Fog simulation for partially transparent objects

In a graphics rendering system for generating an output image from a 3D graphics scene having a viewpoint and graphical object models, a method for simulating fog in the 3D graphics scene, the method comprising:

• for a pixel having a color A and being rasterized for an object in the 3D graphics scene, computing an amount of fog ƒ of color F between the viewpoint and the object, wherein A comprises an opacity component;
• applying the amount of fog ƒ between the viewpoint and the object to the pixel using at least F and A in an atop image operator;
• repeating the above for additional pixels rasterized for the object to compute an image simulating the object in fog;
• simulating motion of a moving object in the scene in a z-direction relative to the viewpoint of the scene by rendering the moving object, including fogging the pixels of the moving object with a new fog amount, to compute a fogged image layer representing the moving object;
• and
• re-using a previously computed fogged image layer by compositing the fogged image layer representing the moving object with the previously computed fogged layer to compute an output image.

In a graphics rendering system for generating an output image from a 3D graphics scene, a fog simulator for applying fog of color F to a pixel comprising a color component A, wherein A comprises an opacity value, the fog simulator comprising:

• an amount of fog calculator for calculating an amount of fog ƒ between the viewpoint and an object;
• and
• a fog applier using A, F and ƒ in an atop image operator to apply the amount of fog ƒ of color F to the pixel comprising a color A, wherein A comprises an opacity value;
• wherein the atop image operator comprises:
• a first multiplier for multiplying the amount of fog ƒ and opacity of the pixel to compute ƒAα;
• a second multiplier for multiplying ƒAα and a fog color F to compute ƒAαF;
• a third multiplier for multiplying (1−ƒ) and pixel color A to compute (1−ƒ)A;
• and
• an adder for adding the results of the second and third multipliers to compute a fogged pixel via the atop image operator.

A graphics rendering system including a fog mode for applying fog to objects rendered as pixels to simulate fog, the graphics rendering system comprising:

• a rasterizer for computing pixel color, opacity and depth values for pixel locations in a view space from geometric primitives in the objects, and for computing an amount of fog ƒ between a viewpoint and an object surface of one of the objects being rendered;
• a fog applicator for applying the amount of fog of a predetermined color to rasterized pixels from the rasterizer using an atop image operator to yield a plurality of fogged pixels from different objects, wherein the fogged pixels are fogged using an atop image operator;
• and
• a compositor for compositing the plurality of fogged pixels from different objects together to compute a composite pixel value, wherein the plurality of fogged pixels are fogged using an atop image operator;
• wherein
• the fog is of a color F;
• rasterized pixels from the rasterizer each comprise color components A, the color components A each comprising an opacity component;
• and
• the fog applicator is operable for applying the amount of fog of a predetermined color to rasterized pixels from the rasterizer by using at least the following (a)-(c) for a pixel being fogged:
• (a) multiplying the color components A, including the opacity component, of the pixel being fogged by a factor (1−ƒ) to compute a first intermediate result, where ƒ is a dissolve factor representing an amount of fog between the viewpoint in the scene and pixel depth of the pixel being fogged;
• (b) multiplying color components of a fog color value F by the opacity component of the pixel being fogged, and by the dissolve factor ƒ to compute a second intermediate result;
• and
• (c) adding the first and second intermediate results to compute a fogged pixel, wherein the fogged pixel is a fogged version of the pixel being fogged.

In a graphics rendering system for generating an output image from a 3D graphics scene having a viewpoint and graphical object models, a method for simulating fog in the 3D graphics scene, the method comprising:

• for a pixel being fogged and rasterized for an object in the 3D graphics scene, wherein the pixel being fogged has color components and an opacity component, computing an amount of fog ƒ of color F between the viewpoint and the object;
• applying the amount of fog ƒ between the viewpoint and the object to the pixel being fogged using an atop image operator by performing at least the following (a) (b) and (c):
• (a) multiplying the color components and the opacity component of the pixel being fogged by a factor (1−ƒ) to compute a first intermediate result, where ƒ is a dissolve factor representing an amount of fog between the viewpoint in the scene and pixel depth of the pixel being fogged;
• (b) multiplying color components of a fog color value F by the opacity component of the pixel being fogged, and by the dissolve factor ƒ to compute a second intermediate result;
• and
• (c) adding the first and second intermediate results to compute a fogged pixel, wherein the fogged pixel is a fogged version of the pixel being fogged;
• and
• repeating the above steps for additional pixels rasterized for the object to compute an image simulating the object in fog.

A computer-readable medium having computer executable instructions for performing the following in a graphics rendering system for generating an output image from a 3D graphics scene having a viewpoint and graphical object models to simulate fog in the 3D graphics scene:

• for a pixel being fogged and rasterized for an object in the 3D graphics scene, wherein the pixel being fogged has color components and an opacity component, computing an amount of fog ƒ of color F between the viewpoint and the object;
• applying the amount of fog ƒ between the viewpoint and the object to the pixel being fogged using an atop image operator by performing at least the following (a) (b) and (c):
• (a) multiplying the color components and the opacity component of the pixel being fogged by a factor (1−ƒ) to compute a first intermediate result, where ƒ is a dissolve factor representing an amount of fog between the viewpoint in the scene and pixel depth of the pixel being fogged;
• (b) multiplying color components of a fog color value F by the opacity component of the pixel being fogged, and by the dissolve factor ƒ to compute a second intermediate result;
• and
• (c) adding the first and second intermediate results to compute a fogged pixel, wherein the fogged pixel is a fogged version of the pixel being fogged;
• and
• repeating the above for additional pixels rasterized for the object to compute an image simulating the object in fog.