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Reputation: 158

JOGL glReadPixel with long paramater

I'm doing some stuff with OpenGL in Java using JOGL and I came across something that surprised me.

There is two glReadPixel functions that points to the same C implementation.

The first one is :

glReadPixels(int x, int y, int width, int height, int format, int type, Buffer pixels)

I understand how it works compared to the C binding but the second one :

glReadPixels(int x, int y, int width, int height, int format, int type, long pixels_buffer_offset)

How can it convert a void* to a long ? Are we really getting the pixels within a long ?

I really do not understand the second binding. I have done some googling and I just found out everyone is using the first one ...

Upvotes: 0

Views: 72

Answers (1)

Reto Koradi
Reto Koradi

Reputation: 54592

The second binding would be used if you want to read the result into a PBO (Pixel Buffer Object). In that case, the last argument is an offset into the buffer. If a buffer is bound to GL_PIXEL_PACK_BUFFER, the last argument of glReadPixels() specifies an offset relative to the start of this buffer.

This is very similar to glVertexAttribPointer(), where the last argument is overloaded in the C/C++ bindings, but Java needs two bindings because it is more type safe. In that case, if a GL_ARRAY_BUFFER binding exists, the last argument is an offset relative to the start of the VBO, otherwise it's a pointer to the data.

Upvotes: 1

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