Using sRGB Framebuffer Attachments in OpenGL for Gamma‑Correct Off‑Screen Rendering

Why use an sRGB-enabled FBO?

When you render off‑screen to a texture that will later be sampled in a shader, the color values are stored exactly as written. If your lighting calculations assume linear RGB but the texture is stored in sRGB space, the final image will appear too dark or too bright. By attaching an sRGB-encoded texture to a Framebuffer Object and enabling GL_FRAMEBUFFER_SRGB, OpenGL automatically converts linear values written by the shader to sRGB when storing them, and later samples perform the inverse conversion. This gives you correct gamma‑correct rendering without extra math in the shader.

Worked configuration: sRGB color attachment

The following code creates an FBO, attaches a texture with an sRGB internal format, checks completeness, and shows where to enable the sRGB conversion. Replace width and height with the desired render target size.

// 1. Generate and bind the FBO
GLuint fbo;
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);

// 2. Create an sRGB texture to serve as color attachment 0
GLuint colorTex;
glGenTextures(1, &colorTex);
glBindTexture(GL_TEXTURE_2D, colorTex);
// GL_SRGB8_ALPHA8 stores 8‑bit sRGB R,G,B and linear alpha
glTexImage2D(GL_TEXTURE_2D, 0, GL_SRGB8_ALPHA8, width, height, 0,
             GL_RGBA, GL_UNSIGNED_BYTE, NULL);
// Typical filtering/wrapping for a render target
 glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
 glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
 glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
 glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

// 3. Attach the texture to the FBO
 glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
                        GL_TEXTURE_2D, colorTex, 0);

// 4. Verify completeness
 GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
 if (status != GL_FRAMEBUFFER_COMPLETE) {
     // Handle error – e.g., unsupported format or mismatched dimensions
 }

// 5. (Optional) Enable automatic sRGB conversion for this FBO
// This affects writes to the attachment while the FBO is bound.
 glEnable(GL_FRAMEBUFFER_SRGB);

// 6. Set viewport matching the attachment size before rendering
 glViewport(0, 0, width, height);
 glClearColor(0.2f, 0.4f, 0.8f, 1.0f); // linear RGB clear color
 glClear(GL_COLOR_BUFFER_BIT);
 // … draw geometry here …

// 7. When finished, disable sRGB conversion and return to default FBO
 glDisable(GL_FRAMEBUFFER_SRGB);
 glBindFramebuffer(GL_FRAMEBUFFER, 0);

// 8. (Optional) Use the generated texture in a later pass
// glActiveTexture(GL_TEXTURE0);
// glBindTexture(GL_TEXTURE_2D, colorTex);
// … sample it in a shader; the sampler will convert sRGB → linear automatically if
//    GL_TEXTURE_2D's internal format is sRGB or you enable GL_TEXTURE_SRGB_DECODE_EXT.

Limits you should query

• Maximum color attachments: GL_MAX_COLOR_ATTACHMENTS. Query with glGetIntegerv(GL_MAX_COLOR_ATTACHMENTS, &value);
• Maximum viewport dimensions: GL_MAX_VIEWPORT_DIMS returns width and height limits. Ensure your width and height are within this range.
• sRGB renderability: Not all implementations support sRGB as a renderable internal format. After attaching, you can check GL_FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING for the attachment; if it returns GL_SRGB the format is sRGB‑encoded, otherwise you fell back to linear.

Common pitfalls and how to avoid them

1. Forgetting to enable GL_FRAMEBUFFER_SRGB.
Without the enable, OpenGL writes linear values directly into the sRGB texture, bypassing the conversion, so the texture stores non‑sRGB data.
2. Using a non‑sRGB internal format.
Formats like GL_RGBA8 are linear; attaching them will not give you automatic gamma correction.
3. Mismatched attachment dimensions.
All attachments to the same FBO must share width and height; otherwise glCheckFramebufferStatus returns GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT.
4. Not disabling GL_FRAMEBUFFER_SRGB before rendering to the default framebuffer.
If left enabled, the window system framebuffer (usually linear) will be treated as sRGB, causing colors to be distorted when presented.
5. Assuming the sampled texture is already linear.
When you later sample the sRGB texture, you must either use a sampler that performs sRGB→linear decode (default for sRGB internal formats in OpenGL 2.1+) or explicitly decode in the shader. Forgetting this step leads to double‑gamma correction.

Verification steps

1. After rendering to the FBO, either glReadPixels from the bound FBO or sample the attached texture in a subsequent pass and compare the result to the expected sRGB‑encoded value. For a clear color of (0.2, 0.4, 0.8) linear, the stored sRGB values are approximately (0.466, 0.631, 0.885) after conversion.
2. Check that glCheckFramebufferStatus(GL_FRAMEBUFFER) returns GL_FRAMEBUFFER_COMPLETE. Any other value indicates an incomplete FBO and should be inspected with glGetError or by examining the status enum.
3. Optionally query GL_MAX_COLOR_ATTACHMENTS and GL_MAX_VIEWPORT_DIMS to ensure your chosen size and attachment count are within limits.
4. To confirm the sRGB path is active, you can query GL_FRAMEBUFFER_SRGB with glIsEnabled while the FBO is bound; it should return GL_TRUE when you have enabled it.