Make Tonemapping::None a passthrough and add Tonemapping::Linear

[stuartparmenter]

Objective

Tonemapping::None doesn't actually turn everything off. The SDR in-shader path still applies color grading and dither, and clamps negative color channels to zero. The HDR work needs a real off switch, for example to send exact calibration patterns to the display unmodified.

Solution

Tonemapping::None becomes a full passthrough. Cameras that used it with ColorGrading or DebandDither can switch to the new Tonemapping::Linear, which keeps grading, dither, and the clamp under an identity tone curve. Default cameras render unchanged.

Testing

Check, clippy, and tests pass

---

This PR was built by me with the assistance of Claude Code w/ Fable 5

[JMS55]

1. What code makes Tonemapping:Linear apply color grading/exposure/etc, but not Tonemapping::None?
2. Not sure how I feel about this PR overall. I get why you need it, but Tonemapping::None not doing color grading and really/especially exposure feels wrong.

[JMS55]

Why linear => None?

[stuartparmenter]

The shader def only picks the tone curve, and TONEMAP_METHOD_NONE means the identity curve. That's exactly what Linear is, so Linear maps to it. None still hits this match because the pipeline is prepared for every view, but the node never dispatches it (so that arm is dead in practice). I can rename the def to TONEMAP_METHOD_LINEAR if that reads better?

[stuartparmenter]

1. What code makes Tonemapping:Linear apply color grading/exposure/etc, but not Tonemapping::None?
2. Not sure how I feel about this PR overall. I get why you need it, but Tonemapping::None not doing color grading and really/especially exposure feels wrong.

1. Grading and exposure run inside tone_mapping() here:

   bevy/crates/bevy_core_pipeline/src/tonemapping.wesl

   Lines 373 to 428 in 17e28cd

   	fn tone_mapping(in: vec4<f32>, in_color_grading: ColorGrading) -> vec4<f32> {
   	var color = max(in.rgb, vec3(0.0));
   	var color_grading = in_color_grading; // So we can take pointers to it.
   	// Rotate hue if needed, by converting to and from HSV. Remember that hue is
   	// an angle, so it needs to be modulo 2π.
   	@if(HUE_ROTATE) {
   	var hsv = rgb_to_hsv(color);
   	hsv.r = (hsv.r + color_grading.hue) % PI_2;
   	color = hsv_to_rgb(hsv);
   	}
   	// Perform white balance correction. Conveniently, this is a linear
   	// transform. The matrix was pre-calculated from the temperature and tint
   	// values on the CPU.
   	@if(WHITE_BALANCE) {
   	color = max(color_grading.balance * color, vec3(0.0));
   	}
   	// Perform the "sectional" color grading: i.e. the color grading that
   	// applies individually to shadows, midtones, and highlights.
   	@if(SECTIONAL_COLOR_GRADING) {
   	color = sectional_color_grading(color, &color_grading);
   	} @else {
   	// If we're not doing sectional color grading, the exposure might still need
   	// to be applied, for example when using auto exposure.
   	color = color * powsafe(vec3(2.0), color_grading.exposure);
   	}
   	// tone_mapping
   	@if(TONEMAP_METHOD_NONE) {
   	color = color;
   	} @elif(TONEMAP_METHOD_REINHARD) {
   	color = tonemapping_reinhard(color.rgb);
   	} @elif(TONEMAP_METHOD_REINHARD_LUMINANCE) {
   	color = tonemapping_reinhard_luminance(color.rgb);
   	} @elif(TONEMAP_METHOD_ACES_FITTED) {
   	color = ACESFitted(color.rgb);
   	} @elif(TONEMAP_METHOD_AGX) {
   	color = applyAgXLog(color);
   	color = applyLUT3D(color, 32.0);
   	} @elif(TONEMAP_METHOD_SOMEWHAT_BORING_DISPLAY_TRANSFORM) {
   	color = somewhat_boring_display_transform(color.rgb);
   	} @elif(TONEMAP_METHOD_TONY_MC_MAPFACE) {
   	color = sample_tony_mc_mapface_lut(color);
   	} @elif(TONEMAP_METHOD_BLENDER_FILMIC) {
   	color = sample_blender_filmic_lut(color.rgb);
   	} @elif(TONEMAP_METHOD_PBR_NEUTRAL) {
   	color = tonemapping_pbr_neutral(color.rgb);
   	}
   	// Perceptual post tonemapping grading
   	color = saturation(color, color_grading.post_saturation);
   	return vec4(color, in.a);
   	}

   
   Linear still enters that function and selects the identity curve, so the grading around it runs. None no longer sets TONEMAP_IN_SHADER, so the shader never calls the function at all.

2. None is already a full passthrough on Hdr cameras:
   

   bevy/crates/bevy_core_pipeline/src/tonemapping/node.rs

   Lines 46 to 52 in 17e28cd

   	if *tonemapping == Tonemapping::None {
   	return;
   	}
   	if !camera.hdr {
   	return;
   	}

   , so Sdr and Hdr cameras already behave differently -- this helps unify them (for both None and Linear)

The camera's Exposure component is applied in the lighting pass and isn't changed. AutoExposure requires Hdr, where None already bypassed it. The only real change here is an SDR camera that pairs None with a custom grading exposure, and switching to Linear solves that.