tailieunhanh - Photoshop 6 for Windows Bible- P13

Photoshop 6 for Windows Bible- P13:If you are reading this foreword, it probably means that you’ve purchased a copy of Adobe Photoshop , and for that I and the rest of the Photoshop team at Adobe thank you. If you own a previous edition of the Photoshop Bible, you probably know what to expect. If not, then get ready for an interesting trip. | Chapter 8 Selections and Paths 329 32 default 60 100 Figure 8-6 Note the results of clicking on a pixel with a brightness value of 140 top row and a brightness value of 10 bottom row with the tolerance set to three different values. Red Green Blue Figure 8-7 Because the yellow Sasquatch sign contains almost no blue it appears most clearly distinguished from its background in the blue channel. So the blue channel is the easiest channel in which to select the sign with the magic wand. 330 Part III Selections Masks and Filters Note Here s one more twist to the Tolerance story The magic wand is affected by the Sample Size option that you select for the eyedropper tool. If you select Point Sample the wand bases its selection solely on the single pixel that you click. But if you select 3 by 3 Average or 5 by 5 Average the wand takes into account 15 or 25 pixels respectively. As you can imagine this option can have a noticeable impact on the extent of the selection that you get from the wand. Try clicking the same spot in your image using each of these Sample Size settings using the same Tolerance value throughout to see what I mean. Making the wand see beyond a single layer The Use All Layers option enables you to create a selection based on pixels from different layers see Chapter 12 for more about layers . Returning to my previous landmass example suppose you set Europe on one layer and North America on the layer behind it so the two continents overlap. Normally if you clicked inside Europe with the magic wand it would select an area inside Europe without extending out into the area occupied by North America on the other layer. Because the wand doesn t even see the contents of other layers anything outside Europe is an empty void. We re talking pre-Columbus Europe here. If you select Use All Layers though the situation changes. Suddenly the wand can see all the layers you can see. If you click on Europe and if North America and Europe contain similar colors the wand .