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Alpha Testing in the GS world

Today, we will discuss the alpha testing feature of the GS. The feature is very basic but awful to emulate. Yet again, GS capability is 10 miles ahead of current GPUs.

Let's first look at the graphics pipeline to understand what alpha testing is. The main purpose of the graphic rendering pipeline is to compute the color of a pixel. But the color isn't enough if you want to render a 3D scene. Standard renderers use a Z-buffer, also called a depth buffer. They are based on a pixel test (note: they ought to be called fragment tests). It is really easy; If the test fails (pixel is rendered behind the current one), you discard the color depth value. If the test passes (pixel is rendered above the current one), you update both, color and depth values.

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Channel Shuffle Effect

I) Introduction

Dear PCSX2 users,

GSdx got various improvements recently, issues were fixed on games such as Metal Gear Solid 3, Gran Turismo 4, Tekken 5, Tales of Legendia and Urban Chaos. All of those games use a certain effect that I have nicknamed as the "Channel Shuffle Effect". All games use a variation of this effect but each time the effect proved to be memory expensive and it was very slow, even with the best computers. The effect is very interesting because it explains the rendering/GPU architecture evolution from a fixed unit to a processor. So I decided to take this opportunity to explain it the best I can.


Read more: Channel Shuffle Effect


Explanation of impossible blend

The goal of blending is to combine two colors. The general equation on a modern GPU is:



coefficient1 * color1 +/- coefficient2 * color2

Color1/Color 2 are either the source color or the destination color.
Coefficient1/Coefficient2 are either the alpha value (transparency) of source/source2/destination, 1 - alpha, or a constant. The GPU will clamp the coefficients to [0;1]

The general equation on the PS2 however is:



(Color1 - Color2) * Coefficient + Color3

Color1/Color2/Color3 are either the source or destination color or zero.

Coefficient is the alpha value (transparency) of the source or destination, or a constant.

Read more: Explanation of impossible blend


Explanation of why hacks are needed to fix upscaling related glitches

First, let's explain aliasing and why people are so keen on removing it with anti-aliasing. Let's start with some math Smile

In the real world, signals are often "continuous." If you take two points A and B and A is very close to B then F(A) is very close to F(B). But a screen is not continuous. If you take two pixels side by side, one can be white while the other is black.

Read more: Explanation of why hacks are needed to fix upscaling related glitches


MMU mini-series part 2 of 4

Hello PCSX2 followers,

After a long delay, this is the second part of the MMU mini-presentation. In the previous chapter we saw that MMU allows giving a virtual address space to a process. Besides, I told you MMU controls the cache behavior (cacheable/uncached accelerated/uncached). First, let me quickly explain the different cache accesses. Then I will introduce the default memory mapping of the PS2.

Read more: MMU mini-series part 2 of 4

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