyph_size * position + glyph_offset;
    out.position = uniforms.projection_matrix * float4(cell_pos.x, cell_pos.y, 0.0f, 1.0f);

    // Calculate the texture coordinate in pixels. This is NOT normalized
    // (between 0.0 and 1.0) and must be done in the fragment shader.
    out.tex_coord = float2(input.glyph_pos) + float2(input.glyph_size) * position;

    out.color = contrasted_color(uniforms.min_contrast, out.color, float4(input.bg_color) / 255.0f);
    break;
  }

  case MODE_STRIKETHROUGH: {
    // Strikethrough Y value is just our thickness
    float2 strikethrough_size = float2(cell_size_scaled.x, uniforms.strikethrough_thickness);

    // Position the strikethrough where we are told to
    float2 strikethrough_offset = float2(cell_size_scaled.x, uniforms.strikethrough_position);

    // Go to the bottom of the cell, take away the size of the
    // strikethrough, and that is our position. We also float it slightly
    // above the bottom.
    cell_pos = cell_pos + strikethrough_offset - (strikethrough_size * position);

    out.position = uniforms.projection_matrix * float4(cell_pos, 0.0f, 1.0);
    break;
  }

  }

  return out;
}

fragment float4 uber_fragment(
  VertexOut in [[ stage_in ]],
  texture2d<float> textureGreyscale [[ texture(0) ]],
  texture2d<float> textureColor [[ texture(1) ]]
) {
  constexpr sampler textureSampler(address::clamp_to_edge, filter::linear);

  switch (in.mode) {
  case MODE_BG:
    return in.color;

  case MODE_FG: {
    // Normalize the texture coordinates to [0,1]
    float2 size = float2(textureGreyscale.get_width(), textureGreyscale.get_height());
    float2 coord = in.tex_coord / size;

    // We premult the alpha to our whole color since our blend function
    // uses One/OneMinusSourceAlpha to avoid blurry edges.
    // We first premult our given color.
    float4 premult = float4(in.color.rgb * in.color.a, in.color.a);
    // Then premult the texture color
    float a = textureGreyscale.sample(textureSampler, coord).r;
    premult = premult * a;
    return premult;
  }

  case MODE_FG_COLOR: {
    // Normalize the texture coordinates to [0,1]
    float2 size = float2(textureColor.get_width(), textureColor.get_height());
    float2 coord = in.tex_coord / size;
    return textureColor.sample(textureSampler, coord);
  }

  case MODE_STRIKETHROUGH:
    return in.color;
  }
}

//-------------------------------------------------------------------
// Image Shader
//-------------------------------------------------------------------
#pragma mark - Image Shader

struct ImageVertexIn {
  // The grid coordinates (x, y) where x < columns and y < rows where
  // the image will be rendered. It will be rendered from the top left.
  float2 grid_pos [[ attribute(1) ]];

  // Offset in pixels from the top-left of the cell to make the top-left
  // corner of the image.
  float2 cell_offset [[ attribute(2) ]];

  // The source rectangle of the texture to sample from.
  float4 source_rect [[ attribute(3) ]];

  // The final width/height of the image in pixels.
  float2 dest_size [[ attribute(4) ]];
};

struct ImageVertexOut {
  float4 position [[ position ]];
  float2 tex_coord;
};

vertex ImageVertexOut image_vertex(
  unsigned int vid [[ vertex_id ]],
  ImageVertexIn input [[ stage_in ]],
  texture2d<uint> image [[ texture(0) ]],
  constant Uniforms &uniforms [[ buffer(1) ]]
) {
  // The size of the image in pixels
  float2 image_size = float2(image.get_width(), image.get_height());

  // Turn the image position into a vertex point depending on the
  // vertex ID. Since we use instanced drawing, we have 4 vertices
  // for each corner of the cell. We can use vertex ID to determine
  // which one we're looking at. Using this, we can use 1 or 0 to keep
  // or discard the value for the vertex.
  //
  // 0 = top-right
  // 1 = bot-right
  // 2 = bot-left
  // 3 = top-left
  float2 position;
  position.x = (vid == 0 || vid == 1) ? 1.0f : 0.0f;
  position.y = (vid == 0 || vid == 3) ? 0.0f : 1.0f;

  // The texture coordinates start at our source x/y, then add the width/height
  // as enabled by our instance id, then normalize to [0, 1]
  float2 tex_coord = input.source_rect.xy;
  tex_coord += input.source_rect.zw * position;
  tex_coord /= image_size;

  ImageVertexOut out;

  // The position of our image starts at the top-left of the grid cell and
  // adds the source rect width/height components.
  float2 image_pos = (uniforms.cell_size * input.grid_pos) + input.cell_offset;
  image_pos += input.dest_size * position;

  out.position = uniforms.projection_matrix * float4(image_pos.x, image_pos.y, 0.0f, 1.0f);
  out.tex_coord = tex_coord;
  return out;
}

fragment float4 image_fragment(
  ImageVertexOut in [[ stage_in ]],
  texture2d<uint> image [[ texture(0) ]]
) {
  constexpr sampler textureSampler(address::clamp_to_edge, filter::linear);

  // Ehhhhh our texture is in RGBA8Uint but our color attachment is
  // BGRA8Unorm. So we need to convert it. We should really be converting
  // our texture to BGRA8Unorm.
  uint4 rgba = image.sample(textureSampler, in.tex_coord);
  return float4(rgba) / 255.0f;
}

//-------------------------------------------------------------------
// Post Shader
//-------------------------------------------------------------------
#pragma mark - Post Shader

struct PostVertexOut {
    float4 position [[ position ]];
};

constant float2 post_pos[4] = { {-1,-1}, {1,-1}, {-1,1}, {1,1 } };

vertex PostVertexOut post_vertex(uint id [[ vertex_id ]]) {
    PostVertexOut out;
    out.position = float4(post_pos[id], 0, 1);
    return out;
}