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#version 120
varying highp vec3 pos;
varying highp vec3 rayDir;
uniform highp sampler3D textureSampler;
uniform highp vec4 colorIndex[256];
uniform highp int color8Bit;
uniform highp vec3 textureDimensions;
uniform highp int sampleCount; // This is the maximum sample count
uniform highp float alphaMultiplier;
uniform highp int preserveOpacity;
uniform highp vec3 minBounds;
uniform highp vec3 maxBounds;
// Ray traveling straight through a single 'alpha thickness' applies 100% of the encountered alpha.
// Rays traveling shorter distances apply a fraction. This is used to normalize the alpha over
// entire volume, regardless of texture dimensions
const highp float alphaThicknesses = 32.0;
const highp float SQRT3 = 1.73205081;
void main() {
vec3 rayStart = pos;
highp vec3 startBounds = minBounds;
highp vec3 endBounds = maxBounds;
if (rayDir.x < 0.0) {
startBounds.x = maxBounds.x;
endBounds.x = minBounds.x;
}
if (rayDir.y > 0.0) {
startBounds.y = maxBounds.y;
endBounds.y = minBounds.y;
}
if (rayDir.z > 0.0) {
startBounds.z = maxBounds.z;
endBounds.z = minBounds.z;
}
// Calculate ray intersection endpoint
highp vec3 rayStop;
highp vec3 invRayDir = 1.0 / rayDir;
highp vec3 t = (endBounds - rayStart) * invRayDir;
highp float endT = min(t.x, min(t.y, t.z));
if (endT <= 0.0)
discard;
rayStop = rayStart + endT * rayDir;
// Convert intersections to texture coords
rayStart = 0.5 * (rayStart + 1.0);
rayStop = 0.5 * (rayStop + 1.0);
highp vec3 ray = rayStop - rayStart;
highp float fullDist = length(ray);
highp float stepSize = SQRT3 / sampleCount;
highp vec3 step = (SQRT3 * normalize(ray)) / sampleCount;
rayStart += (step * 0.001);
highp vec3 curPos = rayStart;
highp float curLen = 0.0;
highp vec4 curColor = vec4(0, 0, 0, 0);
highp float curAlpha = 0.0;
highp vec3 curRgb = vec3(0, 0, 0);
highp vec4 destColor = vec4(0, 0, 0, 0);
highp float totalOpacity = 1.0;
highp float extraAlphaMultiplier = stepSize * alphaThicknesses * alphaMultiplier;
// Raytrace into volume, need to sample pixels along the eye ray until we hit opacity 1
for (int i = 0; i < sampleCount; i++) {
curColor = texture3D(textureSampler, curPos);
if (color8Bit != 0)
curColor = colorIndex[int(curColor.r * 255.0)];
if (curColor.a >= 0.0) {
if (curColor.a == 1.0 && (preserveOpacity == 1 || alphaMultiplier >= 1.0))
curAlpha = 1.0;
else
curAlpha = curColor.a * extraAlphaMultiplier;
highp float nextOpacity = totalOpacity - curAlpha;
// If opacity goes beyond full opacity, we need to adjust current alpha according
// to the fraction of the distance the material is visible, so that we don't get
// box artifacts around texels.
if (nextOpacity < 0.0) {
curAlpha *= totalOpacity / curAlpha;
nextOpacity = 0.0;
}
curRgb = curColor.rgb * curAlpha * (totalOpacity + nextOpacity);
destColor.rgb += curRgb;
totalOpacity = nextOpacity;
}
curPos += step;
curLen += stepSize;
if (curLen >= fullDist || totalOpacity <= 0.0)
break;
}
if (totalOpacity == 1.0)
discard;
// Brighten up the final color if there is some transparency left
if (totalOpacity >= 0.0 && totalOpacity < 1.0)
destColor *= (1.0 - (totalOpacity * 0.5)) / (1.0 - totalOpacity);
destColor.a = (1.0 - totalOpacity);
gl_FragColor = clamp(destColor, 0.0, 1.0);
}
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