OpenGL billboarding vertex shader resets the scale - opengl

Question

I am fairly new to OpenGL and to 3D graphics in general and this is my first divergence away from tutorials to do something on my own. I want to create billboard textures so that these textures will always face the camera. (I plan to use them as position indicators for light sources and so on.)
After some research, I found this website that offers a fairly simple solution for my problem.
Simple Billboarding
I followed their steps and everything worked as expected except 1 thing. My scaling parameter is ignored and the quad that I use to draw the texture on is not being scaled.
Here is my very simple shader code.
#version 400 core
in vec3 position;
in vec2 textureCoordinates;
out vec2 pass_textureCoordinates;
uniform mat4 transformationMatrix;
uniform mat4 projectionMatrix;
uniform mat4 viewMatrix;
void main(void){
mat4 modelView = viewMatrix * transformationMatrix;
modelView[0][0] = 1;
modelView[0][1] = 0;
modelView[0][2] = 0;
modelView[1][0] = 0;
modelView[1][1] = 1;
modelView[1][2] = 0;
modelView[2][0] = 0;
modelView[2][1] = 0;
modelView[2][2] = 1;
gl_Position = projectionMatrix * modelView * vec4(position,1.0);
pass_textureCoordinates = textureCoordinates;
}
And here is the code that generates my vieMatrix and transformationMatrix
public static Matrix4f createTransformationMatrix(BillboardEntity billboardEntity) {
Vector3f translation = billboardEntity.getTranslation();
float rx = billboardEntity.getRotationX();
float ry = billboardEntity.getRotationY();
float rz = billboardEntity.getRotationZ();
float scale = billboardEntity.getScale();
Matrix4f matrix = new Matrix4f();
matrix.setIdentity();
Matrix4f.translate(translation, matrix, matrix);
Matrix4f.rotate((float) Math.toRadians(rx), new Vector3f(1,0,0), matrix, matrix);
Matrix4f.rotate((float) Math.toRadians(ry), new Vector3f(0,1,0), matrix, matrix);
Matrix4f.rotate((float) Math.toRadians(rz), new Vector3f(0,0,1), matrix, matrix);
Matrix4f.scale(new Vector3f(scale, scale, scale), matrix, matrix);
return matrix;
}
public static Matrix4f createViewMatrix(Camera camera) {
Matrix4f viewMatrix = new Matrix4f();
viewMatrix.setIdentity();
Matrix4f.rotate((float) Math.toRadians(camera.getPitch()), new Vector3f(1, 0, 0), viewMatrix,
viewMatrix);
Matrix4f.rotate((float) Math.toRadians(camera.getYaw()), new Vector3f(0, 1, 0), viewMatrix,
viewMatrix);
Vector3f cameraPos = camera.getPosition();
Vector3f negativeCameraPos = new Vector3f(-cameraPos.x, -cameraPos.y, -cameraPos.z);
Matrix4f.translate(negativeCameraPos, viewMatrix, viewMatrix);
return viewMatrix;
}
I should probably mention that I am using Java and LWJGL in my project. Any idea why the scale is being ignored? Also, is this a good approach to make billboard textures ?

Answer 1

I found this post on Google, so I would like to share my solution (if you don't mind).
If you set the upper-left 3x3 part of a 4x4 transform matrix to be an "identity" matrix (all "1"s on the diagonal), you nuke the rotation, but also the scale because they're both stored there.
To fix it, in this particular example, you can use:
void main(){
mat4 modelView = viewMatrix * transformationMatrix;
modelView[0][0] = length(vec3(transformationMatrix[0]));
modelView[0][1] = 0;
modelView[0][2] = 0;
modelView[1][0] = 0;
modelView[1][1] = length(vec3(transformationMatrix[1]));
modelView[1][2] = 0;
modelView[2][0] = 0;
modelView[2][1] = 0;
modelView[2][2] = 1;
gl_Position = projectionMatrix * modelView * vec4(position,1.0);
pass_textureCoordinates = textureCoordinates;
}
It will retain the scaling information from the transform (or model) matrix. I tested it and it works well with any scale set on the CPU side (in .cpp).
However, you should know that length is usually an expensive process. For example, in GLM, it uses sqrt(dot(v, v)) and if you don't know, sqrt is quite expensive, especially if you want MANY billboards in your scene (grass, clouds, particles, etc). So what you can do, instead, is use a uniform that handles the scaling, which is much cheaper:
uniform bool uIsSpherical;
uniform vec2 uSpriteScale; // Using a uniform instead
void main(){
...
modelView[0][0] = uSpriteScale.x;
modelView[0][1] = 0;
modelView[0][2] = 0;
if (uIsSpherical)
{
modelView[1][0] = 0;
modelView[1][1] = uSpriteScale.y;
modelView[1][2] = 0;
}
else
{
modelView[1][1] *= uSpriteScale.y;
}
modelView[2][0] = 0;
modelView[2][1] = 0;
modelView[2][2] = 1;
...
}
The if condition on the second column (modelView[1][...]) is for a cylindrical billboard, suitable for trees in the far distance or other kind of impostors, since you don't want them to tilt backwards if you see them from above, like from an airplane or something.
Now, the problem with doing a billboard in shader, is that on the CPU side, during shadow render passes, you're still using a fixed transform matrix. So the shadow won't follow the rotated quad, it will still "see" it as stationary. The "solution" to that, I've been told, is to use a shadow billboard shader, and do the same calculations there. Apparently it's not worth storing or caching the results, because GPUs are much faster at doing the same things again than doing memory fetches, since the memory is optimized for throughput, not latency. But I have yet to test it. Just keep in mind that by default, shadows won't rotate with the billboard.