"I wrote the shifting logic that way because I've been burnt by a micro-coded shift-by-variable instruction, where shifting by a constant takes 1 cycle, but shifting by a variable breaks down into a little for loop that shifts by 1 n times, over n*k cycles... I can't actually remember whether this is the case on modern PC CPUs or not

"

But why all the bulking up the cose with unsigned int(4) unsigned int(8) when 4 , 8 ... is all you need The shift only takes ints so you could unclutter it a bit at least for that

Also since you own this function couldnt you shorten the variable names (its really fairly obvious what it does and is quite repetative and a simple comment would tell any less knowlegable person what it does

also what is the guts of quantize4() and could you imbed it even further to chop out extraneous operations ???

A table lookup for a non linear conversion function with a byte domain (feeding it the float to int calc directly as subscript) ???

or mutate the inline quantize4() function with the * 255.0 + .5 imbedded inside it return an unsigned int from it and imbed the whole call right into the shift equation sequence (probably need to speed testcompare it to see if any such condensations make any diference besides looking cleaner .... getting rid of the intermediary variables....)

heh, you could also reuse the 0 -7 qA variables for the second set 8 -15

something less bulky like

DXT3AlphaBlock compressDXT3Alpha(vec4 colors[16]) { unsigned int qA0,qA1,qA2,QA3,qA4,qA5,qA6,qA7; DXT3AlphaBlock dxt3Alpha; qA0 = quantize4(int(colors[0].w * 255.0 + .5)); qA1 = quantize4(int(colors[1].w * 255.0 + .5)); qA2 = quantize4(int(colors[2].w * 255.0 + .5)); qA3 = quantize4(int(colors[3].w * 255.0 + .5)); qA4 = quantize4(int(colors[4].w * 255.0 + .5)); qA5 = quantize4(int(colors[5].w * 255.0 + .5)); qA6 = quantize4(int(colors[6].w * 255.0 + .5)); qA7 = quantize4(int(colors[7].w * 255.0 + .5)); dxt3Alpha.alphas[0] = qA0 << 0 | qA1 << 4 | qA2 << 8 | qA3 << 12 | qA4 << 16 | qA5 << 20 | qA6 << 24 | qA7 << 28; qA0 = quantize4(int(colors[8].w * 255.0 + .5)); qA1 = quantize4(int(colors[9].w * 255.0 + .5)); qA2 = quantize4(int(colors[10].w * 255.0 + .5)); qA3 = quantize4(int(colors[11].w * 255.0 + .5)); qA4 = quantize4(int(colors[12].w * 255.0 + .5)); qA5 = quantize4(int(colors[13].w * 255.0 + .5)); qA6 = quantize4(int(colors[14].w * 255.0 + .5)); qA7 = quantize4(int(colors[15].w * 255.0 + .5)); dxt3Alpha.alphas[1] = qA0 << 0 | qA1 << 4 | qA2 << 8 | qA3 << 12 | qA4 << 16 | qA5 << 20 | qA6 << 24 | qA7 << 28; return dxt3Alpha; }

This is a function taht looks like it will be crunching alot of bulk data for texture conversion so doing such (and similar) optimization could add up for the actuual programs

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DXT3AlphaBlock compressDXT3Alpha(vec4 colors[16]) { DXT3AlphaBlock dxt3Alpha; dxt3Alpha.alphas[0] = quantize4(int(colors[0].w * 255.0 + .5)) << 0 | quantize4(int(colors[1].w * 255.0 + .5)) << 4 | quantize4(int(colors[2].w * 255.0 + .5)) << 8 | quantize4(int(colors[3].w * 255.0 + .5)) << 12 | quantize4(int(colors[4].w * 255.0 + .5)) << 16 | quantize4(int(colors[5].w * 255.0 + .5)) << 20 | quantize4(int(colors[6].w * 255.0 + .5)) << 24 | quantize4(int(colors[7].w * 255.0 + .5)) << 28; dxt3Alpha.alphas[1] = quantize4(int(colors[8].w * 255.0 + .5)) << 0 | quantize4(int(colors[9].w * 255.0 + .5)) << 4 | quantize4(int(colors[10].w * 255.0 + .5)) << 8 | quantize4(int(colors[11].w * 255.0 + .5)) << 12 | quantize4(int(colors[12].w * 255.0 + .5)) << 16 | quantize4(int(colors[13].w * 255.0 + .5)) << 20 | quantize4(int(colors[14].w * 255.0 + .5)) << 24 | quantize4(int(colors[15].w * 255.0 + .5)) << 28; return dxt3Alpha; }

wrapper of ((unsigned int) quantize4(...)) possibly needed if the shift is wonky

pointer math on the colors[].w Float ptr with ptr += 4 to eliminate the .array index multiply ???

etc....