Fix PositionScale Scale value passed to shader

[jamierobertson1]

I noticed there is small rendering error when points are scaled in the shader.

Currently the points positions are moved and scaled in the shader as follows:

    #ifdef VSG_POSITION_SCALE
    vec4 vertex = vec4(vsg_PositionScale.xyz + vsg_Vertex * vsg_PositionScale.w, 1.0);
    #else 

Where the vsg_Vertex is an 0-1 float calculate from one of Vulkans UNORM integer types depending on the bits used in the VSGPoints::Settings.

Currently VSGPoints is sending the bricksize as the scaling value.

double brickSize = brickPrecision * pow(2.0, static_cast<double>(settings.bits));

When a brick for a position are calculated, it uses the full pow(2.0,nbits) bit range for a brick (i.e. divides the original position by this), and the remainder makes up the position. As an example (ignoring precision), for 8 bits, each brick will be 256 units in length, with the possible remainder being in the 0-255 range as per image below:

In the shader assuming , a value of 255 (VK_FORMAT_R8G8B8_UNORM) will be interpreted as 1.0:
(from vulkan conversion docs):

The 255 value will then be scaled by the full brick size vsg_PositionScale.w (i.e. the full 256 size) This is incorrect as it should be one precision increment less (as per diagram above), so currently the points are scaled a bit too much on screen.

Simple fix is to subtract one precision increment from the positionScale, scale value in the Brick::createRendering function. The rest of the positionScale stuff calculations seem good, it's literally just the bit used by the shader which needs to be updated.

Change:

vsg::vec4 positionScale(position.x, position.y, position.z, brickSize);
return createRendering(settings, positionScale, pointSize);

to:

vsg::vec4 positionScale(position.x, position.y, position.z, brickSize - brickPrecision);
return createRendering(settings, positionScale, pointSize); 

I verified this by drawing a wireframe box of known size, and then adding VSGPoint points at the corners.

Currently:

After fix:

For testing I just modified the VSGPoints application, to draw the box and points if no input file is supplied:

    if (group->children.empty())
    {
        // add box
        vsg::StateInfo stateInfo;
        stateInfo.wireframe = true;
        auto builder = vsg::Builder::create();
        builder->options = options;

        vsg::GeometryInfo info;
        info.cullNode = true;
        info.dx.set(2.f, 0.0f, 0.0f);
        info.dy.set(0.0f, 2.0f, 0.0f);
        info.dz.set(0.0f, 0.0f, 2.0f);
        auto box = builder->createBox(info, stateInfo);
        group->addChild(box);

        // add vsgpoints for corners
        settings = vsgPoints::Settings::create();
        settings->createType = vsgPoints::CREATE_FLAT;
        settings->precision = 0.001;
        settings->bits = 8;
        auto bricks = vsgPoints::Bricks::create(settings);

        // checked fixed coordinates
        bricks->add(vsg::dvec3(-1.0, -1.0, -1.0), vsg::ubvec4(0, 255, 0, 255));
        bricks->add(vsg::dvec3(-1.0, 1.0, -1.0), vsg::ubvec4(0, 255, 0, 255));
        bricks->add(vsg::dvec3(-1.0, -1.0, 1.0), vsg::ubvec4(0, 255, 0, 255));
        bricks->add(vsg::dvec3(-1.0, 1.0, 1.0), vsg::ubvec4(0, 255, 0, 255));
        bricks->add(vsg::dvec3(1.0, -1.0, 1.0), vsg::ubvec4(0, 255, 0, 255));
        bricks->add(vsg::dvec3(1.0, 1.0, 1.0), vsg::ubvec4(0, 255, 0, 255));
        bricks->add(vsg::dvec3(1.0, -1.0, -1.0), vsg::ubvec4(0, 255, 0, 255));
        bricks->add(vsg::dvec3(1.0, 1.0, -1.0), vsg::ubvec4(0, 255, 0, 255));
        group->addChild(vsgPoints::createSceneGraph(bricks, settings));
    }

Tested on windows