CODEWITHSUNDEEP
c++openglsprite-sheet
Semicolon
Semicolon

Reputation: 163

Techniques for drawing spritesheets in OpenGL with shaders

I'm learning OpenGL with right now and I'd like to draw some sprites to the screen from a sprite sheet. I'm note sure if I'm doing this the right way though.

What I want to do is to build a world out of tiles à la Terraria. That means that all tiles that build my world are 1x1, but I might want things later like entities that are 2x1, 1x2, 2x2 etc.

What I do right now is that I have a class named "Tile" which contains the tile's transform matrix and a pointer to its buffer. Very simple:

Tile::Tile(glm::vec2 position, GLuint* vbo)
{
    transformMatrix = glm::translate(transformMatrix, glm::vec3(position, 0.0f));
    buffer = vbo;
}

Then when I draw the tile I just bind the buffer and update the shader's UV-coords and vertex position. After that I pass the tile's transform matrix to the shader and draw it using glDrawElements:

glEnableVertexAttribArray(positionAttrib);
glEnableVertexAttribArray(textureAttrib);

for(int i = 0; i < 5; i++) 
{
    glBindBuffer(GL_ARRAY_BUFFER, *tiles[i].buffer);

    glVertexAttribPointer(positionAttrib, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(GLfloat), 0);
    glVertexAttribPointer(textureAttrib, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(GLfloat), (void*)(2 * sizeof(GLfloat)));

    glUniformMatrix4fv(transformMatrixLoc, 1, GL_FALSE, value_ptr(tiles[i].transformMatrix));
    glDrawElements(GL_TRIANGLE_STRIP, 4, GL_UNSIGNED_INT, 0);
}

glDisableVertexAttribArray(positionAttrib);
glDisableVertexAttribArray(textureAttrib);

Could I do this more efficiently? I was thinking that I could have one buffer for 1x1 tiles, one buffer for 2x1 tiles etc. etc. and then just have the Tile class contain UVpos and UVsize and then just send those to the shader, but I'm not sure how I'd do that.

I think what I described with one buffer for 1x1 and one for 2x1 sounds like it would be a lot faster.

Upvotes: 3

Views: 2736

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473407

Could I do this more efficiently?

I don't think you could do it less efficiently. You are binding a whole buffer object for each quad. You are then uploading a matrix. For each quad.

The way tilemap drawing (and only the map, not the entities) normally works is that you build a buffer object that contains some portion of the visible screen's tiles. Empty space is rendered as a transparent tile. You then render all of the tiles for that region of the screen, all in one drawing call. You provide one matrix for all of the tiles in that region.

Normally, you'll have some number of such visible regions, to make it easy to update the tiles for that region when they change. Regions that go off-screen are re-used for regions that come on-screen, so you fill them with new tile data.

Upvotes: 1

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