Скачать или смотреть Understanding Texture Mapping and Lighting in OpenGL's Vertex Shader with GLSL

Learn how to effectively utilize `texture mapping` and `per vertex lighting` in OpenGL using GLSL shaders, and understand why certain implementations may lead to unexpected results.
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Mastering Texture Mapping and Lighting in OpenGL with GLSL

Introduction

OpenGL is a powerful graphics library that allows developers to create complex 3D graphics. One common challenge faced by developers is achieving per vertex lighting while implementing texture mapping. A user recently encountered a significant issue when trying to render an object using these techniques in an OpenGL project via QOpenGLWindow. What should have been a vibrant, textured object turned entirely black—a frustrating problem for any developer!

In this guide, we will address this issue by explaining the core concepts of texture mapping and lighting in vertex and fragment shaders, specifically in GLSL (OpenGL Shading Language). We seek to clarify why per vertex lighting might not be the best approach for handling lighting, especially when textures are involved, and provide a solution for achieving proper rendering.

The Core Problem

The developer was trying to achieve per vertex lighting alongside texture mapping, but the output was simply a black object. When using texture mapping in the fragment shader, the output worked as expected, leading to the question: Why did the vertex shader fail to properly implement lighting and texture mapping?

Understanding Vertex vs. Fragment Shaders

Vertex Shader:

Executes per vertex (corner of a primitive).

Outputs interpolated data for fragments (pixels).

Ideal for handling vertex transformations, but not lighting with textures.

Fragment Shader:

Executes per fragment (essentially the pixel).

Can access textures per pixel and implement complex lighting.

Best for incorporating various light models and textures.

What Went Wrong

The key takeaway is that using per vertex lighting for textured models can often lead to undesirable results, such as the black color seen in the rendered object.

Key Reasons for the Issue:

Interpolation Challenges: The vertex shader merely outputs colors determined at the vertices. However, when interpolating these colors for the fragments, more complex calculations like texture mapping become problematic, leading to inaccurate renderings.

Non-linear Light Distribution: Per-vertex lighting only accurately represents constant diffuse light. For dynamic lighting such as specular highlights (shiny spots on surfaces) or varied textures, linear interpolation fails to capture the complexities of light distribution.

Inadequate Shader Logic: The configuration entrusted with interpreting diffuse and specular reflections should ideally reside within the fragment shader due to its capability to process pixel data.

The Solution

To remedy the issue of achieving proper lighting and texture mapping, move your lighting calculations exclusively to the fragment shader. This provides a more accurate representation necessary for lighting effects, particularly when textures are involved.

Steps to Modify Your Shader:

Keep the Vertex Shader Simplified:

Only pass necessary information (e.g., transformed positions, texture coordinates) to the fragment shader.

Do not conduct lighting computations in the vertex shader since they will lack the interpolative precision needed for accurate pixel rendering.

Implement Lighting in the Fragment Shader:

Move all light calculations, including ambient, directional, point, and spot lighting, to the fragment shader.

Use the interpolated fragment values such as normals and texture coordinates to compute the final output, ensuring you calculate lighting effects based on the actual pixel positions.

Example Approach:

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

Conclusion

By shifting lighting calculations from the vertex shader to the fragment shader, you can unlock enhanced renderi