Simple Volume Renderer in Codea.lua

--# Main
-- Slices

function setup()
    displayMode(FULLSCREEN)
    parameter.number("Limit",0,1,.1)
    parameter.integer("LowSlice",-80,80,-80)
    parameter.integer("HighSlice",-80,80,80)
    R = 0
--    VolumePacker()
    v = Volume()
end

function draw()
    background(40, 40, 50)
    perspective(45)
    camera(0,0,500,0,0,0)
    rotate(R,0,1,0)
    v:draw()
end

function touched(touch)
    R = R + touch.deltaX
end

--# Volume
Volume = class()

function Volume:init()
    local sq = {
        vec3(0,0,0), vec3(1,0,0),
        vec3(1,1,0), vec3(1,1,0),
        vec3(0,1,0), vec3(0,0,0)      
    }
    local dz = 1.3
    local hd = 100*dz/2
    self.m = mesh()
    local vs,uvs = {},{}
    self.m.shader = VolumeShader
--    self.m.texture = "Documents:Slices"
--    self.m.shader.texture2 = "Documents:Slices2"
    self:loadTextures()
    for i=1,99 do
        for j,v in ipairs(sq) do
            table.insert(vs, (v - vec3(.5,.5,0))*256 +
                vec3(0,0,i*dz - hd))
            -- texture space divided by 8 to match smaller images
            local x = (math.floor(i)-1)
            local y = math.floor(x*.125)
            local uv = (vec2(v.x,v.y) + vec2(x%8,y))*.125
            table.insert(uvs, uv)
        end
    end 
    self.m.vertices = vs
    self.m.texCoords = uvs
end

function Volume:loadTextures()
    local url = "https://dl.dropbox.com/s/mzz2hce40r8kq35/"
    http.request(url .. "Slices.png", function (data)
        self.m.texture = data
    end)
end

function Volume:draw()
    self.m.shader.limit = Limit
    self.m.shader.lowSlice = LowSlice
    self.m.shader.highSlice = HighSlice
    self.m:draw()
end


--# VolumeShader
VolumeShader = shader()
VolumeShader.vertexProgram = [[
uniform mat4 modelViewProjection;
attribute vec4 position;
attribute vec2 texCoord;
varying highp vec2 vTexCoord;
uniform highp float lowSlice;
uniform highp float highSlice;

void main()
{
    vTexCoord = texCoord;
    if(position.z > lowSlice && position.z < highSlice) {
        gl_Position = modelViewProjection * position;
    } else {
        // move all points of slice to origin to hide it.
        gl_Position = vec4(0.,0.,0.,0.); 
    }
}
]]
VolumeShader.fragmentProgram = [[
uniform lowp sampler2D texture;
uniform lowp sampler2D texture2;
varying highp vec2 vTexCoord;
uniform highp float limit;

void main()
{
    lowp vec4 col;
    if(vTexCoord.y > 1.) {
          col = texture2D(texture2, vTexCoord);
    } else {
          col = texture2D(texture, vTexCoord);
    }

    if(col.r < limit) discard;
    gl_FragColor = col;
}
]]

--# VolumePacker
-- Used to pack several images into one image
VolumePacker = class()

function VolumePacker:init(x)
    local w = 2048
    local img = image(w,w)
    local img2 = image(w,w)
    local x,y = 0,0
    spriteMode(CORNER)
    setContext(img)
    for i=1,99 do
        sprite(self:getSlice(i),x,y)
        x = x + 256
        if x >= w then
            x,y = 0,y + 256
            if y >= w then
                setContext(img2)
                x,y = 0,0
            end
        end
    end
    setContext()
    saveImage("Documents:Slices", img)
    saveImage("Documents:Slices2", img2)
end

function VolumePacker:getSlice(i)
    local id = i
    if i < 10 then id = "00" .. id
    else id = "0" .. id end
    return "Dropbox:slices/mrbrain-8bit" .. id
end