How to Successfully Read from One Texture and Write to Another in WebGL

Learn how to manage textures in WebGL, specifically how to read from one texture and write the output to another texture using advanced techniques with shaders and framebuffers.
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This video is based on the question https://stackoverflow.com/q/70780656/ asked by the user 'Avery' ( https://stackoverflow.com/u/11059069/ ) and on the answer https://stackoverflow.com/a/70794298/ provided by the user 'LJᛃ' ( https://stackoverflow.com/u/978057/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the WebGL Texture Management Challenge

When working with WebGL, especially with its advanced version WebGL2, developers often run into hurdles related to texture management. A common scenario arises when you want to render the output of one program (also known as a shader) directly into another texture. This task may seem straightforward at first, but it can lead to frustrating errors that can stall your development progress.

Consider this example: You have two rendering programs. The first program draws a triangle in a specific color onto a texture, while the second program attempts to read from that texture and render its contents to a new texture. This should work seamlessly, but you may encounter an error stating that the source and destination textures for the draw operation are the same.

In this post, we'll explore how to correctly implement this functionality in WebGL, ensuring that you can read from one texture and write to another without errors.

The Setup

You need to create two shaders and set up two framebuffer objects (FBOs) for this process. Here’s a breakdown of what we’ll cover:

Creating Shaders

Setting Up Framebuffers

Rendering to Textures

1. Creating Shaders

First, let's ensure you have both vertex and fragment shaders defined properly. Shaders are small programs executed on the GPU, and they define how the graphics should be rendered.

Here’s a basic setup for the vertex and fragment shaders:

[[See Video to Reveal this Text or Code Snippet]]

2. Setting Up Framebuffers

Next, we need to set up two framebuffer objects. Framebuffers are off-screen rendering targets that allow you to render textures without displaying them directly to the screen. Here’s how to do this:

Framebuffer for Program 1

[[See Video to Reveal this Text or Code Snippet]]

Framebuffer for Program 2

After unbinding the first framebuffer, create and bind the second framebuffer like this:

[[See Video to Reveal this Text or Code Snippet]]

3. Rendering to Textures

Rendering to each texture requires careful management of your current framebuffer state. To avoid errors when you try to render, ensure you correctly bind the first texture before using Program 2 to read it.

Below is the sequence of actions you should take:

After rendering your triangle in Program 1, unbind the framebuffer.

Switch to Program 2 and specify the texture you want it to read from:

[[See Video to Reveal this Text or Code Snippet]]

Finally, execute your drawing command:

[[See Video to Reveal this Text or Code Snippet]]

Troubleshooting Common Errors

You may encounter an error like GL_INVALID_OPERATION: glDrawArrays: Source and destination textures of the draw are the same. This indicates that WebGL believes you are trying to read from and write to the same texture simultaneously. To prevent this, always ensure that you're binding the texture you want to read from before making draw calls in the second program.

Conclusion

Learning how to manage multiple textures in WebGL can dramatically improve the functionality of your applications. By ensuring that textures are correctly bound before rendering and that framebuffers are managed properly, you will streamline the rendering process, enabling complex graphics operations without errors.

This approach of clearly separating the rendering into two distinct programs, each with its framebuffer and texture targets, allows for expansive creative possibilities in your WebGL applications.

With this guide, you should now have a solid understanding of how to read from one texture and write to another in WebGL. Happy coding!

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