CODEWITHSUNDEEP
opengl
Russ
Russ

Reputation: 11

how to update a certain channel of a texture

I am now using FFMPEG to read a high resolution video (6480*1920) and use opengl to show it

after decoding, I get 3 pointer that point to the Y,U,V.

At first, I use swsscale to convert it rgb and show it, but I find it's too slow. So I directly deal with YUV. My second try is generate 3 one channel texture and convert it to rgb in fragment shader. It is faster, but still cannot achieve 60fps

I find the bottleneck is this function : texture(texy, tex_coord.xy). When the texture is large, it cost a lot of time. So instead of call it 3 times, my idea is to put the YUV in one single texture since a texture can have 4 channel. But I wonder that how can I update a certain channel of a texture.

I try the following code, but it seems do not work. Instead of update a channel, glTexSubImage2D changes the whole texture:

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, frame->width, frame->height,0, GL_RED, GL_UNSIGNED_BYTE, Y);
glTexSubImage2D(GL_TEXTURE_2D,0,0,0,frame->width, frame->height, GL_GREEN,U);
glTexSubImage2D(GL_TEXTURE_2D,0,0,0,frame->width, frame->height, GL_BLUE,V);

So how can I use one texture to pass the YUV data ? I also try that gather the YUV data into one array then generate the texture. But it does not help since it need a lot of time to generate that array.

Any good idea?

Upvotes: 1

Views: 320

Answers (1)

datenwolf
datenwolf

Reputation: 162319

You're approaching this from the wrong angle, since you don't actually understand what is causing the poor performance in the first place. Yes, texture access is a rather expensive operation. But it is not that expensive; I mean, just think about of the amount of texture data that gets pushed around in modern games at very high frame rates.

The problem is not the channel format of the texture, and it is also not the call of GLSL texture.

Your problem is this:

(…) high resolution video (6480*1920)

Plain and simple the dimensions of the frame are outside the range of what the GPU is comfortable working with. Try breaking down the picture into a set of smaller textures. Using glPixelStorei paramters GL_UNPACK_ROW_LENGTH, GL_UNPACK_SKIP_PIXELS and GL_UNPACK_SKIP_ROWS you can select the rectangle inside your source picture to copy.

You don't have to make several draw calls BTW, just select the texture inside the shader based on the target fragment position or texture coordinate.

Unfortunately OpenGL doesn't offer a convenient function to determine the sweet spot, for most GPUs these days the maximum size in either direction for dense textures is 2048. Go above it and in my experience the performance tanks for dense textures.

Sparse textures are an entirely different chapter, and irrelevant for this problem.

And just for the sake of completeness: I take it, that you don't reinitialize the texture for each and every frame with a call to glTexImage2D. Do that only once at the start of the video, then just update the texture(s).

Upvotes: 1

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