java - How to use function glMultiDrawElementsBaseVertex?

Question

I'm currently trying to use function glMultiDrawElementsBaseVertex? in java with lwjgl and opengl but i have some questions.

first, i stored 2 meshes in one vbo then i want to draw them in one draw call.

so i did

int[] count = new int[] {this.modelManager.getElementCount().get(0), this.modelManager.getElementCount().get(1)};
int[] baseVertex  = new int[] {0, this.modelManager.getElementCount().get(1)};
glMultiDrawElementsBaseVertex(GL_TRIANGLES, count, GL_UNSIGNED_INT, this.modelManager.pointerBuffer, baseVertex);

but im not sure about parameters. "getElementCount" return the numbers of indices for a mesh. "baseVertex" if understand correctly it represent how many objects of each mesh i want to draw. And for pointerBuffer im realy no sure about how i construct it but it store all indices of the two meshes.

this.pointerBuffer = PointerBuffer.create(MemoryUtil.memAddress(indicesBuffer), indices.length);

And for PointerBuffer parameter, i read somewhere that pointerBuffer must be a pointer to array of array but i dont know how to do it. Following image is actually my result, one strangly combined of two mesh but i wanted to draw 100 objects of each mesh.

Edit: i forget to say my code works with the following call but it draw only one type of mesh

glDrawElementsInstancedBaseVertex(GL_TRIANGLES, this.modelManager.getElementCount().get(1), GL_UNSIGNED_INT, 0, NB_MAX_OBJECTS, this.modelManager.getElementCount().get(0));

question from:https://stackoverflow.com/questions/65843968/how-to-use-function-glmultidrawelementsbasevertex

Answer 1

basevertex specifies a constant that should be added to each element of indices? when chosing elements from the enabled vertex arrays.
If the index of the mesh starts at 0, the basevertex for the 1st mesh must be 0. For the 2nd mesh it must be the number of indices of the 1st mesh:

int[] count = new int[] {
    this.modelManager.getElementCount().get(0), 
    this.modelManager.getElementCount().get(1)
};
int[] baseVertex  = new int[] {
    0,
    this.modelManager.getElementCount().get(0)
};

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If you want to draw 1 mesh multiple times, you need to use instancing Instancing. Use glDrawElementsInstanced to draw multiple instances of a set of elements.