How to use a 4x4 matrix as a vertex attribute in GLSL?

Question

I am trying to use a 4x4 matrix as a vertex attribute, using this code:

Mat4 matrices[numVerts];

int mtxBoneID = glGetAttribLocation(hProgram, "aMtxBone");

glEnableVertexAttribArray(mtxBoneID + 0);
glEnableVertexAttribArray(mtxBoneID + 1);
glEnableVertexAttribArray(mtxBoneID + 2);
glEnableVertexAttribArray(mtxBoneID + 3);
glVertexAttribPointer(mtxBoneID + 0, 4, GL_FLOAT, GL_FALSE, sizeof(Mat4), ((Vec4*)matrices) + 0);
glVertexAttribPointer(mtxBoneID + 1, 4, GL_FLOAT, GL_FALSE, sizeof(Mat4), ((Vec4*)matrices) + 1);
glVertexAttribPointer(mtxBoneID + 2, 4, GL_FLOAT, GL_FALSE, sizeof(Mat4), ((Vec4*)matrices) + 2);
glVertexAttribPointer(mtxBoneID + 3, 4, GL_FLOAT, GL_FALSE, sizeof(Mat4), ((Vec4*)matrices) + 3);

// shader:
// ...
attribute mat4 aMtxBone;
// ...

But all I get on the screen is garbage.

Answer 1

you may try something like this in your shader use layout

layout(location=x) in mat4 <name>;

x won't be equal to glGetAttribLocation ,you must maintain it by yourself.it is equal to number times you call glVertexAttribPointer . example

layout(location=0) in vec4 in_Position;
layout(location=1) in vec4 in_Color;
layout(location=2) in vec4 in_Normal;

glVertexAttribPointer(0, xxxxxxxx);
glVertexAttribPointer(1, xxxxxx);
glVertexAttribPointer(2, xxxxxx);

Answer 2

So, my answer was deleted because of reasons unknown.

Here I go again, I'll format it differently this time.

I had the EXACT same problem as the question had/has, stuff would be drawn quite messed up. At least that's the description.

What fixed it for me was calling

glDrawElementsInstancedBaseVertex(GL_TRIANGLES,
                                    d->drawCount,
                                    GL_UNSIGNED_INT,
                                    0,
                                    p_objects.size(),
                                    0);

To draw my stuff, just this and nothing more.

HOWEVER!

This MIGHT not be the only issue you have if your stuff is drawn incorrectly. The entire list of stuff you NEED to get correct it immense. So i'm going to put a link here, which has a great tutorial on how to draw lots of stuff. Also, you can get the source code and try it out yourself.

http://ogldev.atspace.co.uk/www/tutorial33/tutorial33.html

Sources here: http://ogldev.atspace.co.uk/ogldev-source.zip

Downvote me again and delete my answer, it's content hasn't changed at all from what it was.

Ban me from the site, your loss.

Answer 3

First you need to create VBO ( glGenBuffers ) for your matrix, then bind it ( glBindBuffer ) as current, then use

glVertexAttribPointer(mtxBoneID + 0, 4, GL_FLOAT, GL_FALSE, 0, 0);
glVertexAttribPointer(mtxBoneID + 1, 4, GL_FLOAT, GL_FALSE, 0, 4);
glVertexAttribPointer(mtxBoneID + 2, 4, GL_FLOAT, GL_FALSE, 0, 8);
glVertexAttribPointer(mtxBoneID + 3, 4, GL_FLOAT, GL_FALSE, 0, 12);

instead of your glVertexAttribPointer calls.

Answer 4

It looks like your offsets are off. It should be

((Vec4*)matrices) + sizeOf(Vec4)*i

instead of

((Vec4*)matrices) + i