CODEWITHSUNDEEP
objective-cmacosopengl
hockeyman
hockeyman

Reputation: 1183

if vertex array count > 1000, glDrawArrays becomes slow?

I have painting app. Mouse event coordinates are stored to VertexArray. Then vertex array is being drawn to screen. My code structure looks like this

// I get mouse event coordinates and store them to VertexArray
glPushMatrix();
//some new matrix settings
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer);
glClear(GL_COLOR_BUFFER_BIT);
//now I draw first full size textured quad and later I draw vertexArray

glDrawArrays(.....);

//and now I draw second full size textured quad on top of first quad ant that what have been drawn from vertex array

glPopMatrix();
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);


//immediately after that I draw FBO to screen:
glBindTexture(GL_TEXTURE_2D, fbTexture);
//Code for drawing textured quad
glBindTexture(GL_TEXTURE_2D, 0);

So everything is redrawn every time when new mouse event coordinate is being registered. And if there are more than 1000 coordinates, drawing becomes really slow. Where could be my problem? I thing 1000 vertices for OpenGL is not much

Upvotes: 2

Views: 1351

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 474296

It's not the number of vertices; it's how you're sending them.

First, you never defined "really slow"; often times people will mistakenly think that a change from 400fps to 300fps is "slow". It's not. It only represents a render time increase from 2.5ms-per-frame to 3.3ms, a change of less than a single millisecond. Non-trivial, but probably not something to be too concerned over.

It's always important to measure performance in terms of render time, not FPS.

That being said, your main problem is that you're drawing a single quad at a time. Each one coming from a separate glDrawArrays command. That's not necessarily a good thing, especially if you change state between drawing commands (like binding a texture and so forth).

If you're doing that, then you need to find ways to avoid doing that. What you want to do is render a lot of quads all with one draw calls. This means you have to use the same texture for all of them.

The common solution to this problem is to make a larger texture that has multiple images in different locations. This is commonly called a "texture atlas" (Google that for the details). Each quad would have texture coordinates for the particular image it renders. Text is often drawn in such a way, where each letter (glyph) is stored in the same texture.

Upvotes: 3

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