//

struct TerminalCell
{
    // cell index into GlyphTexture, should be two 16-bit (x,y) values packed: "x | (y << 16)"
    uint GlyphIndex;

    // 0xAABBGGRR encoded colors, nonzero alpha for Foreground indicates to render colored-glyph
    // which means use RGB values from GlyphTexture directly as output, not as  ClearType blending weights
    uint Foreground;
    uint Background;
};

cbuffer ConstBuffer : register(b0)
{
    uint2 CellSize;
    uint2 TermSize;
}; 

// TermSize.x * TermSize.y amount of cells to render as output
StructuredBuffer<TerminalCell> Cells : register(t0);

// RGB blending weights for ClearType
// or alpha-premultipled RGB values for colored glyphs
Texture2D<float4> GlyphTexture : register(t1);

RWTexture2D<float4> Output : register(u0);

float3 GetColor(uint i)
{
    int r = i & 0xff;
    int g = (i >> 8) & 0xff;
    int b = (i >> 16) & 0xff;
    return float3(r, g, b) / 255.0;
}

uint2 GetGlyphPos(uint GlyphIndex)
{
    return uint2(GlyphIndex & 0xffff, GlyphIndex >> 16) * CellSize;
}

// dispatch with (TermSize*CellSize+7)/8 groups for x,y and 1 for z
[numthreads(8, 8, 1)]
void shader(uint3 Id: SV_DispatchThreadID)
{
    uint2 ScreenPos = Id.xy;

    // index of cell on screen
    uint2 CellIndex = ScreenPos / CellSize;

    // pixel position in cell
    uint2 CellPos = ScreenPos % CellSize;

    TerminalCell Cell = Cells[CellIndex.y * TermSize.x + CellIndex.x];

    // position where glyph starts in texture
    uint2 GlyphPos = GetGlyphPos(Cell.GlyphIndex);

    // absolute pixel location in texture to use for output
    uint2 PixelPos = GlyphPos + CellPos;

    float4 Tex = GlyphTexture[PixelPos];

    float3 Background = GetColor(Cell.Background);
    float3 Foreground = GetColor(Cell.Foreground);

    bool ColoredGlyph = (Cell.Foreground >> 24) != 0;

    float3 Color;
    if (ColoredGlyph)
    {
        // colored glyphs are alpha premultiplied
        Color = Background * (1.0 - Tex.a) + Tex.rgb;
    }
    else
    {
        // TODO: proper ClearType blending
        Color = lerp(Background, Foreground, Tex.rgb);
    }

    Output[ScreenPos] = float4(Color, 1);
}