CODEWITHSUNDEEP
openglglsl
svcop3
svcop3

Reputation: 19

shader storage block length returns zero or less

I am getting zero or less when I try to get the length of an indeterminate-length array in a shader storage block.

Setting up the storage:

geometryBuffer = ByteBuffer.allocateDirect(12* 4)
    .order(ByteOrder.nativeOrder());

geometryBuffer.putFloat(1f);
geometryBuffer.putFloat(0.25f);
geometryBuffer.putFloat(0.5f);
geometryBuffer.putFloat(0.75f);
geometryBuffer.putFloat(1.1f);
geometryBuffer.putFloat(0.35f);
geometryBuffer.putFloat(0.6f);
geometryBuffer.putFloat(0.85f);
geometryBuffer.putFloat(1.2f);
geometryBuffer.putFloat(0.45f);
geometryBuffer.putFloat(0.7f);
geometryBuffer.putFloat(0.95f);

geometryBuffer.flip();

geometryBufferId = GL15.glGenBuffers();
GL15.glBindBuffer(GL43.GL_SHADER_STORAGE_BUFFER, geometryBufferId);
System.out.println("bb" + GL11.glGetError());
GL15.glBufferData(GL43.GL_SHADER_STORAGE_BUFFER, geometryBuffer, GL15.GL_STATIC_DRAW);
System.out.println("bd" + GL11.glGetError());
GL30.glBindBufferRange(GL43.GL_SHADER_STORAGE_BUFFER, 0, geometryBufferId, 0, 36);
System.out.println("br" + GL11.glGetError());
GL15.glBindBuffer(GL43.GL_SHADER_STORAGE_BUFFER, 0);

The frag shader:

#version 430

out vec4 outColour;

layout(std430, binding=0) buffer Geometry {
    vec4 a;
    vec4 myVec[];
} ; 

void main() {
    // if (myVec[0].y == 0.35) {
    if (myVec.length() == -1) {
        outColour = vec4 (1,0,0,1);
    } else {
        outColour = vec4(0,0,1,1);
    }
}

I expect length to be 2 but the length returned in this example is -1. I can retrieve the values in the array as the test if (myVec[0].y == 0.35) also returns red.

According to https://www.opengl.org/wiki/Interface_Block_(GLSL) and the GLSL spec v4.5 section 4.1.9, the size is calculated at runtime from the size of the buffer or range, minus all the sized components in the block and then divided by the size of an individual array entry. What I seem to be getting is zero minus the number of vec4 units before the indeterminate storage in the block.

An alternative would be to pass a uniform with the array size, but why doesn't length() return the correct value? I'm running on a GTX660M

Upvotes: 0

Views: 166

Answers (1)

svcop3
svcop3

Reputation: 19

I cant claim the credit for this as it was provided by someone on a GL forum, but to close this question out, the answer is that referencing myVec.length() is not enough, you have to actually reference the array so this works :

if (myVec.length() == 2 && myVec[0].y == 0.35) {
...

Without actually referencing an array entry in the shader, the compiler presumably optimizes it out, thus returning a length of zero at runtime.

Upvotes: 2

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